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Vol. 2, by Kempster Miller and George Patterson and Charles Thom and Robert Millikan and Samuel McMeen
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Title: Cyclopedia of Telephony and Telegraphy, Vol. 2
A General Reference Work on Telephony, etc. etc.
Author: Kempster Miller
George Patterson
Charles Thom
Robert Millikan
Samuel McMeen
Release Date: August 15, 2010 [EBook #33437]
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK TELEPHONY AND TELEGRAPHY, VOL 2 ***
Produced by Ronald Holder, Stephen H. Sentoff and the
Online Distributed Proofreading Team at https://www.pgdp.net
[Illustration: THOMAS A. EDISON Pioneer Electrical Investigator and
Inventor of Numerous Telegraph, Telephone, Lighting, and Other
Electrical Devices.]
Cyclopedia
of
Telephony and Telegraphy
_A General Reference Work on_
TELEPHONY, SUBSTATIONS, PARTY LINE SYSTEMS, PROTECTION, MANUAL
SWITCHBOARDS, AUTOMATIC SYSTEMS, POWER PLANTS, SPECIAL
SERVICE FEATURES, CONSTRUCTION, ENGINEERING,
OPERATION, MAINTENANCE, TELEGRAPHY, WIRELESS
TELEGRAPHY AND TELEPHONY, ETC.
_Prepared by a Corps of_
TELEPHONE AND TELEGRAPH EXPERTS, AND ELECTRICAL ENGINEERS OF
THE HIGHEST PROFESSIONAL STANDING
_Illustrated with over Two Thousand Engravings_
FOUR VOLUMES
CHICAGO
AMERICAN SCHOOL OF CORRESPONDENCE
1919
COPYRIGHT, 1911, 1912,
BY
AMERICAN SCHOOL OF CORRESPONDENCE
COPYRIGHT, 1911, 1912
BY
AMERICAN TECHNICAL SOCIETY
Entered at Stationers' Hall, London
All Rights Reserved
Authors and Collaborators
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head, Department of Electrical Engineering, University of Michigan
* * * * *
CHARLES THOM
Chief of Quadruplex Department, Western Union Main Office, New York City
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Associate Professor of Physics, University of Chicago
Member, Executive Council, American Physical Society
* * * * *
SAMUEL G. McMEEN
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
LAWRENCE K. SAGER, S.B., M.P.L.
Patent Attorney and Electrical Expert
Formerly Assistant Examiner, U.S. Patent Office
* * * * *
GLENN M. HOBBS, Ph.D.
Secretary, American School of Correspondence
Formerly Instructor in Physics, University of Chicago
American Physical Society
* * * * *
CHARLES G. ASHLEY
Electrical Engineer and Expert in Wireless Telegraphy and Telephony
* * * * *
A. FREDERICK COLLINS
Editor, _Collins Wireless Bulletin_
Author of "Wireless Telegraphy, Its History, Theory, and Practice"
* * * * *
FRANCIS B. CROCKER, E.M., Ph.D.
Head, Department of Electrical Engineering, Columbia University
Past-President, American Institute of Electrical Engineers
* * * * *
MORTON ARENDT, E.E.
Instructor in Electrical Engineering, Columbia University, New York
* * * * *
EDWARD B. WAITE
Head, Instruction Department, American School of Correspondence
American Society of Mechanical Engineers
Western Society of Engineers
* * * * *
DAVID P. MORETON, B.S., E.E.
Associate Professor of Electrical Engineering, Armour Institute of
Technology
American Institute of Electrical Engineers
* * * * *
LEIGH S. KEITH, B.S.
Managing Engineer with McMeen and Miller, Electrical Engineers and
Patent Experts Chicago
Associate Member, American Institute of Electrical Engineers
* * * * *
JESSIE M. SHEPHERD, A.B.
Associate Editor, Textbook Department, American School of Correspondence
* * * * *
ERNEST L. WALLACE, B.S.
Assistant Examiner, United States Patent Office, Washington, D. C.
* * * * *
GEORGE R. METCALFE, M.E.
Editor, _American Institute of Electrical Engineers_
Formerly Head of Publication Department, Westinghouse Elec. & Mfg. Co.
* * * * *
J. P. SCHROETER
Graduate, Munich Technical School
Instructor in Electrical Engineering, American School of Correspondence
* * * * *
JAMES DIXON, E.E.
American Institute of Electrical Engineers
* * * * *
HARRIS C. TROW, S.B., _Managing Editor_
Editor-in-Chief, Textbook Department, American School of Correspondence
Authorities Consulted
The editors have freely consulted the standard technical literature of
America and Europe in the preparation of these volumes. They desire to
express their indebtedness particularly to the following eminent
authorities, whose well-known works should be in the library of every
telephone and telegraph engineer.
Grateful acknowledgment is here made also for the invaluable
co-operation of the foremost engineering firms and manufacturers in
making these volumes thoroughly representative of the very best and
latest practice in the transmission of intelligence, also for the
valuable drawings, data, suggestions, criticisms, and other courtesies.
* * * * *
ARTHUR E. KENNELY, D.Sc.
Professor of Electrical Engineering, Harvard University.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
HENRY SMITH CARHART, A.M., LL.D.
Professor of Physics and Director of the Physical Laboratory, University
of Michigan.
Author of "Primary Batteries," "Elements of Physics," "University
Physics," "Electrical Measurements," "High School Physics," etc.
* * * * *
FRANCIS B. CROCKER, M.E., Ph.D.
Head of Department of Electrical Engineering, Columbia University, New
York; Past-President, American Institute of Electrical Engineers.
Author of "Electric Lighting;" Joint Author of "Management of Electrical
Machinery."
* * * * *
HORATIO A. FOSTER
Consulting Engineer; Member of American Institute of Electrical
Engineers; Member of American Society of Mechanical Engineers.
Author of "Electrical Engineer's Pocket-Book."
* * * * *
WILLIAM S. FRANKLIN, M.S., D.Sc.
Professor of Physics, Lehigh University.
Joint Author of "The Elements of Electrical Engineering," "The Elements
of Alternating Currents."
* * * * *
LAMAR LYNDON, B.E., M.E.
Consulting Electrical Engineer; Associate Member of American Institute
of Electrical Engineers; Member, American Electro-Chemical Society.
Author of "Storage Battery Engineering."
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Professor of Physics, University of Chicago.
Joint Author of "A First Course in Physics," "Electricity, Sound and
Light," etc.
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert; of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago.
Author of "American Telephone Practice."
* * * * *
WILLIAM H. PREECE
Chief of the British Postal Telegraph.
Joint Author of "Telegraphy," "A Manual of Telephony," etc.
* * * * *
LOUIS BELL, Ph.D.
Consulting Electrical Engineer; Lecturer on Power Transmission,
Massachusetts Institute of Technology.
Author of "Electric Power Transmission," "Power Distribution for
Electric Railways," "The Art of Illumination," "Wireless Telephony,"
etc.
* * * * *
OLIVER HEAVISIDE, F.R.S.
Author of "Electro-Magnetic Theory," "Electrical Papers," etc.
* * * * *
SILVANUS P. THOMPSON, D.Sc., B.A., F.R.S., F.R.A.S.
Principal and Professor of Physics in the City and Guilds of London
Technical College.
Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
"Polyphase Electric Currents and Alternate-Current Motors," "The
Electromagnet," etc.
* * * * *
ANDREW GRAY, M.A., F.R.S.E.
Author of "Absolute Measurements in Electricity and Magnetism."
* * * * *
ALBERT CUSHING CREHORE, A.B., Ph.D.
Electrical Engineer; Assistant Professor of Physics, Dartmouth College;
Formerly Instructor in Physics, Cornell University.
Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of
"Alternating Currents."
* * * * *
J. J. THOMSON, D.Sc., LL.D., Ph.D., F.R.S.
Fellow of Trinity College, Cambridge University; Cavendish Professor of
Experimental Physics, Cambridge University.
Author of "The Conduction of Electricity through Gases," "Electricity
and Matter."
* * * * *
FREDERICK BEDELL, Ph.D.
Professor of Applied Electricity, Cornell University.
Author of "The Principles of the Transformer;" Joint Author of
"Alternating Currents."
* * * * *
DUGALD C. JACKSON, C.E.
Head of Department of Electrical Engineering, Massachusetts Institute of
Technology; Member, American Institute of Electrical Engineers, etc.
Author of "A Textbook on Electromagnetism and the Construction of
Dynamos;" Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.
Professor of Electro-Mechanics, Columbia University, New York.
Author of "Propagation of Long Electric Waves," and "Wave-Transmission
over Non-Uniform Cables and Long-Distance Air Lines."
* * * * *
FRANK BALDWIN JEWETT, A.B., Ph.D.
Transmission and Protection Engineer, with American Telephone &
Telegraph Co.
Author of "Modern Telephone Cable," "Effect of Pressure on Insulation
Resistance."
* * * * *
ARTHUR CROTCH
Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical
Schools, Norwich, Eng.
Author of "Telegraphy and Telephony."
* * * * *
JAMES ERSKINE-MURRAY, D.Sc.
Fellow of the Royal Society of Edinburgh; Member of the Institution of
Electrical Engineers.
Author of "A Handbook of Wireless Telegraphy."
* * * * *
A. H. McMILLAN, A.B., LL.B.
Author of "Telephone Law, A Manual on the Organization and Operation of
Telephone Companies."
* * * * *
WILLIAM ESTY, S.B., M.A.
Head of Department of Electrical Engineering, Lehigh University.
Joint Author of "The Elements of Electrical Engineering."
* * * * *
GEORGE W. WILDER, Ph.D.
Formerly Professor of Telephone Engineering, Armour Institute of
Technology.
Author of "Telephone Principles and Practice," "Simultaneous Telegraphy
and Telephony," etc.
* * * * *
WILLIAM L. HOOPER, Ph.D.
Head of Department of Electrical Engineering, Tufts College.
Joint Author of "Electrical Problems for Engineering Students."
* * * * *
DAVID S. HULFISH
Technical Editor, _The Nickelodeon_; Telephone and Motion-Picture
Expert; Solicitor of Patents.
Author of "How to Read Telephone Circuit Diagrams."
* * * * *
J. A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.
Professor of Electrical Engineering in University College, London; Late
Fellow and Scholar of St. John's College, Cambridge; Fellow of
University College, London.
Author of "The Alternate-Current Transformer," "Radiotelegraphy and
Radiotelephony," "Principles of Electric Wave Telegraphy," "Cantor
Lectures on Electrical Oscillations and Electric Waves," "Hertzian Wave
Wireless Telegraphy," etc.
* * * * *
F. A. C. PERRINE, A.M., D.Sc.
Consulting Engineer; Formerly President, Stanley Electric Manufacturing
Company; Formerly Professor of Electrical Engineering, Leland Stanford,
Jr. University.
Author of "Conductors for Electrical Distribution."
* * * * *
A. FREDERICK COLLINS
Editor, _College Wireless Bulletin_.
Author of "Wireless Telegraphy, Its History, Theory and Practice,"
"Manual of Wireless Telegraphy," "Design and Construction of Induction
Coils," etc.
* * * * *
SCHUYLER S. WHEELER, D.Sc.
President, Crocker-Wheeler Co.; Past-President, American Institute of
Electrical Engineers.
Joint Author of "Management of Electrical Machinery."
* * * * *
CHARLES PROTEUS STEINMETZ
Consulting Engineer, with the General Electric Co.; Professor of
Electrical Engineering, Union College.
Author of "The Theory and Calculation of Alternating-Current Phenomena,"
"Theoretical Elements of Electrical Engineering," etc.
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head of Department of Electrical Engineering, University of Michigan.
Joint Author of "Electrical Measurements."
* * * * *
WILLIAM MAVER, Jr.
Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the
American Institute of Electrical Engineers.
Author of "American Telegraphy and Encyclopedia of the Telegraph,"
"Wireless Telegraphy."
* * * * *
JOHN PRICE JACKSON, M.E.
Professor of Electrical Engineering, Pennsylvania State College.
Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
AUGUSTUS TREADWELL, Jr., E.E.
Associate Member, American Institute of Electrical Engineers.
Author of "The Storage Battery, A Practical Treatise on Secondary
Batteries."
* * * * *
EDWIN J. HOUSTON, Ph.D.
Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor
of Thomson-Houston System of Arc Lighting; Electrical Expert and
Consulting Engineer.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
WILLIAM J. HOPKINS
Professor of Physics in the Drexel Institute of Art, Science, and
Industry, Philadelphia.
Author of "Telephone Lines and their Properties."
[Illustration: GROSSE POINT EXCHANGE RACK Detroit Home Telephone
Company, Detroit, Mich. _The Dean Electric Co._]
[Illustration: LINE SIDE OF LARGE MAIN DISTRIBUTING FRAME]
Foreword
The present day development of the "talking wire" has annihilated both
time and space, and has enabled men thousands of miles apart to get into
almost instant communication. The user of the telephone and the
telegraph forgets the tremendousness of the feat in the simplicity of
its accomplishment; but the man who has made the feat possible knows
that its very simplicity is due to the complexity of the principles and
appliances involved; and he realizes his need of a practical, working
understanding of each principle and its application. The Cyclopedia of
Telephony and Telegraphy presents a comprehensive and authoritative
treatment of the whole art of the electrical transmission of
intelligence.
The communication engineer--if so he may be called--requires a knowledge
both of the mechanism of his instruments and of the vagaries of the
current that makes them talk. He requires as well a knowledge of plants
and buildings, of office equipment, of poles and wires and conduits, of
office system and time-saving methods, for the transmission of
intelligence is a business as well as an art. And to each of these
subjects, and to all others pertinent, the Cyclopedia gives proper space
and treatment.
The sections on Telephony cover the installation, maintenance, and
operation of all standard types of telephone systems; they present
without prejudice the respective merits of manual and automatic
exchanges; and they give special attention to the prevention and
handling of operating "troubles." The sections on Telegraphy cover both
commercial service and train dispatching. Practical methods of wireless
communication--both by telephone and by telegraph--are thoroughly
treated.
The drawings, diagrams, and photographs incorporated into the Cyclopedia
have been prepared especially for this work; and their instructive value
is as great as that of the text itself. They have been used to
illustrate and illuminate the text, and not as a medium around which to
build the text. Both drawings and diagrams have been simplified so far
as is compatible with their correctness, with the result that they tell
their own story and always in the same language.
The Cyclopedia is a compilation of many of the most valuable Instruction
Papers of the American School of Correspondence, and the method adopted
in its preparation is that which this School has developed and employed
so successfully for many years. This method is not an experiment, but
has stood the severest of all tests--that of practical use--which has
demonstrated it to be the best yet devised for the education of the
busy, practical man.
In conclusion, grateful acknowledgment is due to the staff of authors
and collaborators, without whose hearty co-operation this work would
have been impossible.
Table of Contents
VOLUME II
MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_[A] Page[B] 11
Common-Battery Switchboards--Line Signals--Cord
Circuit--Lamps--Mechanical Signals--Relays--Jacks--Switchboard
Assembly--Transfer Switchboard--Transfer Lines--Handling
Transfers--Multiple Switchboard--Busy Test--Influence of
Traffic--Magneto-Multiple Switchboard--Multiple Boards: Series,
Branch-Terminal, Modern Magneto, Common-Battery--Western Electric No. 1
Relay Board--Western Electric No. 10 Board--Types of Multiple
Boards--Apparatus--Trunking--Western Electric and Kellogg Trunk Circuits
AUTOMATIC SYSTEMS _By K. B. Miller and S. G. McMeen_ Page 135
Automatic vs. Manual--Operation--Selecting Switch--Line Switch--Trunking
Systems--Two- and Three-Wire Systems--Subscriber's Station
Apparatus--First and Second Selector Operation--Connector--Release after
Conversation--Multi-Office System--Automatic Sub-Offices--Rotary
Connector--Party Lines--Two-Wire Automatic System--Lorimer
System--Central-Office Apparatus--Operation--Automanual
System--Operation--Subscriber's Apparatus--Operator's
Equipment--Switching Equipment--Distribution of Calls--Connection--Speed
POWER PLANTS AND BUILDINGS _By K. B. Miller and S. G. McMeen_ Page 227
Currents Employed--Types--Operator's Transmitter Supply--Ringing-Current
Supply--Auxiliary Signaling Current--Primary Sources--Duplicate
Apparatus--Storage Batteries--Power
Switchboards--Circuits--Central-Office Building--Arrangement of
Apparatus--Manual Offices--Automatic Offices
SPECIAL SERVICE FEATURES _By K. B. Miller and S. G. McMeen_ Page 271
Private-Branch Exchanges--Switchboards--Supervision--With Automatic
Offices--Battery Supply--Ringing Current--Inter-Communicating
Systems--Magneto System--Common-Battery Systems--Types--Long-Distance
Switching--Operator's Orders--Trunking--Way Stations--Traffic--Measured
Service--Charging--Rates--Toll Service--Local Service
TELEGRAPH AND RAILWAY WORK _By K. B. Miller and S. G. McMeen_ Page 321
Phantom, Simplex, and Composite Circuits--Ringing--Railway
Composite--Telephone Train Dispatching--Railroad
Conditions--Transmitting Orders--Apparatus--Telephone Equipment--Types
of Circuits--Test Boards--Blocking Sets--Dispatching on Electric
Railways
REVIEW QUESTIONS Page 359
INDEX Page 373
[Footnote A: For professional standing of authors, see list of Authors
and Collaborators at front of volume.]
[Footnote B: For page numbers, see foot of pages.]
[Illustration: PORTION OF TERMINAL ROOM OF LARGE COMMON-BATTERY OFFICE
Prospect Office, New York Telephone Co.]
CHAPTER XXII
THE SIMPLE COMMON-BATTERY SWITCHBOARD
=Advantages of Common-Battery Operation.= The advantages of the
common-battery system of operation, alluded to in Chapter XIII, may be
briefly summarized here. The main gain in the common-battery system of
supply is the simplification of the subscribers' instruments, doing away
with the local batteries and the magneto generators, and the
concentration of all these many sources of current into one single
source at the central office. A considerable saving is thus effected
from the standpoint of maintenance, since the simpler common-battery
instrument is not so likely to get out of order and, therefore, does not
have to be visited so often for repairs, and the absence of local
batteries, of course, makes the renewal of the battery parts by members
of the maintenance department, unnecessary. Another decided advantage in
the common-battery system is the fact that the centralized battery
stands ready always to send current over the line when the subscriber
completes the circuit of the line at his station by removing his
receiver from its hook. The common-battery system, therefore, lends
itself naturally to the purposes of automatic signaling, since it is
only necessary to place at the central office a device in the circuit of
each line that will be responsive to the current which flows from the
central battery when the subscriber removes his receiver from its hook.
It is thus that the subscriber is enabled automatically to signal the
central office when he desires a connection; and as will be shown, it is
by the same sort of means, associated with the cord circuits used in
connecting his line with some other line, that the operator is
automatically notified when a disconnection is desired, the cessation of
current through the subscriber's line when he hangs up his receiver
being made to actuate certain responsive devices which are associated
with the cord at that time connected with his line, and which convey the
proper disconnect signal to the operator.
Concentration of sources of energy into a single large unit, the
simplification of the subscriber's station equipment, and the ready
adaptability to automatic signaling from the subscriber to the central
office are, therefore, the reasons for the existence of the
common-battery system.
=Common Battery vs. Magneto.= It must not be supposed, however, that the
common-battery system always has advantages over the magneto system, and
that it is superior to the magneto or local-battery system for all
purposes. It is the outward attractiveness of the common-battery system
and the arguments in its favor, so readily made by over-zealous
salesmen, that has led, in many cases, to the adoption of this system
when the magneto system would better have served the purpose of utility
and economy.
To say the least, the telephone transmission to be had from
common-battery systems is no better than that to be had from
local-battery systems, and as a rule, assuming equality in other
respects, it is not as good. It is perhaps true, however, that under
average conditions common-battery transmission is somewhat better,
because whereas the local batteries at the subscribers' stations in the
local-battery system are not likely to be in uniformly first-class
condition, the battery in a common-battery system will be kept up to its
full voltage except under the grossest neglect.
The places in which the magneto, or local-battery, system is to be
preferred to the common-battery system, in the opinion of the writers,
are to be found in the small rural communities where the lines have a
rather great average length; where a good many subscribers are likely to
be found on some of the lines; where the sources of electrical power
available for charging storage batteries are likely either not to exist,
or to be of a very uncertain nature; and where it is not commercially
feasible to employ a high-grade class of attendants, or, in fact, any
attendant at all other than the operator at the central office.
In large or medium-sized exchanges it is always possible to procure
suitable current for charging the storage batteries required in
common-battery systems, and it is frequently economical, on account of
the considerable quantity of energy that is thus used, to establish a
generating plant in connection with the central office for developing
the necessary electrical energy. In very small rural places there are
frequently no available sources of electrical energy, and the expense of
establishing a power plant for the purpose cannot be justified. But even
if there is an electric light or railway system in the small town, so
that the problem of available current supply does not exist, the
establishment of a common-battery system with its storage battery and
the necessary charging machinery requires the daily attendance at the
central office of some one to watch and care for this battery, and this,
on account of the small gross revenue that may be derived from a small
telephone system, often involves a serious financial burden.
There is no royal road to a proper decision in the matter, and no sharp
line of demarcation may be drawn between the places where common-battery
systems are superior to magneto and _vice versâ_. It may be said,
however, that in the building of all new telephone plants having over
about 500 local subscribers, the common-battery system is undoubtedly
superior to the magneto. If the plant is an old one, however, and is to
be re-equipped, the continuance of magneto apparatus might be justified
for considerably larger exchanges than those having 500 subscribers.
Telephone operating companies who have changed over the equipment of old
plants from magneto to common battery have sometimes been led into
rather serious difficulty, owing to the fact that their lines, while
serving tolerably well for magneto work, were found inadequate to meet
the more exacting demands of common-battery work. Again in an old plant
the change from magneto to common-battery equipment involves not only
the change of switchboards, but also the change of subscribers'
instruments that are otherwise good, and this consideration alone often,
in our opinion, justifies the replacing of an old magneto board with a
new magneto board, even if the exchange is of such size as to demand a
small multiple board.
Where the plant to be established is of such size as to leave doubt as
to whether a magneto or a common-battery switchboard should be employed,
the questions of availability of the proper kind of power for charging
the batteries, the proper kind of help for maintaining the batteries and
the more elaborate central-office equipment, the demands and previous
education of the public to be served, all are factors which must be
considered in reaching the decision.
It is not proper to say that anything like all exchanges having fewer
than 500 local lines, should be equipped with magneto service. Where all
the lines are short, where suitable power is available, and where a good
grade of attendants is available--as, for instance, in the case of
private telephone exchanges that serve some business establishment or
other institution located in one building or a group of buildings--the
common-battery system is to be recommended and is largely used, even
though it may have but a dozen or so subscribers' lines. It is for such
uses, and for use in those regular public-service exchange systems where
the conditions are such as to warrant the common-battery system, and yet
where the number of lines and the traffic are small enough to be handled
by such a small group of operators that any one of them may reach over
the entire face of the board, that the simple non-multiple
common-battery system finds its proper field of usefulness.
=Line Signals.= The principles and means by which the subscriber is
enabled to call the central-office operator in a common-battery system
have been referred to briefly in Chapter III. We will review these at
this point and also consider briefly the way in which the line signals
are associated with the connective devices in the subscribers' lines.
_Direct-Line Lamp._ The simplest possible way is to put the line signal
directly in the circuit of the line in series with the central-office
battery, and so to arrange the jack of the corresponding line that the
circuit through the line signal will be open when the operator inserts a
plug into that jack. This arrangement is shown in Fig. 307 where the
subscriber's station at the left is indicated in the simplest of its
forms. It is well to repeat here that in all common-battery manual
systems, the subscriber's station equipment, regardless of the
arrangement or type of its talking and signaling apparatus, must have
these features: First, that the line shall be normally open to direct
currents at the subscriber's station; second, that the line shall be
closed to direct currents when the subscriber removes his receiver from
its hook in making or in answering a call; third, that the line
normally, although open to direct currents, shall afford a proper path
for alternating or varying currents through the signal receiving device
at the sub-station. The subscriber's station arrangement shown in Fig.
307, and those immediately following, is the simplest arrangement that
possesses these three necessary features for common-battery service.
[Illustration: Fig. 307. Direct-Line Lamp]
Considering the arrangement at the central office, Fig. 307, the two
limbs of the line are permanently connected to the tip and sleeve
contacts of the jack. These two main contacts of the jack normally
engage two anvils so connected that the tip of the jack is ordinarily
connected through its anvil to ground, while the sleeve of the jack is
normally connected through its anvil to a circuit leading through the
line signal--in this case a lamp--and the common battery, and thence to
ground. The operation is obvious. Normally no current may flow from the
common battery through the signal because the line is open at the
subscriber's station. The removal of the subscriber's receiver from its
hook closes the circuit of the line and allows the current to flow
through the lamp, causing it to glow. When the operator inserts the plug
into the jack, in response to the call, the circuit through the lamp is
cut off at the jack and the lamp goes out.
This arrangement, termed the direct-line lamp arrangement, is largely
used in small common-battery telephone systems where the lines are very
short, such as those found in factories or other places where the
confines of the exchange are those of a building or a group of
neighboring buildings. Many of the so-called private-branch exchanges,
which will be considered more in detail in a later chapter, employ this
direct-line lamp arrangement.
[Illustration: Fig. 308. Direct-Line Lamp with Ballast]
_Direct-Line Lamp with Ballast._ Obviously, however, this direct-line
lamp arrangement is not a good one where the lines vary widely in length
and resistance. An incandescent lamp, as is well known, must not be
subjected to too great a variation in current. If the current that is
just right in amount to bring it to its intended degree of illumination
is increased by a comparatively small amount, the life of the lamp will
be greatly shortened, and too great an increase will result in the
lamp's burning out immediately. On the other hand, a current that is too
small will not result in the proper illumination of the lamp, and a
current of one-half the proper normal value will just suffice to bring
the lamp to a dull red glow. With lines that are not approximately
uniform in length and resistance the shorter lines would afford too
great a flow of current to the lamps and the longer lines too little,
and there is always the danger present, unless means are taken to
prevent it, that if a line becomes short-circuited or grounded near the
central office, the lamp will be subjected to practically the full
battery potential and, therefore, to such a current as will burn it out.
One of the very ingenious and, we believe, promising methods that has
been proposed to overcome this difficulty is that of the iron-wire
ballast, alluded to in Chapter III. This, it will be remembered,
consists of an iron-wire resistance enclosed in a vacuum chamber and so
proportioned with respect to the flow of current that it will be
subjected to a considerable heating effect by the amount of current that
is proper to illuminate the lamp. As has already been pointed out,
carbon has a negative temperature coefficient, that is, its resistance
decreases when heated. Iron, on the other hand, has a positive
temperature coefficient, its resistance increasing when heated. When
such an iron-wire ballast is put in series with the incandescent lamp
forming the line signal, as shown in Fig. 308, it is seen that the
resistance of the carbon in the lamp filament and of the iron in the
ballast will act in opposite ways when the current increases or
decreases. An increase of current will tend to heat up the iron wire of
the ballast and, therefore, increase its resistance, and the ballast is
so proportioned that it will hold the current that may flow through the
lamp within the proper maximum and minimum limits, regardless of the
resistance of the line in which the lamp is used. This arrangement has
not gone into wide use up to the present time.
_Line Lamp with Relay._ By far the most common method of associating the
line lamp with the line is to employ a relay, of which the actuating
coil is in the line circuit, this relay serving to control a local
circuit containing the battery and the lamp. This arrangement and the
way in which these parts are associated with the jack are clearly
indicated in Fig. 309. Here the relay may receive any amount of current,
from the smallest which will cause it to pull up its armature, to the
largest which will not injure its winding by overheat. Relays may be
made which will attract their armatures at a certain minimum current and
which will not burn out when energized by currents about ten times as
large, and it is thus seen that a very large range of current through
the relay winding is permissible, and that, therefore, a very great
latitude as to line resistance is secured. On the other hand, it is
obvious that the lamp circuit, being entirely local, is of uniform
resistance, the lamp always being subjected, in the arrangement shown,
to practically the full battery potential, the lamp being selected to
operate on that potential.
[Illustration: Fig. 309. Line Lamp with Relay]
_Pilot Signals._ In the circuits of Figs. 307, 308, and 309, but a
single line and its associated apparatus is shown, and it may not be
altogether clear to the uninitiated how it is that the battery shown in
those figures may serve, without interference of any function, a larger
number of lines than one. It is to be remembered that this battery is
the one which serves not only to operate the line signals, but also to
supply talking current to the subscribers and to supply current for the
operation of the cord-circuit signals after the cord circuits are
connected with the lines.
In Fig. 310 this matter is made clear with respect to the association of
this common battery with the lines for operating the line signals, and
also another important feature of common-battery work is brought out,
viz, the pilot lamp and its association with a group of line lamps.
Three subscribers' lines only are shown, but this serves clearly to
illustrate the association of any larger number of lines with the common
battery. Ignoring at first the pilot relay and the pilot lamp, it will
be seen that each of the tip-spring anvils of the jacks is connected to
a common wire _1_ which is grounded. Each of the sleeve-contact anvils
is connected through the coil of the line relay to another common wire
_2_, which connects with the live side of the common battery. Obviously,
therefore, this arrangement corresponds with that of Fig. 309, since the
battery may furnish current to energize any one of the line relays upon
the closure of the circuit of the corresponding line. Each of the relay
armatures in Fig. 310 is connected to ground.
Here we wish to bring out an important thing about telephone circuit
diagrams which is sometimes confusing to the beginner, but which really,
when understood, tends to prevent confusion. The showing of a separate
ground for each of the line-relay armatures does not mean that literally
each one of these armatures is connected by a separate wire to earth,
and it is to be understood that the three separate grounds shown in
connection with these relay armatures is meant to indicate just such a
set of affairs as is shown in connection with the tip-spring anvils of
the jacks, all of which are connected to a common wire which, in turn,
is grounded. Obviously, the result is the same, but in the case of this
particular diagram it is seen that a great deal of crossing of lines is
prevented by showing a separate ground at each one of the relay
armatures. The same practice is followed in connection with the common
battery. Sometimes it is very inconvenient in a complicated diagram to
run all of the wires that are supposed to connect with one terminal of
the battery across the diagram to represent this connection. It is
permissible, therefore, and in fact desirable, that separate battery
symbols be shown wherever by so doing the diagram will be simplified,
the understanding being, in the absence of other information or of other
indications, that the same battery is referred to, just as the same
ground is referred to in connection with the relay armatures in the
figure under discussion.
Each line lamp in Fig. 310 is shown connected on one hand to its
corresponding line relay contact and on the other hand to a common wire
which leads through the winding of the pilot relay to the live side of
the battery. It is obvious here that whenever any one of the line relays
attracts its armature the local circuit containing the corresponding
lamp and the common battery will be closed and the lamp illuminated.
Whenever any line relay operates, the current, which is supplied to its
lamp, must come through the pilot-relay winding, and if a number of line
relays are energized, then the current flow of the corresponding lamps
must flow through this relay winding. Therefore, this relay winding must
be of low resistance, so that the drop through its winding may not be
sufficient to interfere with the proper burning of the lamps, even
though a large number of lamps be fed simultaneously through it. The
pilot relay must be so sensitive that the current, even through one
lamp, will cause it to attract its armature. When it does attract its
armature it causes illumination of the pilot lamp in the same way that
the line relays cause the illumination of the line lamps.
The pilot lamp, which is commonly associated with a group of line lamps
that are placed on any one operator's position of the switchboard, is
located in a conspicuous place in the switchboard cabinet and is
provided with a larger lens so as to make a more striking signal. As a
result, whenever any line lamp on a given position lights, the pilot
lamp does also and serves to attract the attention, even of those
located in distant portions of the room, to the fact that a call exists
on that position of the board, the line lamp itself, which is
simultaneously lighted, pointing out the particular line on which the
call exists.
Pilot lamps, in effect, perform similar service to the night alarm in
magneto boards, but, of course, they are silent and do not attract
attention unless within the range of vision of the operator. They are
used not only in connection with line lamps, but also in connection with
the cord-circuit lamps or signals, as will be pointed out.
[Illustration: Fig. 311. Battery Supply Through Impedance Coils]
[Illustration: Fig. 312. Battery Supply through Repeating Coils]
[Illustration: Fig. 313. Battery Supply with Impedance Coils and
Condensers]
=Cord Circuit.= _Battery Supply._ Were it not for the necessity of
providing for cord-circuit signals in common-battery switchboards, the
common-battery cord circuit would be scarcely more complex than that for
magneto working. Stripped of all details, such as signals, ringing and
listening keys, and operator's equipment, cord circuits of three
different types are shown in Figs. 311, 312, and 313. These merely
illustrate the way in which the battery is associated with the cord
circuits and through them with the line circuits for supplying current
for talking purposes to the subscribers. It is thought that this matter
will be clear in view of the discussion of the methods by which current
is supplied to the subscribers' transmitters in common-battery systems
as discussed in Chapter XIII. While the arrangements in this respect of
Figs. 311, 312, and 313 illustrate only three of the methods, these
three are the ones that have been most widely and successfully used.
_Supervisory Signals._ The signals that are associated with the cord
circuits are termed supervisory signals because of the fact that by
their means the operator is enabled to supervise the condition of the
lines during times when they are connected for conversation. The
operation of these supervisory signals may be best understood by
considering the complete circuits of a simple switchboard and must be
studied in conjunction with the circuits of the lines as well as those
of the cords.
[Illustration: Fig. 314. Simple Common-Battery Switchboard]
_Complete Circuit._ Such complete circuits are shown in Fig. 314. The
particular arrangement indicated is that employed by the Kellogg
Company, and except for minor details may be considered as typical of
other makes also. Two subscribers' lines are shown extending from
Station A and Station B, respectively, to the central office. The line
wires are shown terminating in jacks in the same manner as indicated in
Figs. 307, 308, and 309, and their circuits are normally continued from
these jacks to the ground on one side and to the line relay and battery
on the other. The jack in this case has three contacts adapted to
register with three corresponding contacts in each of the plugs. The
thimble of the jack in this case forms no part of the talking circuit
and is distinct from the two jack springs which form the line terminals.
It and the auxiliary contact _1_ in each of the plugs with which it
registers, are solely for the purpose of co-operating in the control of
the supervisory signals.
The tip and sleeve strands of the cord are continuous from one plug to
the other except for the condensers. The two batteries indicated in
connection with the cord circuit are separate batteries, a
characteristic of the Kellogg system. One of these batteries serves to
supply current to the tip and sleeve strand of the cord circuit through
the two windings _3_ and _4_, respectively, of the supervisory relay
connected with the answering side of the cord circuit, while the other
battery similarly supplies current through the windings _5_ and _6_ of
the supervisory relay associated with the calling side of the cord
circuit. The windings of these relays, therefore, act as impedance coils
and the arrangement by which battery current is supplied to the cord
circuits and, therefore, to the lines of the connected subscribers, is
seen to be the combined impedance coil and condenser arrangement
discussed in Chapter XIII.
As soon as a plug is inserted into the jack of a line, the line relay
will be removed from the control of the line, and since the two strands
of the cord circuit now form continuations of the two line conductors,
the supervisory relay will be substituted for the line relay and will be
under control of the line. Since all of the current which passes to the
line after a plug is inserted must pass through the cord-circuit
connection and through the relay windings, and since current can only
flow through the line when the subscriber's receiver is off its hook, it
follows that the supervisory relays will only be energized after the
corresponding plug has been inserted into a jack of the line and after
the subscriber has removed his receiver. Unlike the line relays, the
supervisory relays open their contacts to break the local circuits of
the supervisory lamps _7_ and _8_ when the relay coils are energized,
and to close them when de-energized; but the armatures of the
supervisory relays do alone control the circuits of the supervisory
lamps. These circuits are normally held open in another place, that is,
between the plug contacts _1_ and the jack thimbles. It is only,
therefore, when a plug is inserted into a jack and when the supervisory
relay is de-energized, that the supervisory lamp may be lighted. When a
plug is inserted into a jack and when the corresponding supervisory
relay is de-energized, the circuit may be traced from ground at the
cord-circuit batteries through the left-hand battery, for instance,
through lamp _7_, thence through the contacts of the supervisory relay
to the contact _1_ of the plug, thence through the thimble of the jack
to ground. When a plug is inserted into the jack, therefore, the
necessary arrangements are completed for the supervisory lamp to be
under the control of the subscriber. Under this condition, whenever the
subscriber's receiver is on its hook, the circuit of the line will be
broken, the supervisory relay will be de-energized, and the supervisory
lamp will be lighted. When, on the other hand, the subscriber's receiver
is off its hook, the circuit of the line will be complete, the
supervisory relay will be energized, and the supervisory lamp will be
extinguished.
_Salient Features of Supervisory Operation._ It will facilitate the
student's understanding of the requirements and mode of operation of
common-battery supervisory signals in manual systems, whether simple or
multiple, if he will firmly fix the following facts in his mind. In
order that the supervisory signal may become operative at all, some act
must be performed by the operator--this being usually the act of
plugging into a jack--and then, until the connection is taken down, the
supervisory signal is under the control of the subscriber, and it is
displayed only when the subscriber's receiver is placed on its hook.
_Cycle of Operations._ We may now trace through the complete cycle of
operations of the simple common-battery switchboard, the circuits of
which are shown in Fig. 314. Assume all apparatus in its normal
condition, and then assume that the subscriber at Station A removes his
receiver from its hook. This pulls up the line relay and lights the line
lamp, the pilot relay also pulling up and lighting the common pilot lamp
which is not shown. In response to this call, the operator inserts the
answering plug and throws her listening key _L.K._ The operator's
talking set is thus bridged across the cord circuit and she is enabled
to converse with the calling subscriber. The answering supervisory lamp
_7_ did not light when the operator inserted the answering plug into the
jack, because, although the contacts in the lamp circuit were closed by
the plug contact _1_ engaging the thimble of the jack, the lamp circuit
was held open by the attraction of the supervisory relay armature, the
subscriber's receiver being off its hook. Learning that the called-for
subscriber is the one at Station B, the operator inserts the calling
plug into the jack at that station and presses the ringing key _R.K._,
in order to ring the bell. The act of plugging in, it will be
remembered, cuts off the line-signaling apparatus from connection with
that line. As the subscriber at Station B was not at his telephone when
called and his receiver was, therefore, on its hook, the insertion of
the calling plug did not energize the supervisory relay coils _5_ and
_6_, and, therefore, that relay did not attract its armature. The
supervisory lamp _8_ was thus lighted, the circuit being from ground
through the right-hand cord-circuit battery, lamp _8_, back contacts of
the supervisory relay, third strand of the cord to contact _1_ of the
calling plug, and thence to ground through the thimble of the jack. The
lighting of this lamp is continued until the party at Station B responds
by removing his receiver from its hook, which completes the line
circuit, energizes relay windings _5_ and _6_, causes that relay to
attract its armature, and thus break the circuit of the lamp _8_. Both
supervisory lamps remain out as long as the two subscribers are
conversing, but when either one of them hangs up his receiver the
corresponding supervisory relay becomes de-energized and the
corresponding lamp lights. When both of the lamps become illuminated,
the operator knows that both subscribers are through talking and she
takes down the connection.
Countless variations have been worked in the arrangement of the line and
cord circuits, but the general mode of operation of this particular
circuit chosen for illustration is standard and should be thoroughly
mastered. The operation of other arrangements will be readily understood
from an inspection of the circuits, once the fundamental mode of
operation that is common to all of them is well in mind.
=Lamps.= The incandescent lamps used in connection with line and
supervisory signals are specially manufactured, but differ in no sense
from the larger lamps employed for general lighting purposes, save in
the details of size, form, and method of mounting. Usually these lamps
are rated at about one-third candle-power, although they have a somewhat
larger candle-power as a rule. They are manufactured to operate on
various voltages, the most usual operating pressures being 12, 24, and
48 volts. The 24-volt lamp consumes about one-tenth of an ampere when
fully illuminated, the lamp thus consuming about 2.4 watts. The 12- and
48-volt lamps consume about the same amount of energy and corresponding
amounts of current.
[Illustration: Fig. 315. Switchboard Lamp]
_Lamp Mounting._ The usual form of screw-threaded mounting employed in
lamps for commercial lighting was at first applied to the miniature
lamps used for switchboard work, but this was found unsatisfactory and
these lamps are now practically always provided with two contact strips,
one on each side of the glass bulb, these strips forming respectively
the terminals for the two ends of the filament within. Such a
construction of a common form of lamp is shown in Fig. 315, where these
terminals are indicated by the numerals _1_ and _2_, _3_ being a dry
wooden block arranged between the terminals at one end for securing
greater rigidity between them.
[Illustration: Fig. 316. Line Lamp Mounting]
The method of mounting these lamps is subject to a good deal of
variation in detail, but the arrangement is always such that the lamp is
slid in between two metallic contacts forming terminals of the circuit
in which the lamp is to operate. Such an arrangement of springs and the
co-operating mounting forming a sort of socket for the reception of
switchboard lamps is referred to as a _lamp jack_. These are sometimes
individually mounted and sometimes mounted in strips in much the same
way that jacks are mounted in strips. A strip of lamp jacks as
manufactured by the Kellogg Company is shown in Fig. 316. The
opalescent lens is adapted to be fitted in front of the lamp after it
has been inserted into the jack. Fig. 317 gives an excellent view of an
individually-mounted lamp jack with its lamp and lens, this also being
of Kellogg manufacture. This figure shows a section of the plug shelf
which is bored to receive a lamp. In order to protect the lamps and
lenses from breakage, due to the striking of the plugs against them, a
metal shield is placed over the lens, as shown in this figure, this
being so cut away as to allow sufficient openings for the light to shine
through. Sometimes instead of employing lenses in front of the lamps, a
flat piece of translucent material is used to cover the openings of the
lamp, this being protected by suitable perforated strips of metal. A
strip of lamp jacks employing this feature is shown in Fig. 318, this
being of Dean manufacture. An advantage of this for certain types of
work is that the flat translucent plate in front of the lamp may readily
carry designating marks, such as the number of the line or something to
indicate the character of the line, which marks may be readily changed
as required.
[Illustration: Fig. 317. Supervisory Lamp Mounting]
[Illustration: Fig. 318. Line Lamp Mounting]
[Illustration: Fig. 319. Individual Lamp Jacks]
In the types made by some manufacturers the only difference between the
pilot lamp and the line lamp is in the size of the lens in front of it,
the jack and the lamp itself being the same for each, while others use a
larger lamp for the pilot. In Fig. 319 are shown two individual lamp
jacks, the one at the top being for supervisory lamps and the one at the
bottom being provided with a large lens for serving as a pilot lamp.
[Illustration: TERMINAL ROOM APPARATUS IN PROCESS OF INSTALLATION
Installed by Dean Electric Company at Detroit, Mich.]
=Mechanical Signals.= As has been stated the so-called mechanical
signals are sometimes used in small common-battery switchboards instead
of lamps. Where this is done the coil of the signal, if it is a line
signal, is substituted in the line circuit in place of the relay coil.
If the signals are used in connection with cord circuits for supervisory
signals, their coils are put in the circuit in place of the supervisory
relay coils. (These signals are referred to in Chapter III in connection
with Fig. 23.) They are so arranged that the attraction of the armature
lifts a target on the end of a lever, and this causes a display of color
or form. The release of the armature allows this target to drop back,
thus obliterating the display. Such signals, often called _visual
signals_ and _electromagnet signals_, should be distinguished from the
drops considered in connection with magneto switchboards in which the
attraction of the armature causes the display of the signal by the
falling of a drop, the signal remaining displayed until restored by some
other means, the restoration depending in no wise on when the armature
is released.
_Western Electric._ The mechanical signal of the Western Electric
Company, shown in Fig. 320, has a target similar to that shown in Fig.
254 but without a latch. It is turned to show a different color by the
attraction of the armature and allowed to resume its normal position
when the armature is released.
[Illustration: Fig. 320. Mechanical Signal]
_Kellogg._ Fig. 321 gives a good idea of a strip of mechanical signals
as manufactured by the Kellogg Company. This is known as the _gridiron_
signal on account of the cross-bar striping of its target. The white
bars on the target normally lie just behind the cross-bars on the shield
in front, but a slight raising of the target--about one-eighth of an
inch--exposes these white bars to view, opposite the rectangular
openings in the front shield.
[Illustration: Fig. 321. Strip of Gridiron Signals]
_Monarch._ In Fig. 322 is shown the visual signal manufactured by the
Monarch Telephone Company.
[Illustration: Fig. 322. Mechanical Signal]
=Relays.= The line relays for common-battery switchboards likewise
assume a great variety of forms. The well-known type of relay employed
in telegraphy would answer the purpose well but for the amount of room
that it occupies, as it is sometimes necessary to group a large number
of relays in a very small space. Nearly all present-day relays are of
the single-coil type, and in nearly all cases the movement of the
armature causes the movement of one or more switching springs, which are
thus made to engage or disengage their associated spring or springs. One
of the most widely used forms of relays has an L-shaped armature hung
across the front of a forwardly projecting arm of iron, on the
knife-edge corner of which it rocks as moved by the attraction of the
magnet. The general form of this relay was illustrated in Fig. 95.
Sometimes this relay is made up in single units and frequently a large
number of such single units are mounted on a single mounting plate. This
matter will be dealt with more in detail in the discussion of
common-battery multiple switchboards. In other cases these relays are
built _en bloc_, a rectangular strip of soft iron long enough to afford
space for ten relays side by side being bored out with ten cylindrical
holes to receive the electromagnets. The iron of the block affords a
return path for the lines of force. The L-shaped armatures are hung over
the front edge of this block, so that their free ends lie opposite the
magnet cores within the block. This arrangement as employed by the
Kellogg Company is shown in two views in Figs. 323 and 324.
[Illustration: Fig. 323. Strip of Relays]
[Illustration: Fig. 324. Strip of Relays]
A bank of line relays especially adapted for small common-battery
switchboards as made by the Dean Company, is shown in Fig. 325.
[Illustration: Fig. 325. Bank of Relays]
=Jacks.= The jacks in common-battery switchboards are almost always
mounted in groups of ten or twenty, the arrangement being similar to
that discussed in connection with lamp strips. Ordinarily in
common-battery work the jack is provided with two inner contacts so as
to cut off both sides of the signaling circuit when the operator plugs
in. A strip of such jacks is shown in Fig. 326.
[Illustration: Fig. 326. Strip of Cut-Off Jacks]
Ringing and listening keys for simple common-battery switchboards differ
in no essential respect from those employed in magneto boards.
[Illustration: Fig. 327. Details of Lamp, Plug, and Key Mounting]
=Switchboard Assembly.= The general assembly of the parts of a simple
common-battery switchboard deserves some attention. The form of the
switchboard need not differ essentially from that employed in magneto
work, but ordinarily the cabinet is somewhat smaller on account of the
smaller amount of room required by its lamps and jacks. An excellent
idea of the line jacks and lamps, plugs, keys, and supervisory signals
may be obtained from Fig. 327, which is a detail view taken from a
Kellogg board. In the vertical panel of the board above the plug shelf
are arranged the line jacks and the lamps in rows of twenty each, each
lamp being immediately beneath its corresponding jack. Such jacks are
ordinarily mounted on 1/2-inch centers both vertically and horizontally,
so that a group of one hundred lamps and line jacks will occupy a space
only slightly over 10 by 5 inches. Such economy of space is not required
in the simple magneto board, because the space might easily be made
larger without in any way taxing the reach of the operator. The reason
for this comparatively close mounting is a result, not of the
requirements of the simple non-multiple common-battery board itself, but
of the fact that the jack strips and lamp strips, which are required in
very large numbers in multiple boards, have to be mounted extremely
close together, and as the same lamp strips and jack strips are often
available for simple switchboards, an economy in manufacture is effected
by adherence to the same general dimensions.
[Illustration: Fig. 328. Simple Common-Battery Switchboard with
Removable Relay Panel]
A rear view of a common form of switchboard cabinet, known as the
_upright type_ and manufactured by the Dean Company, is shown in Fig.
328. In this all the relays are mounted on a hinged rack, which, when
opened out as indicated, exposes the wiring to view for inspection or
repairs. Access to both sides of the relays is thus given to the
repairman who may do all his work from the rear of the board without
disturbing the operator.
Fig. 329 shows a three-position cabinet of Kellogg manufacture, this
being about the limit in size of boards that could properly be called
simple. Obviously, where a switchboard cabinet must be made of greater
length than this, _i. e._, than is required to accommodate three
operators, it becomes too long for the operators to reach all over it
without undue effort or without moving from their seats. The so-called
_transfer board_ and the _multiple board_ (to be considered in
subsequent chapters), constitute methods of relief from such a condition
in larger exchanges.
[Illustration: Fig. 329. Three-Position Lamp Board]
CHAPTER XXIII
TRANSFER SWITCHBOARD
When the traffic originating in a switchboard becomes so great as to
require so many operators that the board must be made so long that any
one of the operators cannot reach over its entire face, the simple
switchboard does not suffice. Either some form of transfer switchboard
or of multiple switchboard must be used. In this chapter the transfer
switchboard will be briefly discussed.
The transfer switchboard is so named because its arrangement is such
that some of the connections through it are handled by means of two
operators, the operator who answers the call transferring it to another
operator who completes the connection desired.
=Limitations of Simple Switchboard.= Conceive a number of simple magneto
switchboards, or a number of common-battery switchboards, arranged side
by side, their number being so great as to form, by their combination, a
board too long for the ordinary cords and plugs to reach between its
extremities. On each of these simple switchboards, which we will say are
each of the one-position type, there terminates a group of subscribers'
lines so great in number, considering the traffic on them, that the
efforts of one operator will just about be taxed to properly attend to
their calls during the busiest hours of the day. If, now, these
subscribers would be sufficiently accommodating to call for no other
subscribers than those whose lines terminate on the same switchboard
section or on one of the immediately adjacent switchboard sections, all
would be well, but subscribers will not be so restricted. They demand
universal service; that is, they demand the privilege of having their
own lines connected with the line of any other person in the exchange.
Obviously, in the arrangement just conceived, any operator may answer
any call originating at her own board and complete the connection with
the desired subscriber if that subscriber's jack terminates on her own
section or on one of the adjacent ones. Beyond that she is powerless
unless other means are provided.
=Transfer Lines.= In the transfer board these other means consist in the
provision of groups of local trunk lines or transfer lines extending
from each switchboard position to each other non-adjacent switchboard
position. When an operator receives a call for some line on a
non-adjacent position, having answered this call with her answering
plug, she inserts the calling plug into the jack of one of these
transfer lines that leads to the proper other section. The operator at
that section is notified either verbally or by signal, and she completes
the connection between the other end of the transfer line and the line
of the called subscriber; the connection between the two subscribers
thus being effected through the cords of the two operators in question
linked together by the transfer line. Such a transfer line as just
described, requiring the connection at each of its ends by one of the
plugs of the operator's cord pair, is termed a _jack-ended trunk_ or a
_jack-ended transfer line_ because each of its ends terminates in a
jack.
[Illustration: Fig. 330. Jack-Ended Transfer Circuit]
There is another method of accomplishing the same general result by the
employment of the so-called _plug-ended trunk_ or _plug-ended transfer
line_. In this the trunk or transfer line terminates at one end, the
answering end, in a jack as before, and the connection is made with it
by the answering operator by means of the calling plug of the pair with
which she answered the originating call. The other end of this trunk,
instead of terminating in a jack, ends in a plug and the second operator
involved in the connection, after being notified, picks up this plug and
inserts it in the jack of the called subscriber, thus completing the
connection without employing one of her regular cord pairs.
_Jack-Ended Trunk._ In Fig. 330 are shown the circuits of a commonly
employed jack-ended trunk for transfer boards. The talking circuit, as
usual, is shown in heavy lines and terminates in the tip and sleeve of
the transfer jacks at each end. The auxiliary contacts in these jacks
and the circuits connecting them are absolutely independent of the
talking circuit and are for the purpose of signaling only, the
arrangement of the jacks being such that when a plug is inserted, the
spring _1_ will break from spring _2_ and make with spring _3_.
Obviously, the insertion of a plug in either of the jacks will establish
such connections as to light both lamps, since the engagement of spring
_1_ with spring _3_ in either of the jacks will connect both of the
lamps in multiple across the battery, this connection including always
the contacts _1_ and _2_ of the other jack. From this it follows that
the insertion of a plug in the other end of the trunk will, by breaking
contact between springs _1_ and _2_, put out both the lamps. One plug
inserted will, therefore, light both lamps; two plugs inserted or two
plugs withdrawn will extinguish both lamps.
[Illustration: Fig. 331. Jack-Ended Transfer Circuit]
If an operator located at one end of this trunk answers a call and finds
that the called-for subscriber's line terminates within reach of the
operator near the other end of this trunk, she will insert a calling
plug, corresponding to the answering plug used in answering a call, into
the jack of this trunk and thus light the lamp at both its ends. The
operator at the other end upon seeing this transfer lamp illuminated
inserts one of her answering plugs into the jack, and by means of her
listening key ascertains the number of the subscriber desired, and
immediately inserts her calling plug into the jack of the subscriber
wanted and rings him in the usual manner. The act of this second
operator in inserting her answering plug into the jack extinguishes the
lamp at her own end and also at the end where the call originated, thus
notifying the answering operator that the call has been attended to. As
long as the lamps remain lighted, the operators know that there is an
unattended connection on that transfer line. Such a transfer line is
called a _two-way_ line or a _single-track_ line, because traffic over
it may be in either direction. In Fig. 331 is shown a trunk that
operates in a similar way except that the two lamps, instead of being
arranged in multiple, are arranged in series.
[Illustration: Fig. 332. Jack- and Plug-Ended Transfer Circuit]
_Plug-Ended Trunk._ In Fig. 332 is shown a plug-ended trunk, this
particular arrangement of circuits being employed by the Monarch Company
in its transfer boards. This is essentially a one-way trunk, and traffic
over it can pass only in the direction of the arrow. Traffic in the
opposite direction between any two operators is handled by another trunk
or group of trunks similar to this but "pointed" in the other direction.
For this reason such a system is referred to as a _double-track_ system.
The operation of signals is the same in this case as in Fig. 330, except
that the switching device at the left-hand end of the trunk instead of
being associated with the jack is associated with the plug seat, which
is a switch closely associated with the seat of a plug so as to be
operated whenever the plug is withdrawn from or replaced in its seat.
The operation of this arrangement is as follows: Whenever an operator at
the right-hand end of this trunk receives a call for a subscriber whose
line terminates within the reach of the operator at the left-hand end of
the trunk, she inserts the calling plug of the pair used in answering
the calling subscriber into the jack of the trunk, and thus lights both
of the trunk lamps. The operator at the other end of the trunk, seeing
the trunk lamp lighted, raises the plug from its seat and, having
learned the wishes of the calling subscriber, inserts this plug into the
jack of the called subscriber without using one of her regular pairs.
When she raised the trunk plug from its seat, she permitted the long
spring _1_ of the plug seat switch to rise, thus extinguishing both
lamps and giving the signal to the originating operator that the trunk
connection has received attention. On taking down the connection, the
withdrawal of the plug from the right hand of the trunk lights both
lamps, and the restoring of the trunk plug to its normal seat again
extinguishes both lamps.
=Plug-Seat Switch.= The plug-seat switch is a device that has received a
good deal of attention not only for use with transfer systems, but also
for use in a great variety of ways with other kinds of manual switching
systems. The placing of a plug in its seat or withdrawing it therefrom
offers a ready means of accomplishing some switching or signaling
operation automatically. The plug-seat switch has, however, in spite of
its possibilities, never come into wide use, and so far as we are aware
the Monarch Telephone Manufacturing Company is the only company of
prominence which incorporates it in its regular output. The Monarch
plug-switch mechanism is shown in Fig. 333, and its operation is
obvious. It may be stated at this point that one of the reasons why the
plug-seat switch has not been more widely adopted for use, is the
difficulty that has been experienced due to lint from the switchboard
cords collecting on or about the contact points. In the construction
given in the detailed cut, upper part, Fig. 333, is shown the means
adopted by the Monarch Company for obviating this difficulty. The
contact points are carried in the upper portion of an inverted cup
mounted on the under side of the switchboard shelf, and are thus
protected, in large measure, from the damaging influence of dust and
lint.
[Illustration: Fig. 333. Plug-Seat Switch]
[Illustration: Fig. 334. Order-Wire Arrangement]
=Methods of Handling Transfers.= One way of giving the number of the
called subscriber to the second operator in a transfer system is to
have that operator listen in on the circuit after it is continued to her
position and receive the number either from the first operator or from
the subscriber. Receiving it from the first operator has the
disadvantage of compelling the first operator to wait on the circuit
until the second operator responds; receiving it from the subscriber has
the disadvantage of sometimes being annoying to him. This, however, is
to be preferred to the loss of time on the part of the originating
operator that is entailed by the first method. A better way than either
of these is to provide between the various operators working in a
transfer system, a so-called _order-wire_ system. An order wire, as
ordinarily arranged, is a circuit terminating at one end permanently in
the head receiver of an operator, and terminating at the other end in a
push button which, when depressed, will connect the telephone set of the
operator at that end with the order wire. The operator at the
push-button end of the order wire may, therefore, at will, communicate
with the other operator in spite of anything that the other operator may
do. An order-wire system suitable for transfer switchboards consists in
an order wire leading from each operator's receiver to a push button at
each of the other operator's positions, so that every operator has it
within her power to depress a key or button and establish communication
with a corresponding operator. When, therefore, an operator in a
transfer system answers a call that must be completed through a transfer
circuit, she establishes connection with that transfer circuit and then
informs the operator at the other end of that circuit by order wire of
the number of the trunk and the number of the subscriber with which that
trunk is to be connected. Fig. 334 shows a system of order-wire buttons
by means of which each operator may connect her telephone set with that
of every other operator in the room, the number in this case being
confined to three. Assuming that each pair of wires leading from the
lower portion of this figure terminates respectively in the operator's
talking apparatus of the three respective operators, then it is obvious
that operator No. 1, by depressing button No. 2, will connect her
telephone set with that of operator No. 2; likewise that any operator
may communicate with any other operator by depressing the key bearing
the corresponding number.
=Limitations of Transfer System.= It may be stated that the transfer
system at present has a limited place in the art of telephony. The
multiple switchboard has outstripped it in the race for popular approval
and has demonstrated its superiority in practically all large manual
exchange work. This is not because of lack of effort on the part of
telephone engineers to make the transfer system a success in a broad
way. A great variety of different schemes, all embodying the fundamental
idea of having one operator answer the call and another operator
complete it through a trunk line, have been tried. In San Francisco, the
Sabin-Hampton system was in fairly successful service and served many
thousands of lines for a number of years. It was, however, afterwards
replaced by modern multiple switchboards.
_Examples of Obsolete Systems._ The Sabin-Hampton system was unique in
many respects and involved three operators in each connection. It was
one of the very first systems which employed automatic signaling
throughout and did away with the subscribers' generators. It did not,
however, dispense with the subscribers' local batteries.
Another large transfer system, used for years in an exchange serving at
a time as many as 5,000, was employed at Grand Rapids, Michigan. This
was later replaced by an automatic switchboard.
[Illustration: Fig. 335. Three-Position Transfer Switchboard]
=Field of Usefulness.= The real field of utility for the transfer system
today is to provide for the growth of simple switchboards that have
extended beyond their originally intended limits. By the adding of
additional sections to the simple switchboard and the establishment of a
comparatively cheap transfer system, the simple boards may be made to do
continued service without wasting the investment in them by discarding
them and establishing a completely new system. However, switchboards are
sometimes manufactured in which the transfer system is included as a
part of the original equipment. In Fig. 335 is shown a three-position
transfer switchboard, manufactured by the Monarch Telephone Company. At
first glance the switchboard appears to be exactly like those described
in Chapter XXI, but on close observation, the transfer jacks and signals
may be seen in the first and third positions, just below the line jacks
and signals. There is no transfer equipment in the second position of
this switchboard because the operator at that position is able to reach
the jacks of all the lines and, therefore, is able to complete all calls
originating on her position without the use of any transfer equipment.
Referring to Fig. 301, which illustrates a two-position simple
switchboard, it may readily be seen that if the demands for telephone
service in the locality in which this switchboard is installed should
increase so as to require the addition of more switchboard positions,
this switchboard could readily be converted to a transfer switchboard by
placing the necessary transfer jacks and signals in the vacant space
between the line jacks and clearing-out drops.
[Illustration: CABLE TURNING SECTIONS, BETWEEN A AND B BOARDS Cortlandt
Office, New York Telephone Co.]
CHAPTER XXIV
PRINCIPLES OF THE MULTIPLE SWITCHBOARD
=Field of Utility.= The multiple switchboard, unlike the transfer board,
provides means for each operator to complete, without assistance, a
connection with any subscriber's line terminating in the switchboard no
matter how great the number of lines may be. It is used only where the
simple switchboard will not suffice; that is, where the number of lines
and the consequent traffic is so great as to require so many operators
and, therefore, so great a length of board as to make it impossible for
any one operator to reach all over the face of the board without moving
from her position.
=The Multiple Feature.= The fundamental feature of the multiple
switchboard is the placing of a jack for every line served by the
switchboard within the reach of every operator. This idea underlying the
multiple switchboard may be best grasped by merely considering the
mechanical arrangement and grouping of parts without regard to their
details of operation. The idea is sometimes elusive, but it is really
very simple. If the student at the outset will not be frightened by the
very large number of parts that are sometimes involved in multiple
switchboards, and by the great complexity which is apparent in the
wiring and in the action of these parts; and will remember that this
apparent complexity results from the great number of repetitions of the
same comparatively simple group of apparatus and circuits, much will be
done toward a mastery of the subject.
The multiple switchboard is divided into sections, each section being
about the width and height that will permit an ordinary operator to
reach conveniently all over its face. The usual width of a section
brought about by this limitation is from five and one-half to six feet.
Such a section affords room for three operators to sit side by side
before it. Now each line, instead of having a single jack as in the
simple switchboard, is provided with a number of jacks and one of these
is placed on each of the sections, so that each one of the operators may
have within her reach a jack for each line. It is from the fact that
each line has a multiplicity of jacks, that the term multiple
switchboard arises.
_Number of Sections._ Since there is a jack for each line on each
section of the switchboard, it follows that on each section there are as
many jacks as there are lines; that is, if the board were serving 5,000
lines there would be 5,000 jacks. Let us see now what it is that
determines the number of sections in a multiple switchboard. In the
final analysis, it is the amount of traffic that arises in the busiest
period of the day. Assume that in a particular office serving 5,000
lines, the subscribers call at such a very low rate that even at the
busiest time of the day only enough calls are made to keep, say, three
operators busy. In this case there would be no need for the multiple
switchboard, for a single section would suffice. The three operators
seated before that section would be able to answer and complete the
connections for all of the calls that arose. But subscribers do not call
at this exceedingly low rate. A great many more calls would arise on
5,000 lines during the busiest hour than could be handled by three
operators and, therefore, a great many more operators would be required.
Space has to be provided for these operators to work in, and as each
section accommodates three operators the total number of sections must
be at least equal to the total number of required operators divided by
three.
Let us assume, for instance, that each operator can handle 200 calls
during the busy hour. Assume further that during the busy hour the
average number of calls made by each subscriber is two. One hundred
subscribers would, therefore, originate 200 calls within this busy hour
and this would be just sufficient to keep one operator busy. Since one
operator can handle only the calls of one hundred subscribers during the
busy hour, it follows that as many operators must be employed as there
are hundreds of subscribers whose lines are served in a switchboard, and
this means that in an exchange of 5,000 subscribers, 50 operators'
positions would be required, or 16-2/3 sections. Each of these sections
would be equipped with the full 5,000 jacks, so that each operator could
have a connection terminal for each line.
_The Multiple._ These groups of 5,000 jacks, repeated on each of the
sections are termed multiple jacks, and the entire equipment of these
multiple jacks and their wiring is referred to as the multiple. It will
be shown presently that the multiple jacks are only used for enabling
the operator to connect with the called subscriber. In other words these
jacks are for the purpose of enabling each operator to have within her
reach any line that may be called for regardless of what line originates
the call. We will now consider what arrangements are provided for
enabling the operator to receive the signal indicating a call and what
provisions are made for her to answer the call in response to such a
signal.
=Line Signals.= Obviously it is not necessary to have the line signals
repeated on each section of the board as are the multiple jacks. If a
line has one definite place on the switchboard where its signal may be
received and its call may be answered, that suffices. Each line,
therefore, in addition to having its multiple jacks distributed one on
each section of the switchboard, has a line signal and an individual
jack immediately associated with it, located on one only of the
sections. This signal usually is in the form of a lamp and is termed the
line signal, and this jack is termed the answering jack since it is by
means of it that the operator always answers a call in response to the
line signal.
_Distribution of Line Signals._ It is evident that it would not do to
have all of these line signals and answering jacks located at one
section of the board for then they would not be available to all of the
operators. They are, therefore, distributed along the board in such a
way that one group of them will be available to one operator, another
group to another operator, and so on; the number of answering jacks and
signals in any one group being so proportioned with respect to the
number of calls that come in over them during the busy hour that it will
afford just about enough calls to keep the operator at that position
busy.
We may summarize these conditions with respect to the jack and
line-signal equipment of the multiple switchboard by saying that each
line has a multiple jack on each section of the board and in addition to
this has on one section of the board an answering jack and a line
signal. These answering jacks and line signals are distributed in groups
along the face of the board so that each operator will receive her
proper quota of the originating calls which she will answer and, by
virtue of the multiple jack, be able to complete the connections with
the desired subscribers without moving from her position.
=Cord Circuits.= Each operator is also provided with a number of pairs
of cords and plugs with proper supervisory or clearing-out signals and
ringing and listening keys, the arrangement in this respect being
similar to that already described in connection with the simple
switchboard.
=Guarding against Double Connections.= From what has been said it is
seen that a call originating on a given line may be answered at one
place only, but an outgoing connection with that line may be made at any
position. This fact that a line may be connected with when called for at
any one of the sections of the switchboard makes necessary the provision
that two or more connections will not be made with the same line at the
same time. For instance, if a call came in over a line whose signal was
located on the first position of the switchboard for a connection with
line No. 1,000, the operator at the first position would connect this
calling line with No. 1,000 through the multiple jack on the first
section of the switchboard. Assume now that some line, whose signal was
located on the 39th position of the switchboard, should call also for
line No. 1,000 while that line was still connected with the first
calling subscriber. Obviously confusion would result if the operator at
the 39th position, not knowing that line No. 1,000 was already busy,
should connect this second line with it, thereby leaving both of the
calling subscribers connected with line No. 1,000, and as a result all
of these three subscribers connected together.
The provisions for suitable means for preventing the making of a
connection with a line that is already switched at some other section of
the switchboard, has offered one of the most fertile fields for
invention in the whole telephone art. The ways that have been proposed
for accomplishing this are legion. Fortunately common practice has
settled on one general plan of action and that is to so arrange the
circuits that whenever a line is switched at one section, such an
electrical condition will be established on the forward contacts of all
of its multiple jacks that any operator at any other section in
attempting to make a connection with that line will be notified of the
fact that it is already switched by an audible signal, which she will
receive in her head receiver. On the other hand the arrangement is such
that when a line is not busy, _i. e._, it is not switched at any of the
positions of the switchboard, the operator on attempting to make a
connection with such a line will receive no such guarding signal and
will, therefore, proceed with the connection.
We may liken a line in a multiple switchboard to a lane having a number
of gates giving access to it. One of these gates--the answering jack--is
for the exclusive use of the proprietor of that lane. All of the other
gates to the lane--the multiple jacks--are for affording means for the
public to enter. But whenever any person enters one of these gates, a
signal is automatically put up at all of the other gates forbidding any
other person to enter the lane as long as the first person is still
within.
[Illustration: Fig. 336. Principle of Multiple Switchboard]
=Diagram Showing Multiple Board Principle.= For those to whom the
foregoing description of the multiple board is not altogether clear, the
diagram of Fig. 336 may offer some assistance. Five subscribers' lines
are shown running through four sections of a switchboard. Each of these
lines is provided with a multiple jack on each section of the board.
Each line is also provided with an answering jack and a line signal on
one of the sections of the board. Thus the answering jacks and the line
signals of lines _1_ and _2_ are shown in Section I, that of line _4_ is
shown in Section II, that of line _3_ in Section III, and that of line
_5_ in Section IV. At Section I, line _1_ is shown in the condition of
having made a call and having had this call answered by the operator
inserting one of her plugs into its answering jack. In response to the
instructions given by the subscriber, the operator has inserted the
other plug of this same pair in the multiple jack of line _2_, thus
connecting these two lines for conversation. At Section III, line _3_ is
shown as having made a call, and the operator as having answered by
inserting one of her plugs into the answering jack. It happens that the
subscriber on line _3_ requests a connection with line _1_, and the
condition at Section III is that where the operator is about to apply
the tip of the calling plug to the jack of line _1_ to ascertain whether
or not that line is busy. As before stated, when the contact is made
between the tip of the calling plug and the forward contact of the
multiple jack, the operator will receive a click in the ear (by means
that will be more fully discussed in later chapters), this click
indicating to her that line _1_ is not available for connection because
it is already switched at some other section of the switchboard.
=Busy Test.= The busy signal, by which an operator in attempting to make
a connection is informed that the line is already busy, has assumed a
great variety of forms and has brought forth many inventions. It has
been proposed by some that the insertion of a plug into any one of the
jacks of a line would automatically close a little door in front of each
of the other jacks of the line, therefore making it impossible for any
other operator to insert a plug as long as the line is in use. It has
been proposed by others to ring bells or to operate buzzers whenever the
attempt was made by an operator to plug into a line that was already in
use. Still others have proposed to so arrange the circuits that the
operator would get an electric shock whenever she attempted to plug into
a busy line. The scheme that has met with universal adoption, however,
is that the operator shall, when the tip of her calling plug touches the
forward contact of the jack of a line that is already switched, receive
a click in her telephone which will forbid her to insert the plug. The
absence of this click, or silence in her telephone, informs her that she
may safely make the connection.
_Principle._ The means by which the operator receives or fails to
receive this click, according to whether the line is busy or idle, vary
widely, but so far as the writers are aware they all have one
fundamental feature in common. The tip of the calling plug and the test
contact of all of the multiple jacks of an idle line must be absolutely
at the same potential before the test, so that no current will flow
through the test circuit when the test is actually made. The test
thimbles of all the jacks of a busy line must be at a different
potential from the tip of the test plug so that a current will flow and
a click result when the test is made.
_Potential of Test Thimbles._ It has been found an easy matter to so
arrange the contacts in the jacks of a multiple switchboard that
whenever the line is idle the test thimbles of that line will be a
certain potential, the same as that of all the unused calling plug tips.
It has also been easy to so arrange these contacts that the insertion of
a plug into any one of the jacks will, by virtue of the contacts
established, change the potential of all the test thimbles of that line
so that they will be at a different potential from that of the tips of
the calling plugs. It has not been so easy, however, to provide that
these conditions shall exist under all conditions of practice. A great
many busy tests that looked well on paper have been found faulty in
practice. As is always the case in such instances, this has been true
because the people who considered the scheme on paper did not foresee
all of the conditions that would arise in practice. Many busy-test
systems will operate properly while everything connected with the
switchboard and the lines served by it remains in proper order. But no
such condition as this can be depended on in practice. Switchboards, no
matter how perfectly made and no matter with how great care they may be
installed and maintained, will get out of order. Telephone lines will
become grounded or short-circuited or crossed or opened. Such
conditions, in a faulty busy-test system, may result in a line that is
really idle presenting a busy test, and thus barring the subscriber on
that line from receiving calls from other lines just as completely as if
his line were broken. On the other hand, faulty conditions either in the
switchboard or in the line may make a line that is really busy, test
idle, and thus result in the confusion of having two or more subscribers
connected to the same line at the same time.
_Busy-Test Faults._ To show how elusive some of the faults of a busy
test may be, when considered on paper, it has come within the
observation of the writers that a new busy-test system was thought well
enough of by a group of experienced engineers to warrant its
installation in a group of very large multiple switchboards, the cost of
which amounted to hundreds of thousands of dollars, and yet when so
installed it developed that a single short-circuited cord in a position
would make the test inoperative on all the cords of that
position--obviously an intolerable condition. Luckily the remedy was
simple and easily applied.
In a well-designed busy-test system there should be complete silence
when the test is made of an idle line, and always a well-defined click
when the test is made of a busy line. The test on busy lines should
result in a uniform click regardless of length of lines or the condition
of the apparatus. It does not suffice to have a little click for an idle
line and a big click for a busy line, as practice has shown that this
results in frequent errors on the part of the operators.
Good operating requires that the tip of the calling plug be tapped
against the test thimble several times in order to make sure of the
state of the called line.
In some multiple switchboards the arrangement has been such that the
jacks of a line would test busy as soon as the subscriber on that line
removed his receiver from its hook to make a call, as well as while any
plug was in any jack of that line. The advocates of this added feature,
in connection with the busy test, have claimed that the receiver, when
removed from its hook in making a call, should make the line test busy
and that a line should not be connected with when the subscriber's
receiver was off its hook any more than it should be when it was already
connected with at some other section of the switchboard. While it is
true that a line may be properly termed busy when the subscriber has
removed his receiver in order to make a call, it is not true that there
is any real necessity for guarding against a connection with it while he
is waiting for the operator to answer. Leaving the line unguarded for
this brief period may result in the subscriber, who intended to make the
call, having to defer his call until he has conversed with the party who
is trying to reach him. This cannot be said to be a detriment to the
service, however, since the second party gets the connection he desires
much sooner than he otherwise would, and the first party may still make
his first intended call as soon as he has disposed of the party who
reached him while he was waiting for his own operator to answer. It may
be said, therefore, in connection with this matter of making the line
test busy as soon as a subscriber has removed his receiver from the
hook, that it is not considered an essential, and in case of those
switchboard systems which naturally work out that way it is not
considered a disadvantage.
=Field of Each Operator.= It was stated earlier in this chapter that as
each section accommodated three operators, the total number of sections
in a switchboard will be at least one-third the total number of required
operators. This thought needs further development, for to stop at that
statement is to arrive somewhat short of the truth. In order to do this
it is necessary to consider the field in the multiple, reached by each
operator. The section is of such size, or should be, that an operator
seated in the center position of it may, without undue effort, reach all
over the multiple. But the operator at the right-hand position cannot
reach the extreme left portion of the multiple of that section, nor can
the operator at the left reach the extreme right. How then may each
operator reach a jack for every line? Remembering that the multiple
jacks are arranged exactly the same in each section, each jack always
occupying the same relative position, it is easy to see that while the
operator at a right-hand position of a section cannot reach the
left-hand third of the multiple in her own section, she may reach the
left-hand third of the multiple in the section at her right, and this,
together with the center and right-hand thirds of her own section,
represents the entire number of lines. So it is with the left-hand
operator at any section, she reaches two-thirds of all the lines in the
multiple of her own section and one-third in that of the section at her
left.
_End Positions._ This makes it necessary to inquire about the operators
at the end positions of the entire board. To provide for these the
multiple is extended one-third of a section beyond them, so as to supply
at the ends of the switchboard jacks for those lines which the end
operators cannot reach on their own sections. Sometimes instead of
adding these end sections to the multiple for the end operators, the
same result is accomplished by using only the full and regular sections
of the multiple, and leaving the end positions without operators'
equipment, as well as without answering jacks, line signals, and cords
and plugs, so that in reality the end operator is at the middle position
of the end section. This, in our opinion, is the better practice, since
it leaves the sections standard, and makes it easier to extend the
switchboard in length, as it grows, by the mere addition of new sections
without disturbing any of the old multiple.
=Influence of Traffic.= We wish again to emphasize the fact that it is
the traffic during the busiest time of day and not the number of lines
that determine the size of a multiple switchboard so far as its length
is concerned. The number of lines determines the size of the multiple in
any one section, but it is the amount of traffic, the number of calls
that are made in the busiest period, that determines the number of
operators required, and thus the number of positions. Had this now very
obvious fact been more fully realized in the past, some companies would
be operating at less expense, and some manufacturers would have sold
less expensive switchboards.
The whole question as to the number of positions boils down to how many
answering jacks and line signals may be placed at each operator's
position without overburdening the operator with incoming traffic at the
busy time of day. Obviously, some lines will call more frequently than
others, and hence the proper number of answering jacks at the different
positions will vary. Obviously, also, due to changes in the personnel of
the subscribers, the rates of calling of different groups of lines will
change from time to time, and this may necessitate a regrouping of the
line signals and answering jacks on the positions; and changes in the
personnel of the operators or in their skill also demand such
regrouping.
_Intermediate Frame._ The intermediate distributing frame is provided
for this purpose, and will be more fully discussed in subsequent
chapters. Suffice it to say here that the intermediate distributing
frame permits the answering jacks and line signals to be shifted about
among the operators' positions, so that each position will have just
enough originating traffic to keep each of the operators economically
busy during the busiest time of the day.
CHAPTER XXV
THE MAGNETO MULTIPLE SWITCHBOARD
=Field of Utility.= The principles of the multiple switchboard set forth
in the last chapter were all developed long before the common-battery
system came into existence, and consequently all of the first multiple
switchboards were of the magneto type. Although once very widely used,
the magneto multiple switchboard has almost passed out of existence,
since it has become almost universal practice to equip exchanges large
enough to employ multiple boards with common-battery systems.
Nevertheless there is a field for magneto multiple switchboards, and in
this field it has recently been coming into increasing favor. In those
towns equipped with magneto systems employing simple switchboards or
transfer switchboards, and which require new switchboards by virtue of
having outgrown or worn out their old ones, the magneto multiple
switchboard is frequently found to best fit the requirements of economy
and good practice. The reason for this is that by its use the magneto
telephones already in service may be continued, no change being required
outside of the central office. Furthermore, with the magneto multiple
switchboard no provision need be made for a power plant, which, in towns
of small size, is often an important consideration. Again, many
companies operate over a considerable area, involving a collection of
towns and hamlets. It may be that all of these towns except one are
clearly of a size to demand magneto equipment and that magneto equipment
is the standard throughout the entire territory of the company. If,
however, one of the towns, by virtue of growth, demands a multiple
switchboard, this condition affords an additional argument for the
employment of the magneto multiple switchboard, since the same standards
of equipment and construction may be maintained throughout the entire
territory of the operating company, a manifest advantage. On the other
hand, it may be said that the magneto multiple switchboard has no proper
place in modern exchanges of considerable size--say, having upward of
one thousand subscribers--at least under conditions found in the United
States.
Notwithstanding the obsolescence of the magneto multiple switchboard for
large exchanges, a brief discussion of some of the early magneto
multiple switchboards, and particularly of one of the large ones, is
worth while, in that a consideration of the defects of those early
efforts will give one a better understanding and appreciation of the
modern multiple switchboard, and particularly of the modern multiple
common-battery switchboard, the most highly organized of all the manual
switching systems. Brief reference will, therefore, be made to the
so-called series multiple switchboard, and then to the branch terminal
multiple switchboard, which latter was the highest type of switchboard
development at the time of the advent of common-battery working.
[Illustration: Fig. 337. Series Magneto Multiple Switchboard]
=Series-Multiple Board.= In Fig. 337 are shown the circuits of a series
magneto multiple switchboard as developed by the engineers of the
Western Electric Company during the eighties. As is usual, two
subscribers' lines and a single cord circuit are shown. One side of each
line passes directly from the subscriber's station to one side of the
drop, and also branches off to the sleeve contact of each of the jacks.
The other side of the line passes first to the tip spring of the first
jack, thence to the anvil of that jack and to the tip spring of the next
jack, and so on in series through all of the jacks belonging in that
line to the other terminal of the drop coil. Normally, therefore, the
drop is connected across the line ready to be responsive to the signal
sent from the subscriber's generator. The cord circuit is of the
two-conductor type, the plugs being provided with tip and sleeve
contacts, the tips being connected by one of the flexible conductors
through the proper ringing and listening key springs, and the sleeve
being likewise connected through the other flexible conductor and the
other springs of the ringing and listening keys. It is obvious that when
any plug is inserted into a jack, the circuit of the line will be
continued to the cord circuit and at the same time the line drop will be
cut out of the circuit, because of the lifting of the tip spring of the
jack from its anvil. Permanently connected between the sleeve side of
the cord circuit and ground is a retardation coil _1_ and a battery.
Another retardation coil _2_ is connected between the ground and a
point on the operator's telephone circuit between the operator's head
receiver and the secondary of her induction coil. These two retardation
coils have to do with the busy test, the action of which is as follows:
normally, or when a line is not switched at the central office, the test
thimbles will all be at substantially ground potential, _i. e._, they
are supposed to be. The point on the operator's receiver circuit which
is grounded through the retardation coil _2_ will also be of ground
potential because of that connection to ground. In order to test, the
operator always has to throw her listening key _L.K._ into the listening
position. She also has to touch the tip of the calling plug _P_c to a
sleeve or jack of the line that is being tested. If, therefore, a test
is made of an idle or non-busy line, the touching of the tip of the
calling plug with the test thimble of that line will result in no flow
of current through the operator's receiver, because there will be no
difference of potential anywhere in the test circuit, which test circuit
may be traced from the test thimble of the line under test to the tip of
the calling plug, thence through the tip strand of the cord to the
listening key, thence to the outer anvil of the listening key on that
side, through the operator's receiver to ground through the impedance
coil _2_. If, however, the line had already been switched at some other
section by the insertion of either a calling or answering plug, all of
the test thimbles of that line would have been raised to a potential
above that of the ground, by virtue of the battery connected with the
sleeve side of the cord circuit through the retardation coil _1_. If the
operator had made a test of such a line, the tip of her testing plug
would have found the thimble raised to the potential of the battery and,
therefore, a flow of current would occur which would give her the busy
click. The complete test circuit thus formed in testing a busy line
would be from the ungrounded pole of the battery through the impedance
coil _1_ associated with the cord that was already in connection with
the line, thence to the sleeve strand of that cord to the sleeve of the
jack at which the line was already switched, thence through that portion
of the line circuit to which all of the sleeve contacts were connected,
and therefore to the sleeve or test thimble of the jack at which the
test is made, thence through the tip of the calling plug employed in
making the test through the tip side of that cord circuit to the outer
listening key contact of the operator making the test, and thence to
ground through the operator's receiver and the impedance coil _2_. The
resultant click would be an indication to the operator that the line was
already in use and that, therefore, she must not make the connection.
The condenser _3_ is associated with the operator's talking set and with
the extra spring in the listening key _L.K._ in such a manner that when
the listening key is thrown, the tip strand of the cord circuit is
divided and the condenser included between them. This is for the purpose
of preventing any potentials, which might exist on the line with which
the answering plug _P_a was connected, from affecting the busy-test
conditions.
_Operation._ The operation of the system aside from the busy-test
feature is just like that described in connection with the simple
magneto switchboard. Assuming that the subscriber at Station _A_ makes
the call, he turns his hand generator, which throws the drop on his line
at the central office. The operator, seeing the signal, inserts the
answering plug of one of her idle pairs of cords into the answering jack
and throws her listening key _L.K._ This enables the operator to talk
with the calling subscriber, and having found that he desires a
connection with the line extending to Station _B_, she touches the tip
of her calling plug to the multiple jack of that line that is within her
reach, it being remembered that each one of the multiple jacks shown is
on a different section. She leaves the listening key in the listening
position when she does this. If the line is busy, the click will notify
her that she must not make the connection, in which case she informs the
calling subscriber that the line is busy and requests him to call again.
If, however, she received no click, she would insert the calling plug
into the jack, thus completing the connection between the two lines. She
would then press the ringing key associated with the calling plug and
that momentarily disconnects the calling plug from the answering plug
and at the same time establishes connection between the ringing
generator and the called line. The release of the ringing key again
connects the calling and answering plugs and, therefore, connects the
two subscribers' lines ready for conversation. All that is then
necessary is that the called subscriber shall respond and remove his
receiver from its hook, the calling subscriber already having done this.
When the conversation is finished, both of the subscribers (if they
remember it) will operate their ringing generators, which will throw the
clearing-out drop as a signal to the operator for disconnection. If it
should become necessary for the operator to ring back on the line of the
calling subscriber, she may do so by pressing the ringing key associated
with the calling plug.
Frequently this multiple switchboard arrangement was used with grounded
lines, in which case the single line wire extending from the
subscriber's station to the switchboard was connected with the tip
spring of the first jack, the circuit being continued in series through
the jack to the drop and thence to ground through a high non-inductive
resistance.
_Defects._ This series multiple magneto system was used with a great
many variations, and it had a good many defects. One of these defects
was due to the necessary extending of one limb of the line through a
large number of series contacts in the jacks. This is not to be desired
in any case, but it was particularly objectionable in the early days
before jacks had been developed to their present high state of
perfection. A particle of dust or other insulating matter, lodging
between the tip spring and its anvil in any one of the jacks, would
leave the line open, thus disabling the line to incoming signals, and
also for conversation in case the break happened to occur between the
subscriber and the jack that was used in connecting with the line.
Another defect due to the same cause was that the line through the
switchboard was always unbalanced by the insertion of a plug, one limb
of the line always extending clear through the switchboard to the drop
and the other, when the plug was inserted, extending only part way
through the switchboard and being cut off at the jack where the
connection was made. The objection will be apparent when it is
remembered that the wires in the line circuit connecting the multiple
jacks are necessarily very closely bunched together and, therefore,
there is very likely to be cross-talk between two adjacent lines unless
the two limbs of each line are exactly balanced throughout their entire
length.
Again the busy-test conditions of this circuit were not ideal. The fact
that the test rings of the line were connected permanently with the
outside line circuit subjected these test rings to whatever potentials
might exist on the outside lines, due to any causes whatever, such as a
cross with some other wire; thus the test rings of an idle line might by
some exterior cause be raised to such a potential that the line would
test busy. It may be laid down as a fundamental principle in good
multiple switchboard practice that the busy-test condition should be
made independent of any conditions on the line circuit outside of the
central office, and such is not the case in this circuit just described.
[Illustration: CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE Cortlandt
Office, New York Telephone Co.]
=Branch-Terminal Multiple Board.= The next important step in the
development of the magneto multiple switchboard was that which produced
the so-called branch-terminal board. This came into wide use in the
various Bell operating companies before the advent of the common-battery
systems. Its circuits and the principles of operation may be understood
in connection with Fig. 338. In the branch-terminal system there are no
series contacts in the jacks and no unbalancing of the line due to a
cutting off of a portion of the line circuit when a connection was made
with it. Furthermore, the test circuits were entirely local to the
central office and were not likely to be affected by outside conditions
on the line. This switchboard also added the feature of the automatic
restoration of the drops, thus relieving the operator of the burden of
doing that manually, and also permitting the drops to be mounted on a
portion of the switchboard that was not available for the mounting of
jacks, and thus permitting a greater capacity in jack equipment.
[Illustration: Fig. 338. Branch-Terminal Magneto Multiple Switchboard]
Each jack has five contacts, and the answering and multiple jacks are
alike, both in respect to their construction and their connection with
the line. The drops are the electrically self-restoring type shown in
Fig. 263. The line circuits extended permanently from the subscriber's
station to the line winding of the drop and the two limbs of the line
branched off to the tip and sleeve contacts _1_ and _2_ respectively of
each jack. Another pair of wires extended through the multiple parallel
to the line wires and these branched off respectively to the contact
springs _3_ and _4_ of each of the jacks. This pair of wires formed
portions of the drop-restoring circuit, including the restoring coil _6_
and the battery _7_, as indicated. The test thimble _5_ of each of the
jacks is connected permanently with the spring _3_ of the corresponding
jack and, therefore, with the wire which connects through the restoring
coil _6_ of the corresponding drop to ground through the battery _7_.
The plugs were each provided with three contacts. Two of these were the
usual tip and sleeve contacts connected with the two strands of the cord
circuit. The third contact _8_ was not connected with any portion of the
cord circuit, being merely an insulated contact on the plug adapted,
when the plug was fully inserted, to connect together the springs _3_
and _4_. The cord circuit itself is readily understood from the drawing,
having two features, however, which merit attention. One is the
establishing of a grounded battery connection to the center portion of
the winding of the receiver for the purposes of the busy test, and the
other is the provision of a restoring coil and restoring circuit for the
clearing-out drop, this circuit being closed by an additional contact on
the listening key so as to restore the clearing-out drop whenever the
listening key was operated.
_Operation._ An understanding of the operation of this system is easy.
The turning of the subscriber's generator, when the line was in its
normal condition, caused the display of the line signal. The insertion
of the answering plug, in response to this call, did three things: (1)
It extended the line circuit to the tip and sleeve strand of the cord
circuit. (2) It energized the restoring coil _6_ of the drop by
establishing the circuit from the contact spring _3_ through the plug
contact _8_ to the other contact spring _4_, thus completing the circuit
between the two normally open auxiliary wires. (3) The connecting of the
springs _3_ and _4_ established a connection from ground to the test
thimbles of all the jacks on a line, the spring _4_ being always
grounded and the spring _3_ being always connected to the test thimble
_5_.
It is to be noted that on idle lines the test rings are always at the
same potential as the ungrounded pole of the battery _7_, being
connected thereto through the winding _6_ of the restoring coil. On all
busy lines, however, the test rings are dead grounded through the
contact _8_ of the plug that is connected with the line.
The tip of the testing plug at the time of making a test will also be at
the same potential as that of the ungrounded pole of the battery _7_,
since this pole of the battery _7_ is always connected to the center
portion of the operator's receiver winding, and when the listening key
is thrown the tip of the calling plug is connected therewith and is at
the same potential. When, therefore, the operator touches the tip of
the calling plug to the test thimble of an idle line, she will get no
click, since the tip of the plug and the test thimble will be at the
same potential. If, however, the line has already been switched at
another section of the board, there will be a difference of potential,
because the test thimble will be grounded, and the circuit, through
which the current which causes the click flows, may be traced from the
ungrounded pole of the battery _7_ to the center portion of the
operator's receiver, thence through one-half of the winding to the tip
of the calling plug, thence to the test thimble of the jack under test,
thence to the spring _3_ of the jack on another section at which the
connection exists, through the contact _8_ on the plug of that jack to
the spring _4_, and thence to ground and back to the other terminal of
the battery _7_.
_Magnet Windings._ Coils of the line and clearing-out drops by which
these drops are thrown, are wound to such high resistance and impedance
as to make it proper to leave them permanently bridged across the
talking circuit. The necessity for cutting them out is, therefore, done
away with, with a consequent avoidance, in the case of the line drops,
of the provision of series contacts in the jacks.
_Arrangement of Apparatus._ In boards of this type the line and
clearing-out drops were mounted in the extreme upper portion of the
switchboard face so as to be within the range of vision of the operator,
but yet out of her reach. Therefore, the whole face of the board that
was within the limit of the operator's reach was available for the
answering and multiple jacks. A front view of a little over one of the
sections of the switchboard, involving three complete operator's
positions, is shown in Fig. 339, which is a portion of the switchboard
installed by the Western Electric Company in one of the large exchanges
in Paris, France. (This has recently been replaced by a common-battery
multiple board.) In this the line drops may be seen at the extreme top
of the face of the switchboard, and immediately beneath these the
clearing-out drops. Beneath these are the multiple jacks arranged in
banks of one hundred, each hundred consisting of five strips of twenty.
At the extreme lower portion of the jack space are shown the answering
jacks and beneath these on the horizontal shelf, the plugs and keys.
These jacks were mounted on 1/2-inch centers, both vertically and
horizontally and each section had in multiple 90 banks of 100 each,
making 9,000 in all. Subsequent practice has shown that this involves
too large a reach for the operators and that, therefore, 9,000 is too
large a number of jacks to place on one section if the jacks are not
spaced closer than on 1/2-inch centers. With the jack involving as many
parts as that required by this branch terminal system, it was hardly
feasible to make them smaller than this without sacrificing their
durability, and one of the important features of the common-battery
multiple system which has supplanted this branch-terminal magneto system
is that the jacks are of such a simple nature as to lend themselves to
mounting on 3/8-inch centers, and in some cases on 3/10-inch centers.
[Illustration: Fig. 339. Face of Magneto Multiple Switchboard]
=Modern Magneto Multiple Board.= Coming now to a consideration of modern
magneto multiple switchboards, and bearing in mind that such boards are
to be found in modern practice only in comparatively small installations
and then only under rather peculiar conditions, as already set forth, we
will consider the switchboard of the Monarch Telephone Manufacturing
Company as typical of good practice in this respect.
[Illustration: Fig. 340. Monarch Magneto Multiple Switchboard Circuits]
_Line Circuit._ The line and cord circuits of the Monarch system are
shown in Fig. 340. It will be seen that each jack has in all five
contacts, numbered from _1_ to _5_ respectively, of which _1_ and _4_
are the springs which register with the tip and ring contacts of the
plug and through which the talking circuit is continued, while _2_ and
_3_ are series contacts for cutting off the line drop when a plug is
inserted, and _5_ is the test contact or thimble adapted to register
with the sleeve contact on the plug when the plug is fully inserted. The
line circuit through the drop may be traced normally from one side of
the line through the drop coil, thence through all of the pairs of
springs _2_ and _3_ in the jacks of that line, and thence to spring _1_
of the last jack, this spring always being strapped to the spring _2_ in
the last jack, and thence to the other side of the line. All the ring
springs _1_ are permanently tapped on to one side of the line, and all
of the tip springs _4_ are permanently tapped to the other side of the
line. This system may, therefore, properly be called a branch-terminal
system. It is seen that as soon as a plug is inserted into any of the
jacks, the circuit through the drop will be broken by the opening of the
springs _2_ and _3_ in that jack. The drop shown immediately above the
answering jack is so associated mechanically with that jack as to be
mechanically self-restored when the answering plug is inserted into the
answering jack in response to a call. The arrangement in this respect is
the same as that shown in Fig. 259, illustrating the Monarch combined
drop and jack.
_Cord Circuit._ The cord circuit needs little explanation. The tip and
ring strands are the ones which carry the talking current and across
these is bridged the double-wound clearing-out drop, a condenser being
included in series in the tip strand between the two drop windings in
the manner already explained in connection with Fig. 284. The third or
sleeve strand of the cord is continuous from plug to plug, and between
it and the ground there is permanently connected a retardation coil.
_Test._ The test is dependent on the presence or absence of a path to
ground from the test thimbles through some retardation coil associated
with a cord circuit. Obviously, in the case of an idle line there will
be no path to ground from the test thimbles, since normally they are
merely connected to each other and are insulated from everything else.
When, however, a plug is inserted into a multiple or answering jack, the
test thimbles of that line are connected to ground through the
retardation coil associated with the third strand of the plug used in
making the connection. When the operator applies the tip of the calling
plug to a test contact of a multiple jack there will be no path to
ground afforded if the line is idle, while if it is busy the potential
of the tip of the test plug will cause a current to flow to ground
through the impedance coil associated with the plug used in making the
connection. This will be made clearer by tracing the test circuit. With
the listening key thrown this may be traced from the live side of the
battery through the retardation coil _6_, which is common to an
operator's position, thence through the tip side of the listening key to
the tip conductor of the calling cord, and thence to the tip of the
calling plug and the thimble of the jack under test. If the line is idle
there will be no path to ground from this point and no click will
result, but if the line is busy, current will flow from the tip of the
test plug to the thimble of the jack tested, thence by the test wire in
the multiple to the thimble of the jack at which a connection already
exists, and thence to ground through the third strand of the cord used
in making that connection and the impedance coil associated therewith.
The current which flows in this test circuit changes momentarily the
potential of the tip side of the operator's telephone circuit, thus
unbalancing her talking circuit and causing a click.
[Illustration: Fig. 341. Magneto Multiple Switchboard]
If this test system were used in a very large board where the multiple
would extend through a great many sections, there would be some
liability of a false test due to the static capacity of the test
contacts and the test wire running through the multiple. For small
boards, however, where the multiple is short, this system has proven
reliable. A multiple magneto switchboard employing the form of circuits
just described is shown in Fig. 341. This switchboard consists of three
sections of two positions each. The combined answering jacks and drops
may be seen at the lower part of the face of the switchboard and
occupying somewhat over one-half of the jack and drop space. The
multiple jacks are above the answering jacks and drops and it may be
noted that the same arrangement and number of these jacks is repeated in
each section. This switchboard may be extended by adding more sections
and increasing the multiple in those already installed to serve 1,600
lines.
_Assembly._ In connection with the assembly of these magneto multiple
switchboards, as installed by the Monarch Company, Fig. 342 shows the
details of the cord rack at the back of the board. It shows how the ends
of the switchboard cords opposite to the ends that are fastened to the
plugs are connected permanently to terminals on the cord rack, at which
point the flexible conductors are brought out to terminal clips or
binding posts, to which the wires leading from the other portions of the
cord circuit are led. In order to relieve the conductors in the cords
from strain, the outer braiding of the cord at the rack end is usually
extended to form what is called a _strain cord_, and this attached to an
eyelet under the cord rack, so that the weight of the cord and the cord
weights will be borne by the braiding rather than by the conductors.
This leaves the insulated conductors extending from the ends of the
cords free to hang loose without strain and be connected to the
terminals as shown. This method of connecting cords, with variations in
form and detail, is practically universal in all types of switchboards.
[Illustration: Fig. 342. Cord-Rack Connectors]
A detail of the assembly of the drops and jacks in such a switchboard
is shown in Fig. 343. The single pair of clearing-out drops is mounted
in the lower part of the vertical face of the switchboard just above the
space occupied by the plug shelf. Vertical stile strips extend above the
clearing-out drop space for supporting the drops and jacks. A single row
of 10 answering jacks and the corresponding line drops are shown in
place. Above these there would be placed, in the completely assembled
board, the other answering jacks and line signals that were to occupy
this panel, and above these the strips of multiple jacks. The rearwardly
projecting pins from the stile strips are for the support of the
multiple jack strips, these pins supporting the strips horizontally by
suitable multiple clips at the ends of the jack strips; the jack strips
being fastened from the rear by means of nuts engaging the screw
threads on these pins. This method of supporting drops and jacks is one
that is equally adaptable for use in other forms of boards, such as the
simple magneto switchboard.
[Illustration: Fig. 343. Drop and Jack Mounting]
[Illustration: Fig. 344. Keyboard Wiring]
In Fig. 344 is shown a detail photograph of the key shelf wiring in one
of these Monarch magneto switchboards. In this the under side of the
keys is shown, the key shelf being raised on its hinge for that purpose.
The cable, containing all of the insulated wires leading to these keys,
enters the space under the key shelf at the extreme left and from the
rear. It then passes to the right of this space where a "knee" is
formed, after which the cable is securely strapped to the under side of
the key shelf. By this construction sufficient flexibility is provided
for in the cable to permit the raising and lowering of the key shelf,
the long reach of the cable between the "knee" and the point of entry at
the left serving as a torsion member, so that the raising of the shelf
will give the cable a slight twist rather than bend it at a sharp angle.
CHAPTER XXVI
THE COMMON-BATTERY MULTIPLE SWITCHBOARD
=Western Electric No. 1 Relay Board.= The common-battery multiple
switchboard differs from the simple or non-multiple common-battery
switchboard mainly in the provision of multiple jacks and in the added
features which are involved in the provision for a busy test. The
principles of signaling and of supplying current to the subscribers for
talking are the same as in the non-multiple common-battery board. For
purposes of illustrating the practical workings of the common-battery
multiple switchboard, we will take the standard form of the Western
Electric Company, choosing this only because it is the standard with
nearly all the Bell operating companies throughout the United States.
[Illustration: Fig. 345. Line Circuit Western Electric No. 1. Board]
_Line Circuit._ We will first consider the line circuit in simplified
form, as shown in Fig. 345. At the left in this figure the
common-battery circuit is shown at the subscriber's station, and at the
right the central-office apparatus is indicated so far as equipment of a
single line is concerned. In this simplified diagram no attempt has been
made to show the relative positions of the various parts, these having
been grouped in this figure in such a way as to give as clear and simple
an idea as possible of the circuit arrangements. It is seen at a glance
that this is a branch terminal board, the three contacts of each jack
being connected by separate taps or legs to three wires running
throughout the length of the board, these three wires being individual
to the jacks of one line. On this account this line circuit is commonly
referred to as a three-wire circuit. By the same considerations it will
be seen that the switchboard line circuit of the branch-terminal
multiple magneto system, shown in Fig. 338, would be called a four-wire
circuit. It will be shown later that other multiple switchboards in wide
use have a still further reduction in the number of wires running
through the jacks, or through the multiple as it is called, such being
referred to as two-wire switchboards.
The two limbs of the line which extend from the subscriber's circuit,
beside being connected by taps to the tip and sleeve contacts of the
jack respectively, connect with the two back contacts of a cut-off
relay, and when this relay is in its normal or unenergized condition,
these two limbs of the line are continued through the windings of the
line relay and thence one to the ungrounded or negative side of the
common-battery and the other to the grounded side. The subscriber's
station circuit being normally open, no current flows through the line,
but when the subscriber removes his receiver for the purpose of making a
call the line circuit is completed and current flows through the coil of
the line relay, thus energizing that relay and causing it to complete
the circuit of the line lamp. The cut-off relay plays no part in the
operation of the subscriber's calling, but merely leaves the circuit of
the line connected through to the calling relay and battery. The coil of
the cut-off relay is connected to ground on one side and on the other
side to the third wire running through the switchboard multiple and
which is tapped off to each of the test rings on the jacks. As will be
shown later, when the operator plugs into the jack of a line, such a
connection is established that the test ring of that jack will be
connected to the live or negative pole of the common battery, which will
cause current to flow through the coil of the cut-off relay, which will
then operate to _cut off_ both of the limbs of the line from their
normal connection with ground and the battery and the line relay. Hence
the name _cut-off relay_.
The use of the cut-off relay to sever the calling apparatus from the
line at all times when the line is switched serves to make possible a
very much simpler jack than would otherwise be required, as will be
obvious to anyone who tries to design a common-battery multiple system
without a cut-off relay. The additional complication introduced by the
cut-off relay is more than offset by the saving in complexity of the
jacks. It is desirable, on account of the great number of jacks
necessarily employed in a multiple switchboard, that the jacks be of the
simplest possible construction, thus reducing to a minimum their first
cost and making them much less likely to get out of order.
_Cord Circuit._ The cord circuit of the Western Electric standard
multiple common-battery switchboard is shown in Fig. 346. This cord
circuit involves the use of three strands in the flexible cords of both
the calling and the answering plugs. Two of these are the ordinary tip
and ring conductors over which speech is transmitted to the connected
subscriber's wire. The third, the sleeve strand, carries the supervisory
lamps and has associated with it other apparatus for the control of
these lamps and of the test circuit.
[Illustration: Fig. 346. Cord Circuit Western Electric No. 1 Board]
The system of battery feed is the well-known split repeating-coil
arrangement already discussed. The tip strand runs straight through to
the repeating coil, while the ring strand contains, in each case, the
winding of the supervisory relay corresponding to either the calling or
the answering plug. In order that the presence in the talking circuit of
a magnet winding possessing considerable impedance may not interfere
with the talking efficiency, each of these supervisory relay windings is
shunted by a non-inductive resistance. In practice the supervisory relay
windings have each a resistance of about 20 ohms and the shunt around
them each a resistance of about 31 ohms. In the third strand of each
cord is placed a 12-volt supervisory lamp, and in series with it a
resistance of about 80 ohms. Each supervisory relay is adapted, when
energized, to close a 40-ohm shunt about its supervisory lamp. The
arrangement and proportion of these resistances is such that when a plug
is inserted into the jack of a line the lamp will receive current from a
circuit traced from the negative pole of the battery in the center of
the cord circuit through the lamp and the 80-ohm series resistance,
through the third strand of the cord to the test thimble of the jack,
and thence to the positive or grounded pole of the battery through the
third conductor in the multiple and the winding of the cut-off relay.
This current always flows as long as the plug is inserted, and it is
just sufficient to illuminate the lamp when the supervisory relay
armature is not attracted. When, however, the supervisory relay armature
is attracted, the shunting of the lamp by the 40-ohm resistance cuts
down the current to such a degree as to prevent the illumination of the
lamp, although some current still flows through it.
The usual ringing and listening key is associated with the calling plug,
and in some cases a ring-back key is associated with the answering plug,
but this is not standard practice.
_Operation._ The operation of this cord circuit in conjunction with the
line circuit of Fig. 345 may best be understood by reference to Fig.
347. This figure employs a little different arrangement of the line
circuit in order more clearly to indicate how the two lines may be
connected by a cord; a study of the two line circuits, however, will
show that they are identical in actual connections. It is to be
remembered that all of the battery symbols shown in this figure
represent in reality the same battery, separate symbols being shown for
greater simplicity in circuit connections.
We will assume the subscriber at Station _A_ calls for the subscriber at
Station _B_. The operation of the line relay and the consequent lighting
of the line lamp, and also the operation of the pilot relay will be
obvious from what has been stated. The response of the operator by
inserting the answering plug into the answering jack, and the throwing
of her listening key so as to bridge her talking circuit across the cord
in order to place herself in communication with the subscriber, is also
obvious. The insertion of the answering plug into the answering jack
completed the circuit through the third strand of the cord and the
winding of the cut-off relay of the calling line, and this accomplishes
three desirable results. The circuit so completed may be traced from the
negative or ungrounded side of the battery to the center portion of the
cord circuit, thence through the supervisory lamp _1_, resistance _2_,
to the third conductor on the plug, test thimble on the jack, thence
through the winding of the cut-off relay to ground, which forms the
other terminal of the battery. The results accomplished by the closing
of this circuit are: first, the energizing of the cut-off relay to cut
off the signaling portion of the line; second, the flowing of current
through the lamp that is almost sufficient to illuminate it, but not
quite so because of the closure of the shunt about it, for the reason
that will be described; third, the raising of the potential of all the
contact thimbles on the jacks from zero to a potential different from
that of the ground and equal in amount to the fall of potential through
the winding of the cut-off relay. A condition is thus established at the
test rings such that some other operator at some other section in
testing the line will find it busy and will not connect with it.
[Illustration: Fig. 347. Western Electric No. 1 Board]
The reason why the lamp _1_, connected with the answering plug, was not
lighted was that the supervisory relay _3_, associated with the
answering plug, became energized when the operator plugged in, due to
the flow of current from the battery through the calling subscriber's
talking apparatus, this flow of current being permitted by the removal
of the calling subscriber's receiver from its hook. The energizing of
this relay magnet by causing the attraction of its armature, closed the
shunt about the lamp _1_, which shunt contains the 40-ohm resistance
_4_, and thus prevents the lamp from receiving enough current to
illuminate it. Obviously, as soon as the calling subscriber replaces his
receiver on its hook, the supervisory relay _3_ will be de-energized,
the shunt around the lamp will be broken, and the lamp will be
illuminated to indicate to the operator the fact that the subscriber
with whose line her calling plug is connected has replaced his receiver
on its hook.
_Testing--Called Line Idle._ Having now shown how the operator connects
with the calling subscriber's line and how that line automatically
becomes guarded as soon as it is connected with, so that no other
operator will connect with it, we will discuss how the operator tests
the called line and subsequently connects with that line, if it is found
proper to do so. If, on making the test with one of the multiple jacks
of the line leading to Station _B_, that line is idle and free to be
connected with, its test rings will all be at zero potential because of
the fact that they are connected with ground through the cut-off relay
winding with no source of current connected with them. The tip of the
calling plug will also be at zero potential in making this test, because
it is connected to ground through the tip side of the calling-plug
circuit and one winding of the cord-circuit repeating coil. As a result
no flow of current will occur, the operator will receive no click, and
she will know that she is free to connect with the line. As soon as she
does so, by inserting the plug, the third strand of the cord will be
connected with the test thimble of the calling line and the resulting
flow of current will bring about three results, two of which are the
same, and one of which is slightly different from those described as
resulting from the insertion of the answering plug into the jack of the
calling line. First, the cut-off relay will be operated and cut off the
line signaling apparatus from the called line; second, a flow of current
will result through the calling supervisory lamp _5_, which in this case
will be sufficient to illuminate that lamp for the reason that the
called subscriber has not yet responded, the calling supervisory relay
_6_ has, therefore, not yet been energized, and the lamp has not,
therefore, been shunted by its associated resistance _7_; third, the
test thimbles of the called line will be raised to a potential above
that of the earth, and thus the line will be guarded against connection
at another section of the switchboard. As soon as the called subscriber
responds to the ringing current sent out by the operator, current will
flow over the cord circuit and over his line through his transmitter.
This will cause the calling supervisory relay to be energized and the
calling lamp to be extinguished. Both lamps _1_ and _5_ remain
extinguished as long as the connected subscribers are in conversation,
but as soon as either one of them hangs up his receiver the
corresponding lamp will be lighted, due to the de-energization of the
supervisory relay and the breaking of the shunt around the lamp. The
lighting of both lamps associated with a cord circuit is a signal to the
operator for disconnection.
[Illustration: TERMINAL ROOM IN MEDIUM-SIZED MANUAL OFFICE Relay Rack at
Right. This Employs the Kellogg Parallel Arrangement of Frames.]
_Testing--Called Line Busy._ If we now assume that the called line was
already busy, by virtue of being connected with at another section, the
test rings of that line would accordingly all be raised to a potential
above that of the earth. As a result, when the operator applied the tip
of her calling plug to a test thimble on that line, current would flow
from this test thimble through the tip of the calling plug and tip
strand of the cord and through one winding of the cord-circuit repeating
coil to ground. This would cause a slight raising of potential of the
entire tip side of the cord circuit and a consequent momentary flow of
current through the secondary of the operator's circuit bridged across
the cord circuit at that time.
_Operator's Circuit Details._ The details of the operator's talking
circuit shown in Fig. 347 deserve some attention. The battery supply to
the operator's transmitter is through an impedance coil _9_. The
condenser _12_ is bridged around the transmitter and the two primary
windings _10_ and _11_, which windings are in parallel so as to afford a
local circuit for the passage of fluctuating currents set up by the
transmitter. The two primary windings _10_ and _11_ are on separate
induction coils, the secondary windings _13_ and _14_ being, therefore,
on separate cores. The winding _15_, in circuit with the secondary
winding _14_ and the receiver, is a non-inductive winding and is
supposed to have a resistance about equal to the effective resistance to
fluctuating currents of a subscriber's line of average length. Owing to
the respective directions of the primary and secondary windings _10_ and
_11_, _13_ and _14_, the result is that the outgoing currents set up by
the operator's transmitter are largely neutralized in the operator's
receiver. Incoming currents from either of the connected subscribers,
however, pass, in the main, through the secondary coil _13_ and the
operator's receiver, rather than through the shunt path formed by the
secondary _14_, and the non-inductive resistance _15_. This is known as
an "anti-side tone" arrangement, and its object is to prevent the
operator from receiving her own voice transmission so loudly as to make
her ear insensitive to the feebler voice currents coming in from the
subscribers.
_Order-Wire Circuits._ The two keys _16_ and _17_, shown in connection
with the operator's talking circuit in Fig. 347, play no part in the
regular operation of connecting two local lines, as described above.
They are order-wire keys, and the circuits with which they connect lead
to the telephone sets of other operators at distant central offices, and
by pressing either one of these keys the operator is enabled to place
herself in communication over these so-called order-wire circuits with
such other operators. The function and mode of operation of these
order-wire circuits will be described in the next chapter, wherein
inter-office connections will be discussed.
_Wiring of Line Circuit._ The line circuits shown in Figs. 345 and 347
are, as stated, simplified to facilitate understanding, although the
connections shown are those which actually exist. The more complete
wiring of a single line circuit is shown in Fig. 348. The line wires are
shown entering at the left. They pass immediately, upon entering the
central office, through the main distributing frame, the functions and
construction of which will be considered in detail in a subsequent
chapter. The dotted portions of the circuit shown in connection with
this main distributing frame indicate the path from the terminals on one
side of the frame to those on the other through so-called jumper wires.
The two limbs of the line then pass to terminals _1_ and _2_ on one side
of the so-called intermediate distributing frame. Here the circuit of
each limb of the line divides, passing, on the one hand, to the tip and
sleeve springs of all the multiple jacks belonging to that line; and, on
the other hand, through the jumper wires indicated by dotted lines on
the intermediate distributing frame, and thence to the tip and ring
contacts of the answering jack. A consideration of this connection will
show that the actual electrical connections so far as already described
are exactly those of Figs. 345 and 347, although those figures omitted
the main and intermediate distributing frames. Only two limbs of the
line are involved in the main frame. In the intermediate frame the test
wire running through the multiple is also involved. This test wire, it
will be seen, leads from the test thimbles of all the multiple jacks to
the terminal _3_ on the intermediate frame, thence through the jumper
wire to the terminal _6_ of this frame, and to the test thimble of the
answering jack. Here again the electrical connections are exactly those
represented in Figs. 345 and 347, although those figures do not show the
intermediate frame.
The two terminals _4_ and _5_ of the intermediate frame, besides being
connected to the tip and sleeve springs of the answering jack, are
connected to the contacts of the cut-off relay, and thence through the
coils of the line relay to ground on one side and to battery on the
other. Thus the line relay and battery are normally included in the
circuit of the line. The contact _6_ on the intermediate distributing
frame, besides being connected to the test thimble of all the jacks, is
connected through the coil of the cut-off relay to ground, thus
establishing a path by which current is supplied to the cut-off relay
when connection is made to the line at any jack. There is another
contact _7_ on the intermediate distributing frame which merely forms a
terminal for joining one side of the line lamp to the back contact of
the line relay.
_Functions of Distributing Frames._ Since the line circuit thus far
described in connection with Fig. 348 is exactly the same as that of
Fig. 345 in its electrical connections, it becomes obvious that the main
and intermediate distributing frames play no part in the operation of
the circuit any more than a binding post of a telephone plays a part in
its operation. These frames carry terminals for facilitating the
connection of the various wires in the line circuit and, as will be
shown later, for facilitating certain changes in the line connection.
[Illustration: Fig. 348. Line Circuit No. 1 Board]
Remembering that the dotted lines in Fig. 348 indicate jumper wires of
the main and intermediate distributing frames, and that these are in the
nature of temporary or readily changeable connections, and that the full
lines, whether heavy or light, are permanent connections not readily
changeable, it will be seen that the wires leading through the multiple
jacks of a certain line are permanently associated with each other, and
with certain terminals on the main distributing frame and certain other
terminals on the intermediate distributing frame. It will also be seen
that the line lamp and the answering jack, together with the cut-off
relay and line relay, are permanently associated with each other and
with another group of terminals _4_, _5_, _6_, and _7_ on the
intermediate distributing frame. It will also be apparent that by
changing the jumper wires on the main frame, any outside line may be
connected with any different set of line switchboard equipment, and also
that by making changes in the jumper wires on the intermediate frame,
any given answering jack and line lamp with its associated line cut-off
relay may be associated with any set of multiple jacks.
_Pilot Signals._ In a portion of the circuit leading from the battery
that is common to a group of line lamps is the winding of the pilot
relay, which is common to this group of line lamps. This controls, as
already described, the circuit of the pilot lamp common to the same
group of line lamps. In addition, a night-bell circuit is sometimes
provided, this usually being in the form of an ordinary polarized
ringer, the circuit of which is controlled by a night-bell relay common
to the entire office. Normally, this relay is shunted out of the circuit
of the common portion of the lead to the pilot relay contacts by the key
_8_, but when the key _8_ is opened all current that is fed to the pilot
lamps passes through the night-bell relay, and thus, whenever any pilot
lamp is lighted, the night-bell relay will attract its armature and thus
close the circuit of the calling generator through the night bell.
A study of this figure will make clear to the student how the portions
of the circuit that are individual to the line are associated with such
things as the battery, that are common to the entire office, and such as
the pilot relay and lamp, that are common to a group of lines
terminating in one position.
_Modified Relay Windings._ In some cases, the line relay instead of
being double wound, as shown, is made with a single winding, this
winding being normally included between the ring side of the cut-off
relay and the battery, the tip side of the cut-off relay being run
direct to ground. The present practice of the Western Electric Company
is towards the double-wound relay, however, and that is considered
standard in all of their large No. 1 multiple boards, except where the
customer, owing to special reasons, demands a single wound relay on the
ring side of the line. The prime reason for the two-winding line relay
is the lessened click in the calling-subscriber's receiver which occurs
when the operator answers. All line relays prior to 1902 were
single-wound, but after that they were made double and used some turns
of resistance wire to limit the normal calling current.
_Relay Mounting._ In the standard No. 1 relay board of the Western
Electric Company and, in fact, in nearly all common-battery multiple
boards that are manufactured by other companies, the line and cut-off
relays are mounted on separate racks outside the switchboard room and
adjacent to the main and intermediate distributing frames, the wiring
being extended from the relays to the jacks and lamps on the switchboard
proper by means of suitable cables. The Western Electric Company has
recently instituted a departure from this practice in the case of some
of their smaller No. 1 switchboard installations. Where it is thought
that the ultimate capacity required by the board will not be above 3,000
lines, the relay rack is dispensed with and all of the line and cut-off
relays, as well as the supervisory relays, are mounted in the rear of
the switchboard frame. For this purpose the line and cut-off relays are
specially made with the view to securing the utmost compactness. In
still other cases, in switchboards of relatively small ultimate
capacity, they use this small line and cut-off relay mounted on a
separate relay rack, in which case the board is the standard No. 1 board
except for the type of relays. In all of these modifications of the No.
1 board adapted for the use of the smaller and cheaper relays, the line
relay has but a single winding, the small size of the relay winding not
lending itself readily to double winding with the added necessary coil
terminals.
_Capacity Range._ The No. 1 Western Electric board is made in standard
sizes up to an ultimate capacity of 9,600 lines. For all capacities
above 4,900 lines, a 3/8-inch jack, vertical and horizontal face
dimensions, is employed. For this capacity the smaller types of cut-off
and line relays are not employed. Up to ultimate capacities of 4,900
lines, 1/2-inch jacks are employed, and either the small or the large
relays mounted on a separate rack are available. Up to 3,000 lines
ultimate capacity, the 1/2-inch jack is employed, and either the small
or the large cut-off and line relays are available, but in case the
small type is used the purchaser has the option of mounting them on a
separate relay rack, as in ordinary practice, or mounting them in the
switchboard cabinet and dispensing with the relay rack.
=Western Electric No. 10 Board.= The No. 1 common-battery multiple
switchboard, regardless of its size and type of arrangement of line and
cut-off relays, involves two relays for each line, the line relay
energized by the taking of the receiver off its hook, and the cut-off
relay energized by the act of the operator on plugging in and serving to
remove the line relay from the circuit whenever and as long as a plug is
inserted into any jack of the line. This seems to involve a considerable
expense in relays, but this, as has been stated, is warranted by the
greater simplicity in jacks which the use of the cut-off relay makes
possible. In addition to this expense of investment in the line and
cut-off relays, the amount of current required to hold up the cut-off
relays during conversations foots up to a considerable item of expense,
particularly as the system of supervisory signals is one in which the
supervisory lamp takes current not only while burning, but its circuit
takes even more current when the lamp is extinguished during the time of
a connection. For all of these reasons, and some other minor ones, it
was deemed expedient by the engineers of the Western Electric Company to
design a common-battery multiple switchboard for small and medium-sized
exchanges in which certain sacrifices might be made to the end of
accomplishing certain savings. The result has been a type of
switchboard, designated the No. 10, which may be found in a number of
Bell exchanges, it being considered particularly adaptable to
installations of from 500 to 3,000 lines. Although this board has been
subject to a good deal of adverse criticism, and although it seems
probable that even for the cheaper boards the No. 1 type with some of
the modifications just described will eventually supersede this No. 10
board, yet the present extent of use of the No. 10 board and the
instructive features which its type displays warrant its discussion
here.
_Circuits._ The circuits of this switchboard are shown in Fig. 349, this
indicating two-line circuits and a connecting cord circuit, together
with the auxiliary apparatus employed in connection with the operator's
telephone circuit, the pilot and night alarm circuits. The most
noticeable feature is that cut-off jacks are employed, the circuit of
the line normally extending through the sets of jack springs in the
multiple, and answering jacks to the line relay and battery on one side
of the line, and to ground on the other side. Obviously, the additional
complexity of the jack saves the use of a cut-off relay and the relay
equipment of each line consists, therefore, of but a single line relay,
which controls the lamp in an obvious manner.
[Illustration: Fig. 349. Western Electric No. 10 Board]
The cord circuit is of the three-conductor type, the two talking strands
extending to the usual split repeating-coil arrangement, and battery
current for talking purposes being fed through these windings as in the
standard No. 1 board. The supervisory relay is included in the ring
strand of the cord circuit and is shunted by a non-inductive resistance,
so that its impedance will not interfere with the talking currents. The
armature of the supervisory relay closes the lamp contact on its back
stroke, so that the lamp is always held extinguished when the relay is
energized. The supervisory lamp is included in a connection between the
back contact of the supervisory relay and ground, this connection
including the central-office battery. As a result, the illumination of
the supervisory lamp is impossible until a plug has been inserted into a
jack, in which case, assuming the supervisory relay to be de-energized,
the lamp circuit is completed through the wire connecting all of the
test thimbles and the resistance permanently bridged to ground from that
wire.
_Test._ For purposes of the test it is evident that the test rings of an
idle line are always at ground potential, due to their connection to
ground through the resistance coil. It is also evident that the tip of
an unused calling plug will always be at ground potential and,
therefore, that the testing of an idle line will result in no click in
the operator's receiver. When a line is switched, however, the potential
of all the test rings will be raised due to their being connected with
the live pole of the battery through the third strand of the cord. When
the operator in testing touches the test contact of the jack of a busy
line, a current will, therefore, flow from this test contact to the tip
strand of the cord and thence to ground through one of the repeating
coil windings. The potential of the tip side of the cord will,
therefore, be momentarily altered, and this will result in a click in
the operator's receiver bridged across the cord circuit at the time. The
details of the operator's cord circuit and of the pilot lamp and night
alarm circuits will be clear from the diagram.
_Operation._ A brief summary of the operation of this system is as
follows:
The subscriber removes his receiver from its hook, thus drawing up the
armature of the line relay and lighting his line lamp. The operator
answers. The line lamp is extinguished by the falling back of the
line-relay armature, due to the breaking of the relay circuit at the
jack contacts. The subscriber then receives current for his transmitter
through the cord-circuit battery connections. The supervisory relay
connected with the answering cord is not lighted, because, although the
lamp-circuit connection is completed at the jack, the supervisory relay
is operated to hold the lamp circuit open. Conversation ensues between
the operator and the subscriber, after which the operator tests the line
called for with the tip of the calling plug of the pair used in
answering. If the called line is not busy, no click will ensue, because
both the tested ring and the calling plug are at the same potential.
Finding no click, the operator will insert the plug and ring by means of
the ringing key. When the operator plugs in, the supervisory lamp,
associated with the calling plug, becomes lighted because the circuit is
completed at the jack and the supervisory relay remains de-energized,
since the line circuit is open at the subscriber's station. When the
called subscriber responds, the calling supervisory lamp goes out
because of the energization of the supervisory relay. Both lamps remain
out during the conversation, but when either subscriber hangs up, the
corresponding supervisory lamp will be lighted because of the falling
back of the supervisory relay armature.
If the called line is busy, a click will be heard, for the reason
described, and the operator will so inform the calling subscriber. It
goes without saying, that in any multiple-switchboard system a plug may
be found in the actual multiple jack that is reached for, in which case,
although no test will be made, the busy condition will be reported back
to the calling subscriber.
_Economy._ It has been the belief of the Western Electric engineers that
a real economy is accomplished in this type of board by the saving in
relay equipment. It is, of course, apparent at a glance that with a
switchboard long enough and of sections enough, the cost of extra jack
springs and their platinum contacts must become great enough to offset
the saving accomplished by omitting the cut-off relay. This makes it
apparent that if there is any economy in this type of multiple
switchboard, it must be found in the very small boards where there are
but few jacks per line and where the extra cost of the cut-off jack is
not enough to offset the extra cost of an added relay. It is the growing
belief, however, among engineers, that the multiple switchboard must be
very small indeed in order that the added complexity of the cut-off
jacks and wiring may be able to save anything over the two-relay type of
line; and it is believed that where economy is necessary in small
boards, it may be best effected by employing cheaper and more compact
forms of relays and mounting them, if necessary, directly in the
switchboard cabinet.
NOTE. These two standard types of common-battery multiple
switchboards of the Western Electric Company represent the
development through long years of careful work on the part of
the Western Electric and Bell engineers, credit being
particularly due to Scribner, McBerty, and McQuarrie of the
Western Electric Company, and Hayes of the American Telephone
and Telegraph Company.
=Kellogg Two-Wire Multiple Board.= The simplicity in the jacks permitted
by the use of the cut-off relay in the Western Electric common-battery
multiple switchboard for larger exchanges was carried a step further by
Dunbar and Miller in the development of the so-called two-wire
common-battery multiple switchboard, which for many years has been the
standard of the Kellogg Switchboard and Supply Company. The particular
condition which led to the development of the two-wire system was the
demand at that time on the Kellogg Company for certain very large
multiple switchboards, involving as many as 18,000 lines in the
multiple. Obviously, this necessitated a small jack, and obviously a
jack having only two contacts, a tip spring and a sleeve, could be made
more easily and with greater durability of this very small size than a
jack requiring three or more contacts. Other reasons that were
considered were, of course, cheapness in cost of construction and
extreme simplicity, which, other things being equal, lends itself to low
cost of maintenance.
_Line Circuit._ Like the standard Western Electric board for large
offices, the Kellogg two-wire board employs two relays for each line,
the line relay under the control of the subscriber and in turn
controlling the lamp, and a cut-off relay under the control of the
operator and in turn controlling the connection of the line relay with
the line. The line circuit as originally developed and as widely used by
the Kellogg Company is shown in Fig. 350. The extreme simplicity of the
jacks is apparent, as is also the fact that but two wires lead through
the multiple. Another distinguishing feature is, that all of the
multiple and answering jacks are normally cut off from the line at the
cut-off relay, but when the cut-off relay operates it serves, in
addition to cutting off the line relay, to attach the two limbs of the
line to the two wires leading through the multiple and answering jacks.
The control of the line relay by the subscriber's switch hook is clear
from the figure. The control of the cut-off relay is secured by
attaching one terminal of the cut-off relay winding permanently to that
wire leading through the multiple which connects with the sleeve
contacts of the jack, the other terminal of the cut-off relay being
grounded. The way in which this relay is operated will be understood
when it is stated that the sleeve contacts of both the answering and
calling plugs always carry full battery potential and, therefore,
whenever any plug is inserted into any jack, current flows from the
sleeve of the jack through the sleeve contact of the jack to ground,
through the winding of the cut-off relay, which relay becomes energized
and performs the functions just stated. It is seen that the wire
running through the multiple to which the sleeve jack contacts are
attached, is thus made to serve the double purpose of answering as one
side of the talking circuit, and also of performing the functions
carried out by the separate or third wire in the three-wire system. It
will be shown also that, in addition, this wire is made to lend itself
to the purposes of the busy test without any of these functions
interfering with each other in any way.
[Illustration: Fig. 350. Two-Wire Line Circuit]
_Cord Circuit._ The cord circuit in somewhat simplified form is shown in
Fig. 351. Here again there are but two conductors to the plugs and two
strands to the cords. This greater simplicity is in some measure offset
by the fact that four relays are required, two for each plug. This
so-called four-relay cord circuit may be most readily understood by
considering half of it at a time, since the two relays associated with
the answering plug act in exactly the same way as those connected with
the calling plug.
[Illustration: Fig. 351. Two-Wire Cord Circuit]
Associated with each plug of a pair are two relays _1_ and _2_, in the
case of the answering cord, and _3_ and _4_ in the case of the calling
cord. The coils of the relays _1_ and _2_ are connected in series and
bridged across the answering cord, a battery being included between the
coils in this circuit. The coils of the relays _3_ and _4_ are similarly
connected across the calling cord. A peculiar feature of the Kellogg
system is that two batteries are used in connection with the cord
circuit, one of them being common to all answering cords and the other
to all calling cords. The operation of the system would, however, be
exactly the same if a single battery were substituted for the two.
_Supervisory Signals._ Considering the relays associated with the
answering cord, it is obvious that these two relays _1_ and _2_ together
control the circuit of the supervisory lamp _5_, the circuit of this
lamp being closed only when the relay _1_ is de-energized and the relay
_2_ is energized. We will find in discussing the operation of these that
the relay _2_ is wholly under the control of the operator, and that the
relay _1_, after its plug has been connected with a line, is wholly
under the control of the subscriber on that line. It is through the
windings of these two relays that current is fed to the line of the
subscriber connected with the corresponding cord.
When a plug--the answering plug, for instance--is inserted into a jack,
current at once flows from the positive pole of the left-hand battery
through the winding of the relay _2_ to the sleeve of the plug, thence
to the sleeve of the jack and through the cut-off relay to ground. This
at once energizes the supervisory relay _2_ and the cut-off relay
associated with the line. The cut-off relay acts, as stated, to continue
the tip and sleeve wires associated with the jacks to the line leading
to the subscriber, and also to cut off the line relay. The supervisory
relay _2_ acts at the same time to attract its armature and thus
complete its part in closing the circuit of the supervisory lamp.
Whether or not the lamp will be lighted at this time depends on whether
the relay _1_ is energized or not, and this, it will be seen, depends on
whether the subscriber's receiver is off or on its hook. If off its
hook, current will flow through the metallic circuit of the line for
energizing the subscriber's transmitter, and as whatever current goes to
the subscriber's line must flow through the relay _1_, that relay will
be energized and prevent the lighting of the supervisory lamp _5_. If,
on the other hand, the subscriber's receiver is on its hook, no current
will flow through the line, the supervisory relay will not be energized,
and the lamp _5_ will be lighted.
In a nutshell, the sleeve supervisory relay normally prevents the
lighting of the corresponding supervisory lamp, but as soon as the
operator inserts a plug into the jack of the line, the relay _2_
establishes such a condition as to make possible the lighting of the
supervisory lamp, and the lighting of this lamp is then controlled
entirely by the relay _1_, which is, in turn, controlled by the position
of the subscriber's switch hook.
_Battery Feed._ A 2-microfarad condenser is included in each strand of
the cord, and battery is fed through the relay windings to the calling
and called subscribers on opposite sides of these condensers, in
accordance with the combined impedance coil and condenser method
described in Chapter XIII. Here the relay windings do double duty,
serving as magnets for operating the relays and as retardation coils in
the system of battery supply.
_Complete Cord and Line Circuits._ The complete cord and line circuits
of the Kellogg two-wire system are shown in Fig. 352. In the more recent
installations of the Kellogg Company the cord and line circuits have
been slightly changed from those shown in Figs. 350 and 351, and these
changes have been incorporated in Fig. 352. The principles of operation
described in connection with the simplified figures remain, however,
exactly the same. One of the changes is, that the tip side of the lines
is permanently connected to the tips of the jacks instead of being
normally cut off by the cut-off relay, as was done in the system as
originally developed. Another change is, that the line relay is
associated with the tip side of the line, rather than with the sleeve
side, as was formerly done. The cord circuit shown in Fig. 352 shows
exactly the same arrangement of supervisory relays and exactly the same
method of battery feed as in the simplified cord circuit of Fig. 351,
but in addition to this the detailed connections of the operator's
talking set and of her order-wire keys are indicated, and also the
ringing equipment is indicated as being adapted for four-party harmonic
work.
[Illustration: Fig. 352. Kellogg Two-Wire Board]
In connection with this ringing key it may be stated that the springs
_7_, _8_, _9_, and _10_ are individually operated by the pressure of one
of the ringing key buttons, while the spring _17_, connected with the
sleeve side of the calling plug, is always operated simultaneously with
the operation of any one of the other springs. As a result the proper
ringing circuit is established, it being understood that the upper
contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of
their respective ringing generators, the other terminals of which are
grounded. The circuit is, therefore, from the generator, through the
ringing key, out through the tip side of the line, back over the sleeve
side of the line, and to ground through the spring _17_, resistance
_11_, and the battery, which is one of the cord-circuit batteries. The
object of this coil _11_ and the battery connection through it to the
ringing-key spring is to prevent the falling back of the cut-off relay
when the ringing key is operated. This will be clear when it is
remembered that the cut-off relay is energized by battery current fed
over the sleeve strand of the cord, and obviously, since it is necessary
when the ringing key is operated to cut off the supply wire back of the
key, this would de-energize the cut-off relay when the ringing key was
depressed, and the falling back of the cut-off relay contacts would make
it impossible to ring because the sleeve side of the line would be cut
off. The battery supply through the resistance _11_ is, therefore,
substituted on the sleeve strand of the cord for the battery supply
through the normal connection.
_Busy Test._ The busy test depends on all of the test rings being at
zero potential on an idle line and at a higher potential on a busy line.
Obviously, when the line is not switched, the test rings are at zero
potential on account of a ground through the cut-off relay. When,
however, a plug is inserted in either the answering or multiple jacks,
the test rings will all be raised in potential due to being connected
with the live side of the battery through the sleeve strand of the cord.
Conditions on the line external to the central office cannot make an
idle line test busy because, owing to the presence of the cut-off relay,
the sleeve contacts of all the jacks are disconnected from the line when
it is idle. The test circuit from the tip of the calling plug to ground
at the operator's set passes through the tip strand of the cord, thence
through a pair of normally closed extra contacts on the supervisory
relay _4_, thence in series through all the ringing key springs _10_,
_9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_
on the listening key--closed only when the listening key is
operated--and thence to ground through a retardation coil _14_. No
battery or other source of potential exists in this circuit between
ground and the tip of the calling plug and, therefore, the tip is
normally at ground potential. The sleeve ring of the jack being at
ground potential if the line is idle, no current will flow and no click
will be produced in testing such a line. If, however, the line is busy,
the test ring will be at a higher potential and, therefore, current will
flow from the tip of the calling plug to ground over the path just
traced, and this will cause a rise in potential at the terminal of the
condenser _15_ and a momentary flow of current through the tertiary
winding _16_ of the operator's induction coil; hence the click.
[Illustration: SWITCH ROOM OF CITIZENS' TELEPHONE COMPANY, GRAND RAPIDS,
MICH. One of the Earliest Large Automatic Offices.]
Obviously the testing circuit from the tip of the calling plug to ground
at the operator's set is only useful during the time when the calling
plug is not in a jack, and as the tip strand of the calling plug has to
do double duty in testing and in serving as a part of the talking
circuit, the arrangement is made that the testing circuit will be
automatically broken and the talking circuit through the tip strand
automatically completed when the plug is inserted into a jack in
establishing a connection. This is accomplished by means of the extra
contact on the relay _4_, which relay, it will be remembered, is held
energized when its corresponding plug is inserted in a jack. During the
time when the plug is not inserted, this relay is not energized and the
test circuit is completed through the back contact of its right-hand
armature. When connection is made at the jack, this relay becomes
energized and the tip strand of the cord circuit is made complete by the
right-hand lever being pulled against the front contact of this relay.
The keys shown to the right of the operator's set are order-wire keys.
_Summary of Operation._ We may give a brief summary of the operation of
this system as shown in Fig. 352. The left-hand station calls and the
line relay pulls up, lighting the lamp. The operator inserts an
answering plug in the answering jack, thus energizing the cut-off relay
which operates to cut off the line relay and to complete the connection
between the jacks and the external line. The act of plugging in by the
operator also raises the potential of all the test rings so as to guard
the line against intrusion by other callers. The supervisory lamp _5_
remains unlighted because, although the relay _2_ is operated, the relay
_1_ is also operated, due to the calling subscriber's receiver being off
its hook. The operator throws her listening key, communicates with the
subscriber, and, learning that the right-hand station is wanted,
proceeds to test that line. If the line is idle, she will get no click,
because the tip of her calling plug and the tested ring will be at the
same ground potential. She then plugs in and presses the proper
ringing-key button to send out the proper frequency to ring the
particular subscriber on the line--if there be more than one--the
current from the battery through the coil _11_ and spring _17_ serving
during this operation to hold up the cut-off relay.
As soon as the operator plugs in with the calling plug, the supervisory
lamp _6_ lights, assuming that the called subscriber had not already
removed his receiver from its hook, due to the fact that the relay _4_
is energized and the relay _3_ is not. As soon as the called subscriber
responds, the relay _3_ becomes energized and the supervisory lamp goes
out. If the line called for had been busy by virtue of being plugged at
another section, the tip of the operator's plug in testing would have
found the test ring raised to a potential above the ground, and, as a
consequence, current would have flowed from the tip of this plug through
the back contact of the right-hand lever of relay _4_, thence through
the ringing key springs and the auxiliary listening-key springs to
ground through the retardation coil _14_. This would have produced a
click by causing a momentary flow of current through the tertiary
winding _16_ of the operator's set.
_Wiring of Line Circuit._ The more complete wiring diagram of a single
subscriber's line, Fig. 353, shows the placing in the circuits of the
terminals and jumper wires of the main distributing frame and of the
intermediate distributing frame, and also shows how the pilot lamps and
night-alarm circuits are associated with a group of lines. The main
distributing frame occupies the same relative position in this line
circuit as in the Western Electric, being located in the main line
circuit outside of all the switchboard apparatus. The intermediate
distributing frame occupies a different relative position from that in
the Western Electric line. It will be recalled by reference to Fig. 348
that the line lamp and the answering jack were permanently associated
with the line and cut-off relays, such mutations of arrangement as were
possible at the intermediate distributing frame serving only to vary the
connection between the multiple of a line and one of the various groups
of apparatus consisting of an answering jack and line lamp and
associated relays. In the Kellogg arrangement, Fig. 353, the line and
cut-off relays, instead of being permanently associated with the
answering jack and line lamp, are permanently associated with the
multiple jacks, no changes, of which the intermediate or main frames are
capable, being able to alter the relation between a group of multiple
jacks and its associated line and cut-off relays. In this Kellogg
arrangement the intermediate distributing frame may only alter the
connection of an answering jack and line lamp with the multiple and its
permanently associated relays. The pilot and night alarm arrangements of
Fig. 353 should be obvious from the description already given of other
similar systems.
[Illustration: Fig. 353. Kellogg Two-Wire Line Circuit]
=Dean Multiple Board.= In Fig. 354 are shown the circuits of the
multiple switchboard of the Dean Electric Company. The subscriber's
station equipment shown at Station _A_ and Station _B_ will be
recognized as the Wheatstone-bridge circuit of the Dean Company.
_Line Circuit._ The line circuit is easily understood in view of what
has been said concerning the Western Electric line circuit, the line
relay _1_ being single wound and between the live side of the battery
and the ring side of the line. The cut-off relay _2_ is operated
whenever a plug is inserted in a jack and serves to sever the connection
of the line with the normal line signaling apparatus.
_Cord Circuit._ The cord circuit is of the four-relay type, but employs
three conductors instead of two, as in the two-wire system. The relay
_3_, being in series between the battery and the sleeve contact on the
plug, is energized whenever a plug is inserted in the jack, its winding
being placed in series with the cut-off relay of the line with which the
plug is connected. This completes the circuit through the associated
supervisory lamp unless the relay _4_ is energized, the local lamp
circuit being controlled by the back contact of relay _4_ and the front
contact of relay _3_. It is through the two windings of the relay _4_
that current is fed to the subscriber's station, and, therefore, the
armature of this relay is responsive to the movements of the
subscriber's hook. As the relay _3_ holds the supervisory lamp circuit
closed as long as a plug is inserted in a jack of the line, it follows
that during a connection the relay _4_ will have entire control of the
supervisory lamp.
_Listening Key._ The listening key, as usual, serves to connect the
operator's set across the talking strands of the cord circuit, and the
action of this in connection with the operator's set needs no further
explanation.
_Ringing Keys._ The ringing-key arrangement illustrated is adapted for
use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_
each being controlled by a separate button and serving to select the
particular frequency that is to be sent to line. The two springs _9_ and
_10_ always act to open the cord circuit back of the ringing keys,
whenever any one of the selective buttons is depressed, in order to
prevent interference by ringing current with the other operations of the
circuit.
Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356
is an end view of the entire set. In Fig. 355 the listening key is shown
at the extreme right and the four selective buttons at the left. When a
button is released it rises far enough to cause the disengagement of the
contacts, but remains partially depressed to serve as an indication that
it was last used. The group of springs at the extreme left of Fig. 355
are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils
with which those springs co-operate.
[Illustration: Fig. 354. Dean Multiple Board Circuits]
_Test._ The test in this Dean system is simple, and, like the Western
Electric and Kellogg systems, it depends on the raising of the
potential of the test thimbles of all the line jacks of a line when a
connection is made with that line by a plug at any position. When an
operator makes a test by applying the tip of the calling plug to the
test thimble of a busy line, current passes from the test thimble
through the tip strand of the cord to ground through the left-hand
winding of the calling supervisory relay _4_. The drop of potential
through this winding causes the tip strand of the cord to be raised to a
higher potential than it was before, and as a result the upper plate of
the condenser _11_ is thus altered in potential and this change in
potential across the condenser results in a click in the operator's ear.
[Illustration: Fig. 355. Dean Party Line Ringing Key]
[Illustration: Fig. 356. Dean Party Line Ringing Key]
=Stromberg-Carlson Multiple Board.= _Line Circuit._ In Fig. 357 is shown
the multiple common-battery switchboard circuits employed by the
Stromberg-Carlson Telephone Manufacturing Company. The subscriber's line
circuits shown in this drawing are of the three-wire type and, with the
exception of the subscriber's station, are the same as already described
for the Western Electric Company's system.
_Cord Circuit._ The cord circuit employed is of the two-conductor type,
the plugs being so constructed as to connect the ring and thimble
contacts of the jack when inserted. This cord circuit is somewhat
similar to that employed by the Kellogg Switchboard and Supply Company,
shown in Fig. 352, except that only one battery is employed, and that
certain functions of this circuit are performed mechanically by the
inter-action of the armatures of the relays.
_Supervisory Signals._ When the answering plug is inserted in a jack, in
response to a call, the current passing to the subscriber's station and
also through the cut-off relay must flow through the relay _1_, thus
energizing it. As the calling subscriber's receiver is at this time
removed from the hook switch, the path for current will be completed
through the tip of the jack, thence through the tip of the plug, through
relay _2_ to ground, causing relay _2_ to be operated and to break the
circuit of the answering supervisory lamp. The two relays _1_ and _2_
are so associated mechanically that the armature of _1_ controls the
armature of _2_ in such a manner as to normally hold the circuit of the
answering supervisory lamp open. But, however, when the plug is inserted
in a jack, relay _1_ is operated and allows the operation of relay _2_
to be controlled by the hook switch at the subscriber's station. The
supervisory relay _3_ associated with the calling cord is operated when
the calling plug is placed in a jack, and this relay normally holds the
armature of relay _4_ in an operated position in a similar manner as the
armature of relay _1_ controlled that of relay _2_. Supervisory relay
_4_ is under the control of the hook switch at the called subscriber's
station.
_Test._ In this circuit, as in several previously described, when a plug
is inserted in a jack of a line, the thimble contacts of the jacks
associated with that line are raised to a higher potential than that
which they normally have. The operator in testing a busy line, of course
having previously moved the listening key to the listening position,
closes a path from the test thimble of the jack, through the tip of the
calling plug, through the contacts of the relay _4_, the inside springs
of the listening key, thence through a winding of the induction coil
associated with her set to ground. The circuit thus established allows
current to flow from the test thimble of the jack through the winding of
her induction coil to ground, causing a click in her telephone receiver.
The arrangement of the ringing circuit does not differ materially from
that already described for other systems and, therefore, needs no
further explanation.
[Illustration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]
=Multiple Switchboard Apparatus.= Coming now to a discussion of the
details of apparatus employed in multiple switchboards, it may be
stated that much of the apparatus used in the simpler types is capable
of doing duty in multiple switchboards, although, of course,
modification in detail is often necessary to make the apparatus fit the
particular demands of the system in which it is to be used.
_Jacks._ Probably the most important piece of apparatus in the multiple
switchboard is the jack, its importance being increased by the fact that
such very large numbers of them are sometimes necessary. Switchboards
having hundreds of thousands of jacks are not uncommon. The multiple
jacks are nearly always mounted in strips of twenty and the answering
jacks usually in strips of ten, the length of the jack strip being the
same in each case in the same board and, therefore, giving twice as wide
a spacing in the answering as in the multiple jacks. The distance
between centers in the multiple jacks varies from a quarter of an
inch--which is perhaps the extreme minimum--to half an inch, beyond
which larger limit there seems to be no need of going in any case. It is
customary that the jack strip shall be made of the same total thickness
as the distance between the centers of two of its jacks, and from this
it follows that the strips when piled one upon the other give the same
vertical distance between jack centers as the horizontal distance.
In Fig. 358 is shown a strip of multiple and a strip of answering jacks
of Western Electric make, this being the type employed in the No. 1
standard switchboards for large exchanges. In Fig. 359 are shown the
multiple and answering jacks employed in the No. 10 Western Electric
switchboard. The multiple jacks in the No. 1 switchboard are mounted on
3/8-inch centers, the jacks having three branch terminal contacts. The
multiple jacks of the No. 10 switchboard indicated in Fig. 359 are
mounted on 1/2-inch centers, each jack having five contacts as indicated
by the requirement of the circuits in Fig. 349.
In Fig. 360 are shown the answering and multiple jacks of the Kellogg
Switchboard and Supply Company's two-wire system. The extreme simplicity
of these is particularly well shown in the cut of the answering jack,
and these figures also show clearly the customary method of numbering
jacks. In very large multiple boards it has been the practice of the
Kellogg Company to space the multiple jacks on 3/10-inch centers, and in
their smaller multiple work, they employ the 1/2-inch spacing. With the
3/10-inch spacing that company has been able to build boards having a
capacity of 18,000 lines, that many jacks being placed within the reach
of each operator.
In all modern multiple switchboards the test thimble or sleeve contacts
are drawn up from sheet brass or German silver into tubular form and
inserted in properly spaced borings in strips of hard rubber forming the
faces of the jacks. These strips sometimes are reinforced by brass
strips on their under sides. The springs forming the other terminals of
the jack are mounted in milled slots in another strip of hard rubber
mounted in the rear of and parallel to the front strip and rigidly
attached thereto by a suitable metal framework. In this way desired
rigidity and high insulation between the various parts is secured.
[Illustration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]
_Lamp Jacks._ The lamp jacks employed in multiple work need no further
description in view of what has been said in connection with lamp jacks
for simple common-battery boards. The lamp jack spacing is always the
same as the answering jack spacing, so that the lamps will come in the
same vertical alignment as their corresponding answering jacks when the
lamp strips and answering jack strips are mounted in alternate layers.
[Illustration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]
[Illustration: Fig. 360. Answering and Multiple Jacks for Kellogg
Two-Wire Board]
_Relays._ Next in order of importance in the matter of individual parts
for multiple switchboards is the relay. The necessity for reliability of
action in these is apparent, and this means that they must not only be
well constructed, but that they must be protected from dust and moisture
and must have contact points of such a nature as not to corrode even in
the presence of considerable sparking and of the most adverse
atmospheric conditions. Economy of space is also a factor and has led to
the almost universal adoption of the single-magnet type of relay for
line and cut-off as well as supervisory purposes.
[Illustration: Fig. 361. Type of Line Relay]
[Illustration: Fig. 362. Type of Cut-Off Relay]
The Western Electric Company employs different types of relays for line,
cut-off, and supervisory purposes. This is contrary to the practice of
most of the other companies who make the same general type of relay
serve for all of these purposes. A good idea of the type of Western
Electric line relay, as employed in its No. 1 board, may be had from
Fig. 361. As is seen this is of the tilting armature type, the armature
rocking back and forth on a knife-edge contact at its base, the part on
which it rests being of iron and of such form as to practically
complete, with the armature and core, the magnetic circuit. The cut-off
relay, Fig. 362, is of an entirely different type. The armature in this
is loosely suspended by means of a flexible spring underneath two
L-shaped polar extensions, one extending up from the rear end of the
core and the other from the front end. When energized this armature is
pulled away from the core by these L-shaped pieces and imparts its
motion through a hard-rubber pin to the upper pair of springs so as to
effect the necessary changes in the circuit.
[Illustration: Fig. 363. Western Electric Combined Line and Cut-off
Relay]
[Illustration: Fig. 364. Western Electric Supervisory Relay]
[Illustration: Fig. 365. Line Relay No. 10 Board]
Much economy in space and in wiring is secured in the type of
switchboards employing cut-off as well as line relays by mounting the
two relays together and in making of them, in fact, a unitary piece of
apparatus. Since the line relay is always associated with the cut-off
relay of the same line and with no other, it is obvious that this
unitary arrangement effects a great saving in wiring and also secures a
great advantage in the matter of convenience of inspection. Such a
combined cut-off and line relay, employed in the Western Electric No. 1
relay board, is shown in Fig. 363. These are mounted in banks of ten
pairs, a common dust cap of sheet iron covering the entire group.
The Western Electric supervisory relay, Fig. 364, is of the tilting
armature type and is copper clad. The dust cap in this case fits on with
a bayonet joint as clearly indicated. In Fig. 365 is shown the line
relay employed in the Western Electric No. 10 board.
[Illustration: Fig. 366. Kellogg Line and Cut-off Relays]
[Illustration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]
The Kellogg Company employs the type of relay of which the magnetic
circuit was illustrated in Fig. 95. In its multiple boards it commonly
mounts the line and cut-off relays together, as shown in Fig. 366. A
single, soft iron shell is used to cover both of these, thus serving as
a dust shield and also as a magnetic shield to prevent cross-talk
between adjacent relays--an important feature, since it will be
remembered the cut-off relays are left permanently connected with the
talking circuit. Fig. 367, which shows a strip of twenty such pairs of
relays, from five of which the covers have been removed, is an excellent
detail view of the general practice in this respect; obviously, a very
large number of such relays may be mounted in a comparatively small
space. The mounting strip shown in this cut is of heavy rolled iron and
is provided with openings through which the connection terminals--shown
more clearly in Fig. 366--project. On the back of this mounting strip
all the wiring is done and much of this wiring--that connecting adjacent
terminals on the back of the relay strip--is made by means of thin
copper wires without insulation, the wires being so short as to support
themselves without danger of crossing with other wires. When these wires
are adjacent to ground or battery wires they may be protected by
sleeving, so as to prevent crosses.
[Illustration: Fig. 368. Monarch Relay]
An interesting feature in relay construction is found in the relay of
the Monarch Telephone Manufacturing Company shown in Figs. 368 and 369.
The assembled relay and its mounting strip and cap are shown in Fig.
368. This relay is so constructed that by the lifting of a single latch
not only the armature but the coil may be bodily removed, as shown in
Fig. 369, in which the latch is shown in its raised position. As seen,
the armature has an L-shaped projection which serves to operate the
contact springs lying on the iron plate above the coil. The simplicity
of this device is attractive, and it is of convenience not only from the
standpoint of easy repairs but also from the standpoint of factory
assembly, since by manufacturing standard coils with different
characters of windings and standard groups of springs, it is possible to
produce without special manufacture almost any combination of relay.
[Illustration: Fig. 369. Monarch Relay]
=Assembly.= The arrangement of the key and jack equipment in complete
multiple switchboard sections is clearly shown in Fig. 370, which shows
a single three-position section of one of the small multiple
switchboards of the Kellogg Switchboard and Supply Company. The
arrangement of keys and plugs on the key shelf is substantially the same
as in simple common-battery boards. As in the simple switchboards the
supervisory lamps are usually mounted on the hinged key shelf
immediately in the rear of the listening and ringing keys and with such
spacing as to lie immediately in front of the plugs to which they
correspond. The reason for mounting the supervisory lamps on the key
shelf is to make them easy of access in case of the necessity of lamp
renewals or repairs on the wiring. The space at the bottom of the
vertical panels, containing the jacks, is left blank, as this space is
obstructed by the standing plugs in front of it. Above the plugs,
however, are seen the alternate strips of line lamps and answering
jacks, the lamps in each case being directly below the corresponding
answering jacks. Above the line lamps and answering jacks in the two
positions at the right there are blank strips into which additional line
lamps and jacks may be placed in case the future needs of the system
demand it. The space above these is the multiple jack space, and it is
evident from the small number of multiple jacks in this little
switchboard that the present equipment of the board is small. It is also
evident from the amount of blank space left for future installations of
multiple jacks that a considerable growth is expected. Thus, while there
are but four banks of 100 multiple jacks, or 400 in all, there is room
in the multiple for 300 banks of 100 multiple jacks, or 3,000 in all.
The method of grouping the jacks in banks of 100 and of providing for
their future growth is clearly indicated in this figure. The next
section at the right of the one shown would contain a duplicate set of
multiple jacks and also an additional equipment of answering jacks and
lamps.
[Illustration: A MULTIPLE MANUAL SWITCHING BOARD FOR TOLL CONNECTIONS IN
AN AUTOMATIC SYSTEM Multiple Jacks are Provided for Each Line through
Which Toll Connections are Handled Directly.]
[Illustration: Fig. 370. Small Multiple Board Section]
For ordinary local service no operator would sit at the left-hand
position of the section shown, that being the end position, since the
operator there would not be able easily to reach the extreme right-hand
portion of the third position and would have nothing to reach at her
left. This end position in this particular board illustrated is provided
with toll-line equipment, a practice not uncommon in small multiple
boards. To prevent confusion let us assume that the multiple jack space
contains its full equipment of 3,000 jacks on each section. The
operator in the center position of the section shown could easily reach
any one of the jacks on that section. The operator at the third position
could reach any jack on the second and third position of her section,
but could not well reach multiple jacks in the first position. She
would, however, have a duplicate of the multiple jacks in this first
position in the section at her right, _i. e._, in the fourth position,
and it makes no difference on what portion of the switchboard she plugs
into the multiple so long as she plugs into a jack of the right line.
CHAPTER XXVII
TRUNKING IN MULTI-OFFICE SYSTEMS
It has been stated that a single exchange may involve a number of
offices, in which case it is termed a multi-office exchange. In a
multi-office exchange, switchboards are necessary at each office in
which the subscribers' lines of the corresponding office district
terminate. Means for intercommunication between the subscribers in one
office and those in any other office are afforded by inter-office trunks
extended between each office and each of the other offices.
If the character of the community is such that each of the offices has
so few lines as to make the simple switchboard suffice for its local
connections, then the trunking between the offices may be carried out in
exactly the same way as explained between the various simple
switchboards in a transfer system, the only difference being that the
trunks are long enough to reach from one office to another instead of
being short and entirely local to a single office. Such a condition of
affairs would only be found in cases where several small communities
were grouped closely enough together to make them operate as a single
exchange district, and that is rather unusual.
The subject of inter-office trunking so far as manual switchboards are
concerned is, therefore, confined mainly to trunking between a number of
offices each equipped with a manual multiple switchboard.
=Necessity for Multi-Office Exchanges.= Before taking up the details of
the methods and circuits employed in trunking in multi-office systems,
it may be well to discuss briefly why the multi-office exchange is a
necessity, and why it would not be just as well to serve all of the
subscribers in a large city from a single huge switchboard in which all
of the subscribers' lines would terminate. It cannot be denied, when
other things are equal, that it is better to have only one operator
involved in any connection which means less labor and less liability of
error.
The reasons, however, why this is not feasible in really large
exchanges are several. The main one is that of the larger investment
required. Considering the investment first from the standpoint of the
subscriber's line, it is quite clear that the average length of
subscriber's line will be very much greater in a given community if all
of the lines are run to a single office, than will be the case if the
exchange district is divided into smaller office districts and the lines
run merely from the subscribers to the nearest office. There is a direct
and very large gain in this respect, in the multi-office system over the
single office system in large cities, but this is not a net gain, since
there is an offsetting investment necessary in the trunk lines between
the offices, which of course are separate from the subscribers' lines.
Approaching the matter from the standpoint of switchboard construction
and operation, another strong reason becomes apparent for the employment
of more than one office in large exchange districts. Both the
difficulties of operation and the expense of construction and
maintenance increase very rapidly when switchboards grow beyond a
certain rather well-defined limit. Obviously, the limitation of the
multiple switchboard as to size involves the number of multiple jacks
that it is feasible to place on a section. Multiple switchboards have
been constructed in this country in which the sections had a capacity of
18,000 jacks. Schemes have been proposed and put into effect with
varying success, for doubling and quadrupling the capacity of multiple
switchboards, one of these being the so-called divided multiple board
devised by the late Milo G. Kellogg, and once used in Cleveland, Ohio,
and St. Louis, Missouri. Each of these boards had an ultimate capacity
of 24,000 lines, and each has been replaced by a "straight" multiple
board of smaller capacity. In general, the present practice in America
does not sanction the building of multiple boards of more than about
10,000 lines capacity, and as an example of this it may be cited that
the largest standard section manufactured for the Bell companies has an
ultimate capacity of 9,600 lines.
European engineers have shown a tendency towards the opposite practice,
and an example of the extreme in this case is the multiple switchboard
manufactured by the Ericsson Company, and installed in Stockholm, in
which the jacks have been reduced to such small dimensions as to permit
an ultimate capacity of 60,000 lines.
The reasons governing the decision of American engineers in
establishing the practice of employing no multiple switchboards of
greater capacity than about 10,000 lines, briefly outlined, are as
follows: The building of switchboards with larger capacity, while
perfectly possible, makes necessary either a very small jack or some
added complexity, such as that of the divided multiple switchboard,
either of which is considered objectionable. Extremely small jacks and
large multiples introduce difficulties as to the durability of the jacks
and the plugs, and also they tend to slow down the work of operators and
to introduce errors. They also introduce the necessity of a smaller
gauge of wire through the multiple than it has been found desirable to
employ. Considered from the standpoint of expense, it is evident that as
a multiple switchboard increases in number of lines, its size increases
in two dimensions, _i. e._, in length of board and height of section,
and this element of expense, therefore, is a function of the square of
the number of lines.
The matter of insurance, both with respect to the risk as to property
loss and the risk as to breakdown of the service, also points distinctly
in the direction of a plurality of offices rather than one. Both from
the standpoint of risk against fire and other hazards, which might
damage the physical property, and of risk against interruption to
service due to a breakdown of the switchboard itself, or a failure of
its sources of current, or an accident to the cable approaches, the
single office practice is like putting all one's eggs in one basket.
Another factor that has contributed to the adoption of smaller
switchboard capacities is the fact that in the very large cities even a
40,000 line multiple switchboard would still not remove the necessity of
multi-office exchanges with the consequent certainty that a large
proportion of the calls would have to be trunked anyway.
Undoubtedly, one of the reasons for the difference between American and
European practice is the better results that American operating
companies have been able to secure in the handling of calls at the
incoming end of trunks. This is due, no doubt, in part to the
differences in social and economic conditions under which exchanges are
operated in this country and abroad, and also in part to the
characteristics of the English tongue when compared to some of the other
tongues in the matter of ease with which numbers may be spoken. In
America it has been found possible to so perfect the operation of
trunking under proper operating conditions and with good equipment as to
relieve multi-office practice of many of the disadvantages which have
been urged against it.
=Classification.= Broadly speaking there are two general methods that
may be employed in trunking between exchanges. The first and simplest of
these methods is to employ so-called _two-way trunks_. These, as their
name indicates, may be used for completing connections between offices
in either direction, that is, whether the call originates at one end or
the other. The other way is by the use of _one-way trunks_, wherein each
trunk carries traffic in one direction only. Where such is the case, one
end of the trunk is always used for connecting with the calling
subscriber's line and is termed the _outgoing_ end, and the other end is
always used in completing the connection with the called subscriber's
line, and is referred to as the _incoming_ end. Traffic in the other
direction is handled by another set of trunks differing from the first
set only in that their outgoing and incoming ends are reversed.
As has already been pointed out, a system of trunks employing two-way
trunks is called a _single-track system_, and a system involving two
sets of one-way trunks is called a _double-track system_. It is to be
noted that the terms outgoing and incoming, as applied to the ends of
trunks and also as applied to traffic, always refer to the direction in
which the trunk handles traffic or the direction in which the traffic is
flowing with respect to the particular office under consideration at the
time. Thus an _incoming trunk_ at one office is an _outgoing trunk_ at
the other.
_Two-Way Trunks._ Two-way trunks are nearly always employed where the
traffic is very small and they are nearly always operated by having the
_A_-operator plug directly into the jack at her end of the trunk and
displaying a signal at the other end by ringing over the trunk as she
would over an ordinary subscriber's line. The operator at the distant
exchange answers as she would on an ordinary line, by plugging into the
jack of that trunk, and receives her orders over the trunk either from
the originating operator or from the subscriber, and then completes the
connection with the called subscriber. Such trunks are often referred to
as "ring-down" trunks, and their equipment consists in a drop and jack
at each end. In case there is a multiple board at either or both of the
offices, then the equipment at each end of the trunk would consist of a
drop and answering jack, together with the full quota of multiple jacks.
It is readily seen that this mode of operation is slow, as the work that
each operator has to do is the same as that in connecting two local
subscribers, plus the time that it takes for the operators to
communicate with each other over the trunk.
_One-Way Trunks._ Where one-way trunks are employed in the double-track
system, the trunks, assuming that they connect multiple boards, are
provided with multiple jacks only at their outgoing ends, so that any
operator may reach them for an outgoing connection, and at their
incoming ends they terminate each in a single plug and in suitable
signals and ringing keys, the purpose of which will be explained later.
Over such trunks there is no verbal communication between the operators,
the instructions passing between the operators over separate order-wire
circuits. This is done in order that the trunk may be available as much
as possible for actual conversation between the subscribers. It may be
stated at this point that the duration of the period from the time when
a trunk is appropriated by the operators for the making of a certain
connection until the time when the trunk is finally released and made
available for another connection is called the _holding time_, and this
holding time includes not only the period while the subscribers are in
actual conversation over it, but also the periods while the operators
are making the connection and afterwards while they are taking it down.
It may be said, therefore, that the purpose of employing separate order
wires for communication between the operators is to make the holding
time on the trunks as small as possible and, therefore, for the purpose
of enabling a given trunk to take part in as many connections in a given
time as possible.
In outline the operation of a one-way trunk between common-battery,
manual, multiple switchboards is, with modifications that will be
pointed out afterwards, as follows: When a subscriber's line signal is
displayed at one office, the operator in attendance at that position
answers and finding that the call is for a subscriber in another office,
she presses an order-wire key and thereby connects her telephone set
directly with that of a _B_-operator at the proper other office. Unless
she finds that other operators are talking over the order wire, she
merely states the number of the called subscriber, and the _B_-operator
whose telephone set is permanently connected with that order wire merely
repeats the number of the called subscriber and follows this by
designating the number of the trunk which the _A_-operator is to employ
in making the connection. The _A_-operator, thereupon, immediately and
without testing, inserts the calling plug of the pair used in answering
the call into the trunk jack designated by the _B_-operator; the
_B_-operator simultaneously tests the multiple jack of the called
subscriber and, if she finds it not busy, inserts the plug of the
designated trunk into the multiple jack of the called subscriber and
rings his bell by pressing the ringing key associated with the trunk
cord used. The work on the part of the _A_-operator in connecting with
the outgoing end of the trunk and on the part of the _B_-operator in
connecting the incoming end of the trunk with the line goes on
simultaneously, and it makes no difference which of these operators
completes the connection first.
It is the common practice of the Bell operating companies in this
country to employ what is called automatic or machine ringing in
connection with the _B_-operator's work. When the _B_-operator presses
the ringing key associated with the incoming trunk cord, she pays no
further attention to it, and she has no supervisory lamp to inform her
as to whether or not the subscriber has answered. The ringing key is
held down, after its depression by the operator, either by an
electromagnet or by a magnet-controlled latch, and the ringing of the
subscriber's bell continues at periodic intervals as controlled by the
ringing commutator associated with the ringing machine. When the
subscriber answers, however, the closure of his line circuit results in
such an operation of the magnet associated with the ringing key as to
release the ringing key and thus to automatically discontinue the
ringing current.
When a connection is established between two subscribers through such a
trunk the supervision of the connection falls entirely upon the
_A_-operator who established it. This means that the calling supervisory
lamp at the _A_-operator's position is controlled over the trunk from
the station of the called subscriber, the answering supervisory lamp
being, of course, under the control of the calling subscriber as in the
case of a local connection. It is, therefore, the _A_-operator who
always initiates the taking down of a trunk connection, and when, in
response to the lighting of the two lamps, she withdraws her calling
plug from the trunk jack, the supervisory lamp associated with the
incoming end of the trunk at the other office is lighted, and the
_B_-operator obeys it by pulling down the plug.
If, upon testing the multiple jack of the called subscriber's line, the
_B_-operator finds the line to be busy, she at once inserts the trunk
plug into a so-called "busy-back" jack, which is merely a jack whose
terminals are permanently connected to a circuit that is intermittently
opened and closed, and which also has impressed upon it an alternating
current of such a nature as to produce the familiar "buzz-buzz" in a
telephone receiver. The opening and closing of this circuit causes the
calling supervisory lamp of the _A_-operator to flash at periodic
intervals just as if the called subscriber had raised and lowered his
receiver, but more regularly. This is the indication to the _A_-operator
that the line called for is busy. The buzzing sound is repeated back
through the cord circuit of the _A_-operator to the calling subscriber
and is a notification to him that the line is busy.
Sometimes, as is practiced in New York City, for instance, the buzzing
feature is omitted, and the only indication that the calling subscriber
receives that the called-for line is busy is being told so by the
_A_-operator. This may be considered a special feature and it is
employed in New York because there the custom exists of telling a
calling subscriber, when the line he has called for has been found busy,
that the party will be secured for him and that he, the calling
subscriber, will be called, if he desires.
A modification of this busy-back feature that has been employed in
Boston, and perhaps in other places, is to associate with the busy-back
jack at the _B_-operator's position a phonograph which, like a parrot,
keeps repeating "Line busy--please call again." Where this is done the
calling subscriber, _if he understands what the phonograph says_, is
supposed to hang up his receiver, at which time the _A_-operator takes
down the connection and the _B_-operator follows in response to the
notification of her supervisory lamp. The phonograph busy-back scheme,
while ingenious, has not been a success and has generally been
abandoned.
As a rule the independent operating companies in this country have not
employed automatic ringing, and in this case the _B_-operators have
been required to operate their ringing keys and to watch for the
response of the called subscriber. In order to arrange for this, another
supervisory lamp, termed the _ringing lamp_, is associated with each
incoming trunk plug, the going out of this lamp being a notification to
the _B_-operator to discontinue ringing.
=Western Electric Trunk Circuits.= The principles involved in
inter-office trunking with automatic ringing, are well illustrated in
the trunk circuit employed by the Western Electric Company in connection
with its No. 1 relay boards. The dotted dividing line through the center
of Fig. 371 represents the separating space between two offices. The
calling subscriber's line in the first office is shown at the extreme
left and the called subscriber's line in the second office is shown at
the extreme right. Both of these lines are standard multiple switchboard
lines of the form already discussed. The equipment illustrated in the
first office is that of an _A_-board, the cord circuit shown being that
of the regular _A_-operator. The outgoing trunk jacks connecting with
the trunk leading to the other office are, it will be understood,
multipled through the _A_-sections of the board and contain no relay
equipment, but the test rings are connected to ground through a
resistance coil _1_, which takes the place of the cut-off relay winding
of a regular line so far as test conditions and supervisory relay
operation are concerned. The equipment illustrated in the second office
is that of a _B_-board, it being understood that the called subscriber's
line is multipled through both the _A_- and _B_-boards at that office.
The part of the equipment that is at this point unfamiliar to the reader
is, therefore, the cord circuit at the _B_-operator's board. This
includes, broadly speaking, the means: (1) for furnishing battery
current to the called subscriber; (2) for accomplishing the ringing of
the called subscriber and for automatically stopping the ringing when he
shall respond; (3) for performing the ordinary switching functions in
connection with the relays of the called subscriber's line in just the
same way that an _A_-operator's cord carries out these functions; and
(4) for causing the operation of the calling supervisory relay of the
_A_-operator's cord circuit in just the same manner, under control of
the connected called subscriber, as if that subscriber's line had been
connected directly to the _A_-operator's cord circuit.
[Illustration: Fig. 371. Inter-Office Connection--Western Electric
System]
The operation of these devices in the _B_-operator's cord circuit may
be best understood by following the establishment of the connection.
Assuming that the calling subscriber in the first office desires a
connection with the subscriber's line shown in the second office, and
that the _A_-operator at the first office has answered the call, she
will then communicate by order wire with the _B_-operator at the second
office, stating the number of the called subscriber and receiving from
that operator in return the number of the trunk to be employed. The two
operators will then proceed simultaneously to establish the connection,
the _A_-operator inserting the calling plug into the outgoing trunk
jack, and the _B_-operator inserting the trunk plug into the multiple
jack of the called subscriber's line after testing. We will assume at
first that the called subscriber's line is found idle and that both of
the operators complete their respective portions of the work at the same
time and we will consider first the condition of the calling supervisory
relay at the _A_-operator's position.
The circuit of the calling supervisory lamp will have been closed
through the resistance coil _1_ connected with the outgoing trunk jacks
and the lamp will be lighted because, as will be shown, it is not yet
shunted out by the operation of its associated supervisory relay.
Tracing the circuit of the calling supervisory relay of the
_A_-operator's circuit, it will be found to pass from the live side of
the battery to the ring side of the trunk circuit through one winding of
the repeating coil of the _B_-operator's cord; beyond this the circuit
is open, since no path exists through the condenser _2_ bridged across
the trunk circuit or through the normally open contacts of the relay _3_
connected in the talking circuit of the trunk. The association of this
relay _3_ with the repeating coil and the battery of the trunk is seen
to be just the same as that of a supervisory relay in the _A_-operator's
cord, and it is clear, therefore, that this relay _3_ will not be
energized until the called subscriber has responded. When it is
energized it will complete the path to ground through the _A_-operator's
calling supervisory relay and operate to shunt out the _A_-operator's
calling supervisory lamp in just the same manner as if the
_A_-operator's calling plug had been connected directly with the line of
the calling subscriber. In other words, the called subscriber in the
second office controls the relay _3_, which, in turn, controls the
calling supervisory relay of the _A_-operator, which, in turn, shunts
out its lamp.
The connection being completed between the two subscribers, the
_B_-operator depresses one or the other of the ringing keys _5_ or _6_,
according to which party on the line is called, assuming that it is a
two-party line. It will be noticed that the springs of these ringing
keys are not serially arranged in the talking circuit, but the cutting
off of the trunk circuit back of the ringing keys is accomplished by the
set of springs shown just at the left of the ringing keys, which set of
springs _7_ is operated whenever either one of the ringing keys is
depressed. An auxiliary pair of contacts, shown just below the group of
springs _7_, is also operated mechanically whenever either one of the
ringing keys is depressed, and this serves to close one of two normally
open points in the circuit of the ringing-key holding magnet _8_. This
holding magnet _8_ is so arranged with respect to the contacts of the
ringing key that whenever any one of them is depressed by the operator,
it will be held depressed as long as the magnet is energized just the
same as if the operator kept her finger on the key. The other normally
open point in the circuit of the holding magnet _8_ is at the lower pair
of contacts of the test and holding relay _9_. This relay is operated
whenever the trunk plug is inserted in the jack of a called line,
regardless of the position of the subscriber's equipment on that line.
The circuit may be traced from the live side of the battery through the
trunk disconnect lamp _4_, coil _9_, sleeve strand of cord, and to
ground through the cut-off relay of the line. The insertion of the trunk
plug into the jack thus leaves the completion of the holding-magnet
circuit dependent only upon the auxiliary contact on the ringing key,
and, therefore, as soon as the operator presses either one of these
keys, the clutch magnet is energized and the key is held down, so that
ringing current continues to flow at regular intervals to the called
subscriber's station.
The ringing current issues from the generator _10_, but the supply
circuit from it is periodically interrupted by the commutator _11_
geared to the ringing-machine shaft. This periodically interrupted
ringing current passes to the ringing-key contacts through the coil of
the ringing cut-off relay _12_, and thence to the subscriber's line. The
ringing current is, however, insufficient to cause the operation of this
relay _12_ as long as the high resistance and impedance of the
subscriber's bell and condenser are in the circuit. It is, however,
sufficiently sensitive to be operated by this ringing current when the
subscriber responds and thus substitutes the comparatively low
resistance and impedance path of his talking apparatus for the previous
path through his bell. The pulling up of the ringing cut-off relay _12_
breaks a third normally closed contact in the circuit of the holding
coil _8_, de-energizing that coil and releasing the ringing key, thus
cutting off ringing current. There is a third brush on the commutator
_11_ connected with the live side of the central battery, and this is
merely for the purpose of assuring the energizing of the ringing cut-off
relay _12_, should the subscriber respond during the interval while the
commutator _11_ held the ringing current cut off. The relay _12_ may
thus be energized either from the battery, if the subscriber responds
during a period of silence of his ringer, or from the generator _10_, if
the subscriber responds during a period while his bell is sounding; in
either case the ringing current will be promptly cut off by the release
of the ringing key.
The trunk operator's "disconnect lamp" is shown at _4_, and it is to be
remembered that this lamp is lighted only when the _A_-operator takes
down the connection at her end, and also that this lamp is entirely out
of the control of the subscribers, the conditions which determine its
illumination being dependent on the positions of the operators' plugs at
the two ends of the trunk. With both plugs up, the lamp _4_ will receive
current, but will be shunted to prevent its illumination. The path over
which it receives this current may be traced from battery through the
lamp _4_, thence through the coil of the relay _9_ and the cut-off relay
of the called subscriber's line. This current would be sufficient to
illuminate the lamp, but the lamp is shunted by a circuit which may be
traced from the live side of battery through the contact of the relay
_13_, closed at the time, and through the coil of the trunk cut-off
relay coil _14_. The resistance of this coil is so proportioned to the
other parts of the circuit as to prevent the illumination of the lamp
just exactly as in the case of the shunting resistances of the lamps in
the _A_-operator's cord. It will be seen, therefore, that the supply of
current to the trunk disconnect lamp is dependent on the trunk plug
being inserted into the jack of the subscriber's line and that the
shunting out of this lamp is dependent on the energization of the relay
_13_. This relay _13_ is energized as long as the _A_-operator's plug is
inserted into the outgoing trunk jack, the path of the energizing
circuit being traced from the live side of the battery at the second
office through the right-hand winding of this relay, thence over the tip
side of the trunk to ground at the first office. From this it follows
that as long as both plugs are up, the disconnect lamp will receive
current but will be shunted out, and as soon as the _A_-operator pulls
down the connection, the relay _13_ will be de-energized and will thus
remove the shunt from about the lamp, allowing its illumination. The
left-hand winding of the relay _13_ performs no operating function, but
is merely to maintain the balance of the talking circuit, it being
bridged during the connection from the ring side of the trunk to ground
in order to balance the bridge connection of the right-hand coil from
the live side of battery to the tip side of the trunk circuit.
The relay _14_, already referred to as forming a shunt for the trunk
disconnect lamp, has for its function the keeping of the talking circuit
through the trunk open until such time as the relay _13_ operates, this
being purely an insurance against unnecessary ringing of a subscriber in
case the _A_-operator should by mistake plug into the wrong trunk. It is
not, therefore, until the _A_-operator has plugged into the trunk and
the relay _13_ has been operated to cause the energization of the relay
_14_ that the ringing of the called subscriber can occur, regardless of
what the _B_-operator may have done.
The relay _9_ has an additional function to that of helping to control
the circuit of the ringing-key holding magnet. This is the holding of
the test circuit complete until the operator has tested and made a
connection and then automatically opening it. The test circuit of the
_B_-operator's trunk may be traced, at the time of testing, from the
thimble of the multiple jack under test, through the tip of the cord,
thence through the uppermost pair of contacts of the relay _9_ to ground
through a winding of the _B_-operator's induction coil. After the test
has been made and the plug inserted, the relay _9_, which is operated by
the insertion of the plug, acts to open this test circuit and at the
same time complete the tip side of the cord circuit.
In the upper portion of Fig. 371 the order-wire connections, by which
the _A_-operator and the _B_-operator communicate, are indicated. It
must be remembered in connection with these that the _A_-operator only
has control of this connection, the _B_-operator being compelled
necessarily to hear whatever the _A_-operators have to say when the
_A_-operators come in on the circuit.
[Illustration: Fig. 372. Incoming Trunk Circuit]
The incoming trunk circuit employed by the Western Electric Company for
four-party line ringing is shown in Fig. 372, it being necessarily
somewhat modified from that shown in Fig. 371, which is adapted for
two-party line ringing only. In addition to the provision of the
four-party line ringing keys, by which positive or negative pulsating
current is received over either limb of the line, and to the provision
of the regular alternating current ringing key for ringing on single
party lines, it is necessary in the ringing cut-off relay to provide for
keeping the alternating and the pulsating ringing currents entirely
separate. For this reason, the ringing cut-off relay _12_ is provided
with two windings, that at the right being in the path of the
alternating ringing currents that are supplied to the alternating
current key, and that at the left being in the ground return path for
all of the pulsating ringing currents supplied to the pulsating keys.
With this explanation it is believed that this circuit will be
understood from what has been said in connection with Fig. 371. The
operation of the holding coil _8_ is the same in each case, the holding
magnet in Fig. 372 serving to hold depressed any one of the five ringing
keys that may have been used in calling the subscriber.
[Illustration: AUTOMATIC EQUIPMENT, MAIN OFFICE, BERKELEY, CALIFORNIA A
Feature of Interest Here is That the Cement Floor is Treated with a
Filler and Painted, with No Other Covering.]
[Illustration: Fig. 373. Western Electric Trunk Ringing Key]
The standard four-party line, trunk ringing key of the Western Electric
Company is shown in Fig. 373. In this the various keys operate not by
pressure but rather by being pulled by the finger of the operator in
such a way as to subject the key shaft to a twisting movement. The
holding magnet lies on the side opposite to that shown in the figure and
extends along the full length of the set of keys, each key shaft being
provided with an armature which is held by this magnet until the magnet
is de-energized by the action of the ringing cut-off relay.
[Illustration: Fig. 374. Trunk Relay]
[Illustration: Fig. 375. Trunk Relay]
The standard trunk relays employed by the Western Electric Company in
connection with the circuits just described are shown in Figs. 374 and
375. In each case the dust-cap or shield is also shown. The relay of
Fig. 374 is similar to the regular cut-off relay and is the one used for
relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is
somewhat similar to the subscriber's line relay in that it has a tilting
armature, and is the one used at _13_ in Figs. 371 and 372. The trunk
relay _3_ in Figs. 371 and 372 is the same as the _A_-operator's
supervisory relays already discussed.
It has been stated that under certain circumstances _B_-operator's trunk
circuits devoid of ringing keys, and consequently of all keys, may be
employed. This, so far as the practice of the Bell companies is
concerned, is true only in offices where there are no party lines, or
where, as in many of the Chicago offices, the party lines are worked on
the "jack per station" basis. In "jack per station" working, the
selection of the station on a party line is determined by the jack on
which the plug is put, rather than by a ringing key, and hence the
keyless trunk may be employed.
[Illustration: Fig. 376. Keyless Trunk]
A keyless trunk as used in New York is shown in Fig. 376. This has no
manually operated keys whatever, and the relay _17_, when it is
operated, establishes connection between the ringing generator and the
conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in
a manner identical with those bearing corresponding numbers in Fig.
371. As soon as the trunk operator plugs into the multiple jack of the
called subscriber, the relay _16_ will operate for the same reason that
the relay _9_ operated in connection with Fig. 371. The trunk disconnect
lamp will receive current, but if the operator has already established
connection with the other end of the trunk, this lamp will not be
lighted because shunted by the relay _17_, due to the pulling up of the
armature of the relay _13_. The relay _15_ plays no part in the
operation so far described, because of the fact that its winding is
short-circuited by its own contacts and those of relay _12_, when the
latter is not energized. As a result of the operation of the relay _17_,
ringing current is sent to line, the supply circuit including the coil
of the relay _12_. As soon as the subscriber responds to this ringing
current, the armature of the relay _12_ is pulled up, thus breaking the
shunt about the relay _15_, which, therefore, starts to operate in
series with the relay _17_, but as its armatures assume their attracted
position, the relay _17_ is cut out of the circuit, the coil of the
relay _15_ being substituted for that of the relay _17_ in the shunt
path around the lamp _4_. The relay _17_ falls back and cuts off the
ringing current. The relay _15_ now occupies the place with respect to
the shunt around the lamp _4_ that the relay _17_ formerly did, the
continuity of this shunt being determined by the energization of the
relay _13_. When the _A_-operator at the distant exchange withdraws the
calling plug from the trunk jack, this relay _13_ becomes de-energized,
breaking the shunt about the lamp _4_ and permitting the display of that
lamp as a signal to the operator to take down the connection. It may be
asked why the falling back of relay _15_ will not again energize relay
_17_ and thus cause a false ring on the called subscriber. This will not
occur because both the relays _15_ and _17_ depend for their
energization on the closure of the contacts of the relay _13_, and when
this falls back the relay _17_ cannot again be energized even though the
relay _15_ assumes its normal position.
=Kellogg Trunk Circuits.= The provision for proper working of trunk
circuits in connection with the two-wire multiple switchboards is not an
altogether easy matter, owing particularly to the smaller number of
wires available in the plug circuits. It has been worked out in a highly
ingenious way, however, by the Kellogg Company, and a diagram of their
incoming trunk circuit, together with the associated circuits involved
in an inter-office connection, is shown in Fig. 377.
[Illustration: Fig. 377. Inter-Office Connection--Kellogg System]
This figure illustrates a connection from a regular two-wire multiple
subscriber's line in one office, through an _A_-operator's cord circuit
there, to the outgoing trunk jacks at that office, thence through the
incoming trunk circuit at the other office to the regular two-wire
multiple subscriber's line at that second office. The portion of this
diagram to be particularly considered is that of the _B_-operator's cord
circuit. The trunk circuit terminates in the multipled outgoing trunk
jacks at the first office, the trunk extending between offices
consisting, of course, of but two wires. We will first consider the
control of the calling supervisory lamp in the _A_-operator's cord
circuit, it being remembered that this control must be from the called
subscriber's station. It will be noticed that the left-hand armature of
the relay _1_ serves normally to bridge the winding of relay _2_ across
the cord circuit around the condenser _3_. When, however, the relay _1_
pulls up, the coil of relay _4_ is substituted in this bridge connection
across the trunk. The relay _2_ has a very high resistance
winding--about 15,000 ohms--and this resistance is so great that the tip
supervisory relay of the _A_-operator's cord will not pull up through
it. As a result, when this relay is bridged across the trunk circuit,
the tip relay on the calling side of the _A_-operator's cord circuit is
de-energized, just as if the trunk circuit were open, and this results
in the lighting of the _A_-operator's calling supervisory lamp. The
winding of the relay _4_, however, is of low resistance--about 50
ohms--and when this is substituted for the high-resistance winding of
the relay _2_, the tip relay on the calling side of the _A_-operator's
cord is energized, resulting in the extinguishing of the calling
supervisory lamp. The illumination of the _A_-operator's calling
supervisory lamp depends, therefore, on whether the high-resistance
relay _2_, or the low-resistance relay _4_, is bridged across the trunk,
and this depends on whether the relay _1_ is energized or not. The relay
_1_, being bridged from the tip side of the trunk circuit to ground and
serving as the means of supply of battery current to the called
subscriber, is operated whenever the called subscriber's receiver is
removed from its hook. Therefore, the called subscriber's hook controls
the operation of this relay _1_, which, in turn, controls the conditions
which cause the illumination or darkness of the calling supervisory lamp
at the distant office.
Assuming that the _A_-operator has received and answered a call, and has
communicated with the _B_-operator, telling her the number of the
called subscriber, and has received, in turn, the number of the trunk to
be used, and that both operators have put up the connection, then it
will be clear from what has been said that the calling supervisory lamp
of the _A_-operator will be lighted until the called subscriber removes
his receiver from its hook, because the tip relay in the _A_-operator's
cord circuit will not pull up through the 15,000-ohm resistance winding
of the relay _2_. As soon as the subscriber responds, however, the relay
_1_ will be operated by the current which supplies his transmitter. This
will substitute the low-resistance winding of the relay _4_ for the
high-resistance winding of the relay _2_, and this will permit the
energizing of the tip supervisory relay of the _A_-operator and put out
the calling supervisory lamp at her position. As in the Western Electric
circuit, therefore, the control of the _A_-operator's calling
supervisory lamp is from the called subscriber's station and is relayed
back over the trunk to the originating office.
In this circuit, manual instead of automatic ringing is employed,
therefore, unlike the Western Electric circuit, means must be provided
for notifying the B-operator when the calling subscriber has answered.
This is done by placing at the _B_-operator's position a ringing lamp
associated with each trunk cord, which is illuminated when the
_B_-operator places the plug of the incoming trunk into the multiple
jack of the subscriber's line, and remains illuminated until the
subscriber has answered. This is accomplished in the following manner:
when the operator plugs into the jack of the line called, relay _5_ is
energized but is immediately de-energized by the disconnecting of the
circuit of this relay from the sleeve conductor of the cord when the
ringing key is depressed, the selection of the ringing key being
determined by the particular party on the line desired. These ringing
keys have associated with them a set of springs _9_, which springs are
operated when any one of the ringing keys is depressed. Thus, with a
ringing key depressed and the relay _5_ de-energized, the ringing lamp
will be illuminated by means of a circuit as follows: from the live side
of the battery, through the ringing lamp _12_, through the back contact
and armature of the relay _6_, through the armature and contact of relay
_4_, then through the armature and front contact of relay _2_--which at
this time is the relay bridged across the trunk and, therefore,
energized--and thence through the back contact and armature of relay
_5_ to ground. When the subscriber removes his receiver from the hook,
the relay _1_ will become energized as previously described, and will,
therefore, operate relay _6_ to break the circuit of the ringing lamp.
The circuit thus established by the operation of relay _1_ is as
follows: from the live side of battery, through the winding of relay
_6_, through the armature and contact of relay _1_, through the armature
and contact of relay _4_, through the armature and front contact of
relay _2_, thence through the armature and back contact of relay _5_ to
ground. As soon as the _B_-operator notes that the ringing lamp has gone
out, she knows that no further ringing is required on that line, thus
allowing the operation of relay _5_ and accomplishing the locking out of
the ringing lamp during the remainder of that connection. The relay _6_,
after having once pulled up, remains locked up through the rear contact
of the left-hand armature of relay _5_ and ground, until the plug is
removed from the jack.
At the end of the conversation, when the _A_-operator has disconnected
her cord circuit on the illumination of the supervisory signals, both
relays _2_ and _4_ will be in an unoperated condition and will provide a
circuit for illuminating the disconnect lamp associated with the
_B_-operator's cord. This circuit may be traced as follows: from battery
through the disconnect lamp, through the armatures and contacts of
relays _2_ and _4_, thence through the front contact and armature of
relay _5_ to ground, thus illuminating the disconnect lamp. The ringing
lamp will not be re-illuminated at this time, due to the fact that it
has been previously locked out by relay _6_. The operator then removes
the plug from the jack of the line called, and the apparatus in the
trunk circuit is restored to normal condition.
In the circuit shown only keys are provided for ringing two parties.
This circuit, however, is not confined to the use of two-party lines,
but may be extended to four parties by simply duplicating the ringing
keys and by connecting them with the proper current for selectively
ringing the other stations.
The method of determining as to whether the called line is free or busy
is similar to that previously described for the _A_-operator's cord
circuit when making a local connection, and differs only in the fact
that in the case of the trunk cord the test circuit is controlled
through the contacts of a relay, whereas in the case of the
_A_-operator's cord, the test circuit was controlled through the
contacts of the listening key. The function of the resistance _10_ and
the battery connected thereto is the same as has been previously
described.
The general make-up of trunking switchboard sections is not greatly
different from that of the ordinary switchboard sections where no
trunking is involved. In small exchanges where ring-down trunks are
employed, the trunk line equipment is merely added to the regular jack
and drop equipment of the switchboard. In common-battery multiple
switchboards the _A_-boards differ in no respect from the standard
single office multiple boards, except that immediately above the
answering jacks and below the multiple there are arranged in suitable
numbers the jacks of the outgoing trunks.
Where the offices are comparatively small, the incoming trunk portions
of the _B_-boards are usually merely a continuance of the _A_-boards,
the subscriber's multiple being continuous with and differing in no
respect from that on the _A_-sections. Instead of the usual pairs of
_A_-operators' plugs, cords, and supervisory equipment, there are on the
key and plug shelves of these _B_-sections the incoming trunk plugs and
their associated equipment.
In large offices it is customary to make the _B_-board entirely separate
from the _A_-board, although the general characteristics of construction
remain the same. The reason for separate _A_- and _B_-switchboards in
large exchanges is to provide for independent growth of each without the
growth of either interfering with the other.
A portion of an incoming trunk, or _B_-board, is shown in Fig. 378. The
multiple is as usual, and, of course, there are no outgoing trunk jacks
nor regular cord pairs. Instead the key and plug shelves are provided
with the incoming-trunk plug equipments, thirty of these being about the
usual quota assigned to each operator's position.
In multi-office exchanges, employing many central offices, such, for
instance, as those in New York or Chicago, it is frequently found that
nearly all of the calls that originate in one office are for subscribers
whose lines terminate in some other office. In other words, the number
of calls that have to be trunked to other offices is greatly in excess
of the number of calls that may be handled through the multiple of the
_A_-board in which they originate. It is not infrequent to have the
percentage of trunked calls run as high as 75 per cent of the total
number of calls originating in any one office, and in some of the
offices in the larger cities this percentage runs higher than 90 per
cent.
[Illustration: Fig. 378. Section of Trunk Switchboard]
[Illustration: Fig. 379. Section of Partial Multiple Switchboard]
This fact has brought up for consideration the problem as to whether,
when the nature of the traffic is such that only a very small portion of
the calls can be handled in the office where they originate, it is worth
while to employ the multiple terminals for the subscribers' lines on the
_A_-boards. In other words, if so great a proportion as 90 per cent of
the calls have to be trunked any way, is it worth while to provide the
great expense of a full multiple on all the sections of the _A_-board in
order to make it possible to handle the remaining 10 per cent of the
calls directly by the _A_-operators?
As a result of this consideration it has been generally conceded that
where such a very great percentage of trunking was necessary, the full
multiple of the subscribers' lines on each section was not warranted,
and what is known as the partial multiple board has come into existence
in large manual exchanges. In these the regular subscribers' multiple is
entirely omitted from the _A_-board, all subscribers' calls being
handled through outgoing trunk jacks connected by trunks to _B_-boards
in the same as well as other offices. In these partial multiple
_A_-boards, the answering jacks are multipled a few times, usually
twice, so that calls on each line may be answered from more than one
position. This multipling of answering jacks does not in any way take
the place of the regular multipling in full multiple boards, since in no
case are the calls completed through the multiple jacks. It is done
merely for the purpose of contributing to team work between the
operators.
A portion of such a partial multiple _A_-board is shown in Fig. 379.
This view shows slightly more than one section, and the regular
answering jacks and lamps may be seen at the bottom of the jack space
just above the plugs. Above these are placed the outgoing trunk jacks,
those that are in use being indicated with white designation strips.
Above the outgoing trunk jacks are placed the multiples of the answering
jacks, these not being provided with lamps.
The partial multiple _A_-section of Fig. 379 is a portion of the
switchboard equipment of the same office to which the trunking section
shown in Fig. 378 belongs. That this is a large multiple board may be
gathered from the number of multiple jacks in the trunking section,
8,400 being installed with room for 10,500. That the board is a portion
of an equipment belonging to an exchange of enormous proportions may be
gathered from the number of outgoing trunk jacks shown in the _A_-board,
and in the great number of order-wire keys shown between each of the
sets of regular cord-circuit keys. The switchboards illustrated in these
two figures are those of one of the large offices of the New York
Telephone Company on Manhattan Island, and the photographs were taken
especially for this work by the Western Electric Company.
=Cable Color Code.= A great part of the wiring of switchboards
requires to be done with insulated wires grouped into cables.
In the wiring of manual switchboards as described in the seven
preceding chapters, and of automatic and automanual systems and
of private branch-exchange and intercommunicating systems
described in succeeding chapters, cables formed as follows are
widely used:
Tinned soft copper wires, usually of No. 22 or No. 24 B. & S.
gauge, are insulated, first with two coverings of silk, then
with one covering of cotton. The outer (cotton) insulation of
each wire is made of white or of dyed threads. If dyed, the
color either is solid red, black, blue, orange, green, brown,
or slate, or it is striped, by combining one of those colors
with white or a remaining color. The object of coloring the
wires is to enable them to be identified by sight instead of by
electrical testing.
Wires so insulated are twisted into pairs, choosing the colors
of the "line" and "mate" according to a predetermined plan. An
assortment of these pairs then is laid up spirally to form the
cable core, over which are placed certain wrappings and an
outer braid. A widely used form of switchboard cable has paper
and lead foil wrappings over the core, and the outer cotton
braid finally is treated with a fire-resisting paint.
STANDARD COLOR CODE FOR CABLES
+---------------+-------------------------------------------------+
| | MATE |
| LINE WIRE +-------+-------+-------+-----------+-------------+
| | White | Red | Black | Red-White | Black-White |
+---------------+-------+-------+-------+-----------+-------------+
| Blue | 1 | 21 | 41 | 61 | 81 |
| Orange | 2 | 22 | 42 | 62 | 82 |
| Green | 3 | 23 | 43 | 63 | 83 |
| Brown | 4 | 24 | 44 | 64 | 84 |
| Slate | 5 | 25 | 45 | 65 | 85 |
| Blue-White | 6 | 26 | 46 | 66 | 86 |
| Blue-Orange | 7 | 27 | 47 | 67 | 87 |
| Blue-Green | 8 | 28 | 48 | 68 | 88 |
| Blue-Brown | 9 | 29 | 49 | 69 | 89 |
| Blue-Slate | 10 | 30 | 50 | 70 | 90 |
| Orange-White | 11 | 31 | 51 | 71 | 91 |
| Orange-Green | 12 | 32 | 52 | 72 | 92 |
| Orange-Brown | 13 | 33 | 53 | 73 | 93 |
| Orange-Slate | 14 | 34 | 54 | 74 | 94 |
| Green-White | 15 | 35 | 55 | 75 | 95 |
| Green-Brown | 16 | 36 | 56 | 76 | 96 |
| Green-Slate | 17 | 37 | 57 | 77 | 97 |
| Brown-White | 18 | 38 | 58 | 78 | 98 |
| Brown-Slate | 19 | 39 | 59 | 79 | 99 |
| Slate-White | 20 | 40 | 60 | 80 | 100 |
+---------------+-------+-------+-------+-----------+-------------+
The numerals represent the pair numbers in the cable.
The wires of spare pairs usually are designated by solid red
with white mate for first spare pair, and solid black with
white mate for second spare pair. Individual spare wires
usually are colored red-white for first individual spare, and
black-white for second individual spare.
CHAPTER XXVIII
FUNDAMENTAL CONSIDERATIONS OF AUTOMATIC SYSTEMS
=Definition.= The term automatic, as applied to telephone systems, has
come to refer to those systems in which machines at the central office,
under the guidance of the subscribers, do the work that is done by
operators in manual systems. In all automatic telephone systems, the
work of connecting and disconnecting the lines, of ringing the called
subscriber, even though he must be selected from among those on a party
line, of refusing to connect with a line that is already in use, and
informing the calling subscriber that such line is busy, of making
connections to trunk lines and through them to lines in other offices
and doing the same sort of things there, of counting and recording the
successful calls made by a subscriber, rejecting the unsuccessful, and
nearly all the thousand and one other acts necessary in telephone
service, are performed without the presence of any guiding intelligence
at the central office.
The fundamental object of the automatic system is to do away with the
central-office operator. In order that each subscriber may control the
making of his own connections there is added to his station equipment a
call transmitting device by the manipulation of which he causes the
central-office mechanisms to establish the connections he desires.
We think that the automatic system is one of the most astonishing
developments of human ingenuity. The workers in this development are
worthy of particular notice. From occupying a position in popular regard
in common with long-haired men and short-haired women they have recently
appeared as sane, reasonable men with the courage of their convictions
and, better yet, with the ability to make their convictions come true.
The scoffers have remained to pray.
=Arguments Against Automatic Idea.= Naturally there has been a bitter
fight against the automatic. Those who have opposed it have contended:
First: that it is too complicated and, therefore, could be neither
reliable or economical.
Second: that it is too expensive, and that the necessary first cost
could not be justified.
Third: that it is too inflexible and could not adapt itself to special
kinds of service.
Fourth: that it is all wrong from the subscribers' point of view as the
public will not tolerate "doing its own operating."
_Complexity._ This first objection as to complexity, and consequent
alleged unreliability and lack of economy should be carefully analyzed.
It too often happens that a new invention is cast into outer darkness by
those whose opinions carry weight by such words as "it cannot work; it
is too complicated." Fortunately for the world, the patience and
fortitude which men must possess before they can produce meritorious,
though intricate inventions, are usually sufficient to prevent their
being crushed by any such offhand condemnation, and the test of time and
service is allowed to become the real criterion.
It would be difficult to find an art that has gone forward as rapidly as
telephony. Within its short life of a little over thirty years it has
grown from the phase of trifling with a mere toy to an affair of
momentous importance to civilization. There has been a tendency,
particularly marked during recent years, toward greater complexity; and
probably every complicated new system or piece of apparatus has been
roundly condemned, by those versed in the art as it was, as being unable
to survive on account of its complication.
To illustrate: A prominent telephone man, in arguing against the
nickel-in-the-slot method of charging for telephone service once said,
partly in jest, "The Lord never intended telephone service to be given
in that way." This, while a little off the point, is akin to the
sweeping aside of new telephone systems on the sole ground that they are
complicated. These are not real reasons, but rather convenient ways of
disposing of vexing problems with a minimum amount of labor. Important
questions lying at the very root of the development of a great industry
may not be put aside permanently in this offhand way. The Lord has
never, so far as we know, indicated just what his intentions were in the
matter of nickel service; and no one has ever shown yet just what
degree of complexity will prevent a telephone system from working.
It is safe to say that, if other things are equal, the simpler a machine
is, the better; but simplicity, though desirable, is not all-important.
Complexity is warranted if it can show enough advantages.
If one takes a narrow view of the development of things mechanical and
electrical, he will say that the trend is toward simplicity. The
mechanic in designing a machine to perform certain functions tries to
make it as simple as possible. He designs and re-designs, making one
part do the work of two and contriving schemes for reducing the
complexity of action and form of each remaining part. His whole trend is
away from complication, and this is as it should be. Other things being
equal, the simpler the better. A broad view, however, will show that the
arts are becoming more and more complicated. Take the implements of the
art of writing: The typewriter is vastly more complicated than the pen,
whether of steel or quill, yet most of the writing of today is done on
the typewriter, and is done better and more economically. The art of
printing affords even more striking examples.
In telephony, while every effort has been made to simplify the component
parts of the system, the system itself has ever developed from the
simple toward the complex. The adoption of the multiple switchboard, of
automatic ringing, of selective ringing on party lines, of
measured-service appliances, and of automatic systems have all
constituted steps in this direction. The adoption of more complicated
devices and systems in telephony has nearly always followed a demand for
the performance by the machinery of the system of additional or
different functions. As in animal and plant life, so in mechanics--the
higher the organism functionally the more complex it becomes physically.
Greater intricacy in apparatus and in methods is warranted when it is
found desirable to make the machine perform added functions. Once the
functions are determined upon, then the whole trend of the development
of the machine for carrying them out should be toward simplicity. When
the machine has reached its highest stage of development some one
proposes that it be required to do something that has hitherto been done
manually, or by a separate machine, or not at all. With this added
function a vast added complication may come, after which, if it develops
that the new function may with economy be performed by the machine, the
process of simplification again begins, the whole design finally taking
on an indefinable elegance which appears only when each part is so made
as to be best adapted in composition, form, and strength to the work it
is to perform.
Achievements in the past teach us that a machine may be made to do
almost anything automatically if only the time, patience, skill, and
money be brought to bear. This is also true of a telephone system. The
primal question to decide is, what functions the system is to perform
within itself, automatically, and what is to be done manually or with
manual aid. Sometimes great complications are brought into the system in
an attempt to do something which may very easily and cheaply be done by
hand. Cases might be pointed out in which fortunes and life-works have
been wasted in perfecting machines for which there was no real economic
need. It is needless to cite cases where the reverse is true. The matter
of wisely choosing the functions of the system is of fundamental
importance. In choosing these the question of complication is only one
of many factors to be considered.
One of the strongest arguments against intricacy in telephone apparatus
is its greater initial cost, its greater cost of maintenance, and its
liability to get out of order. Greater complexity of apparatus usually
means greater first cost, but it does not necessarily mean greater cost
of up-keep or lessened reliability. A dollar watch is more simple than
an expensive one. The one, however, does its work passably and is thrown
away in a year or so; the other does its work marvelously well and may
last generations, being handed down from father to son. Merely reducing
the number of parts in a machine does not necessarily mean greater
reliability. Frequently the attempt to make one part do several diverse
things results in such a sacrifice in the simplicity of action of that
part as to cause undue strain, or wear, or unreliable action. Better
results may be attained by adding parts, so that each may have a
comparatively simple thing to do.
[Illustration: WESTERN ELECTRIC COMPANY TYPICAL CHARGING OUTFIT AT
DAWSON, GEORGIA]
The stage of development of an art is a factor in determining the degree
of complexity that may be allowed in the machinery of that art. A
linotype machine, if constructed by miracle several hundred years ago,
would have been of no value to the printer's art then. The skill was not
available to operate and maintain it, nor was the need of the public
sufficiently developed to make it of use. Similarly the automatic
telephone exchange would have been of little value thirty years ago. The
knowledge of telephone men was not sufficiently developed to maintain
it, telephone users were not sufficiently numerous to warrant it, and
the public was not sufficiently trained to use it. Industries, like
human beings, must learn to creep before they can walk.
Another factor which must be considered in determining the allowable
degree of complexity in a telephone system is the character of the labor
available to care for and manage it. Usually the conditions which make
for unskilled labor also lend themselves to the use of comparatively
simple systems. Thus, in a small village remote from large cities the
complexity inherent in a common-battery multiple switchboard would be
objectionable. The village would probably not afford a man adequately
skilled to care for it, and the size of the exchange would not warrant
the expense of keeping such a man. Fortunately no such switchboard is
needed. A far simpler device, the plain magneto switchboard--so simple
that the girl who manipulates it may also often care for its
troubles--is admirably adapted to the purpose. So it is with the
automatic telephone system; even its most enthusiastic advocate would be
foolish indeed to contend that for all places and purposes it was
superior to the manual.
These remarks are far from being intended as a plea for complex
telephone apparatus and systems; every device, every machine, and every
system should be of the simplest possible nature consistent with the
functions it has to perform. They are rather a protest against the
broadcast condemnation of complex apparatus and systems just because
they are complicated, and without regard to other factors. Such
condemnation is detrimental to the progress of telephony. Where would
the printing art be today if the linotype, the cylinder press, and other
modern printing machinery of marvelous intricacy had been put aside on
account of the fact that they were more complicated than the printing
machinery of our forefathers?
That the automatic telephone system is complex, exceedingly complex,
cannot be denied, but experience has amply proven that its complexity
does not prevent it from giving reliable service, nor from being
maintained at a reasonable cost.
_Expense._ The second argument against the automatic--that it is too
expensive--is one that must be analyzed before it means anything. It is
true that for small and medium-sized exchanges the total first cost of
the central office and subscribers' station equipment, is greater than
that for manual exchanges of corresponding sizes. The prices at which
various sizes of automatic exchange equipments may be purchased vary,
however, almost in direct proportion to the number of lines, whereas in
manual equipment the price per line increases very rapidly as the number
of lines increases. From this it follows that for very large exchanges
the cost of automatic apparatus becomes as low, and may be even lower
than for manual. Roughly speaking the cost of telephones and
central-office equipment for small exchanges is about twice as great for
automatic as for manual, and for very large exchanges, of about 10,000
lines, the cost of the two for switchboards and telephones is about
equal.
For all except the largest exchanges, therefore, the greater first cost
of automatic apparatus must be put down as one of the factors to be
weighed in making the choice between automatic and manual, this factor
being less and less objectionable as the size of the equipment increases
and finally disappearing altogether for very large equipments. Greater
first cost is, of course, warranted if the fixed charges on the greater
investment are more than offset by the economy resulting. The automatic
screw machine, for instance, costs many times more than the hand screw
machine, but it has largely displaced the hand machine nevertheless.
_Flexibility._ The third argument against the automatic telephone
system--its flexibility--is one that only time and experience has been
able to answer. Enough time has elapsed and enough experience has been
gained, however, to disprove the validity of this argument. In fact, the
great flexibility of the automatic system has been one of its surprising
developments. No sooner has the statement been made that the automatic
system could not do a certain thing than forthwith it has done it. It
was once quite clear that the automatic system was not practicable for
party-line selective ringing; yet today many automatic systems are
working successfully with this feature; the selection between the
parties on a line being accomplished with just as great certainty as in
manual systems. Again it has seemed quite obvious that the automatic
system could not hope to cope with the reverting call problem, _i. e._,
enabling a subscriber on a party line to call back to reach another
subscriber on the same line; yet today the automatic system may do this
in a way that is perhaps even more satisfactory than the way in which it
is done in multiple manual switchboards. It is true that the automatic
system has not done away with the toll operator and it probably never
will be advantageous to require it to do so for the simple reason that
the work of the toll operator in recording the connections and in
bringing together the subscribers is a matter that requires not only
accuracy but judgment, and the latter, of course, no machine can supply.
It is probable also that the private branch-exchange operator will
survive in automatic systems. This is not because the automatic system
cannot readily perform the switching duties, but the private
branch-exchange operator has other duties than the mere building up and
taking down of connections. She is, as it were, a door-keeper guarding
the telephone door of a business establishment; like the toll operator
she must be possessed of judgment and of courtesy in large degree,
neither of which can be supplied by machinery.
In respect to toll service and private branch-exchange service where, as
just stated, operators are required on account of the nature of the
service, the automatic system has again shown its adaptability and
flexibility. It has shown its capability of working in harmony with
manual switchboards, of whatever nature, and there is a growing tendency
to apply automatic devices and automatic principles of operation to
manual switchboards, whether toll or private branch or other kinds, even
though the services of an operator are required, the idea being to do by
machinery that portion of the work which a machine is able to do better
or more economically than a human being.
_Attitude of Public._ The attitude of the public toward the automatic is
one that is still open to discussion; at least there is still much
discussion on it. A few years ago it did seem reasonable to suppose that
the general telephone user would prefer to get his connection by merely
asking for it rather than to make it himself by "spelling" it out on the
dial of his telephone instrument. We have studied this point carefully
in a good many different communities and it is our opinion that the
public finds no fault with being required to make its own connections.
To our minds it is proven beyond question that either the method
employed in the automatic or that in the manual system is satisfactory
to the public as long as good service results, and it is beyond question
that the public may get this with either.
_Subscriber's Station Equipment._ The added complexity of the mechanism
at the subscriber's station is in our opinion the most valid objection
that can be urged against the automatic system as it exists today. This
objection has, however, been much reduced by the greater simplicity and
greater excellence of material and workmanship that is employed in the
controlling devices in modern automatic systems. However, the automatic
system must always suffer in comparison with the manual in respect of
simplicity of the subscriber's equipment. The simplest conceivable thing
to meet all of the requirements of telephone service at a subscriber's
station is the modern common-battery manual telephone. The automatic
telephone differs from this only in the addition of the mechanism for
enabling the subscriber to control the central-office apparatus in the
making of calls. From the standpoint of maintenance, simplicity at the
subscriber's station is, of course, to be striven for since the proper
care of complex devices scattered all over a community is a much more
serious matter than where the devices are centered at one point, as in
the central office. Nevertheless, as pointed out, complexity is not
fatal, and it is possible, as has been proven, to so design and
construct the required apparatus in connection with the subscribers'
telephones as to make them subject to an amount of trouble that is not
serious.
=Comparative Costs.= A comparison of the total costs of owning,
operating, and maintaining manual and automatic systems usually results
in favor of the automatic, except in small exchanges. This seems to be
the consensus of opinion among those who have studied the matter deeply.
Although the automatic usually requires a larger investment, and
consequently a larger annual charge for interest and depreciation,
assuming the same rates for each case, and although the automatic
requires a somewhat higher degree of skill to maintain it and to keep it
working properly than the manual, the elimination of operators or the
reduction in their number and the consequent saving of salaries and
contributory expenses together with other items of saving that will be
mentioned serves to throw the balance in favor of the automatic.
The ease with which the automatic system lends itself to inter-office
trunking makes feasible a greater subdivision of exchange districts into
office districts and particularly makes it economical, where such would
not be warranted in manual working. All this tends toward a reduction in
average length of subscribers' lines and it seems probable that this
possibility will be worked upon in the future, more than it has been in
the past, to effect a considerable saving in the cost of the wire plant,
which is the part of a telephone plant that shows least and costs most.
=Automatic vs. Manual.= Taking it all in all the question of automatic
versus manual may not and can not be disposed of by a consideration of
any single one of the alleged features of superiority or inferiority of
either. Each must be looked at as a practical way of giving telephone
service, and a decision can be reached only by a careful weighing of all
the factors which contribute to economy, reliability, and general
desirability from the standpoint of the public. Public sentiment must
neither be overlooked nor taken lightly, since, in the final analysis,
it is the public that must be satisfied.
=Methods of Operation.= In all of the automatic telephone systems that
have achieved any success whatever, the selection of the desired
subscriber's line by the calling subscriber is accomplished by means of
step-by-step mechanism at the central office, controlled by impulses
sent or caused to be sent by the acts of the subscriber.
_Strowger System._ In the so-called Strowger system, manufactured by the
Automatic Electric Company of Chicago, the subscriber, in calling,
manipulates a dial by which the central-office switching mechanism is
made to build up the connection he wants. The dial is moved as many
times as there are digits in the called subscriber's number and each
movement sends a series of impulses to the central office corresponding
in number respectively to the digits in the called subscriber's number.
During each pause, except the last one, between these series of
impulses, the central-office mechanism operates to shift the control of
the calling subscriber's line from one set of switching apparatus at the
central office to another.
In case a four-digit number is being selected first, the movement of
the dial by the calling subscriber will correspond to the thousands
digit of the number being called, and the resulting movement of the
central-office apparatus will continue the calling subscriber's line
through a trunk to a piece of apparatus capable of further extending his
line toward the line terminals of the thousand subscribers whose numbers
begin with the digit chosen. The next movement of the dial corresponding
to the hundreds digit of the called number will operate this piece of
apparatus to again extend the calling subscriber's line through another
trunk to apparatus representing the particular hundred in which the
called subscriber's number is. The third movement of the dial
corresponding to the tens digit will pick out the group of ten
containing the called subscriber's line, and the fourth movement
corresponding to the units digit will pick out and connect with the
particular line called.
_Lorimer System._ In the Lorimer automatic system invented by the
Lorimer Brothers, and now being manufactured by the Canadian Machine
Telephone Company of Toronto, Canada, the subscriber sets up the number
he desires complete by moving four levers on his telephone so that the
desired number appears visibly before him. He then turns a handle and
the central-office apparatus, under the control of the electrical
conditions thus set up by the subscriber, establishes the connection. In
this system, unlike the Strowger system, the controlling impulses are
not caused by the movement of the subscriber's apparatus in returning to
its normal position after being set by the subscriber. Instead, the
conditions established at the subscriber's station by the subscriber in
setting up the desired number, merely determine the point in the series
of impulses corresponding to each digit at which the stepping impulses
local to the central office shall cease, and in this way the proper
number of impulses in the series corresponding to each digit is
determined.
_Magnet- vs. Power-Driven Switches._ These two systems differ radically
in another respect. In the Strowger system it is the electrical impulses
initiated at the subscriber's apparatus that actually cause the movement
of the switching parts at the central office, these impulses energizing
electromagnets which move the central-office switching devices a step at
a time the desired number of steps. In the Lorimer system the switches
are all power-driven and the impulses under the control of the
subscriber's instrument merely serve to control the application of this
power to the various switching mechanisms. These details will be more
fully dealt with in subsequent chapters.
_Multiple vs. Trunking._ It has been shown in the preceding portion of
this work that the tendency in manual switchboard practice has been away
from trunking between the various sections or positions of a board, and
toward the multiple idea of operating, wherein each operator is able to
complete the connection with any line in the same office without
resorting to trunks or to the aid of other operators. Strangely enough
the reverse has been true in the development of the automatic system. As
long as the inventors tried to follow the most successful practice in
manual working, failure resulted. The automatic systems of today are
essentially trunking systems and while they all involve multiple
connections in greater or less degree, all of them depend fundamentally
upon the extending of the calling line by separate lengths until it
finally reaches and connects with the called line.
_Grouping of Subscribers._ In this connection we wish to point out here
two very essential features without which, so far as we are aware, no
automatic telephone system has been able to operate successfully. The
first of these is the division of the total number of lines in any
office of the exchange into comparatively small groups and the
employment of correspondingly small switch units for each group. Many of
the early automatic systems that were proposed involved the idea of
having each switch capable in itself of making connection with any line
in the entire office. As long as the number of lines was small--one
hundred or thereabouts--this might be all right, but where the lines
number in the thousands, it is readily seen that the switches would be
of prohibitive size and cost.
_Trunking between Groups._ This feature made necessary the employment of
trunk connections between groups. By means of these the lines are
extended a step at a time, first entering a large group of groups,
containing the desired subscriber; then entering the smaller group
containing that subscriber; and lastly entering into connection with the
line itself. The carrying out of this idea was greatly complicated by
the necessity of providing for many simultaneous connections through the
switchboard. It was comparatively easy to accomplish the extension of
one line through a series of links or trunks to another line, but it
was not so easy to do this and still leave it possible for any other
line to pick out and connect with any other idle line without
interference with the first connection. A number of parallel paths must
be provided for each possible connection. Groups of trunks are,
therefore, provided instead of single trunks between common points to be
connected. The subscriber who operates his instrument in making a call
knows nothing of this and it is, of course, impossible for him to give
any thought to the matter as to which one of the possible paths he shall
choose. It was by a realization of these facts that the failures of the
past were turned into the successes of the present. The subscriber by
setting his signal transmitter was made to govern the action of the
central-office apparatus in the selection of the proper _group_ of
trunks. The group being selected, the central-office apparatus was made
to act at once _automatically_ to pick out and connect with _the first
idle trunk of such group_. Thus, we may say _that the subscriber by the
act performed on his signal transmitter, voluntarily chooses the group
of trunks, and immediately thereafter the central-office apparatus
without the volition of the subscriber picks out the first idle one of
this group of trunks so chosen_. This fundamental idea, so far as we are
aware, underlies all of the successful automatic telephone-exchange
systems. It provides for the possibility of many simultaneous
connections through the switchboard, and it provides against the
simultaneous appropriation of the same path by two or more calling
subscribers and thus assures against interference in the choice of the
paths.
_Outline of Action._ In order to illustrate this point we may briefly
outline the action of the Strowger automatic system in the making of a
connection. Assume that the calling subscriber desires a connection with
a subscriber whose line bears the number 9,567. The subscriber in making
the call will, by the first movement of his dial, transmit nine impulses
over his line. This will cause the selective apparatus at the central
office, that is at the time associated with the calling subscriber's
line, to move its selecting fingers opposite a group of terminals
representing the ends of a group of trunk lines leading to apparatus
employed in connecting with the ninth thousand of the subscribers'
lines.
While the calling subscriber is getting ready to transmit the next
digit, the automatic apparatus, without his volition, starts to pick out
the first idle one of the group of trunks so chosen. Having found this
it connects with it and the calling subscriber's line is thus extended
to another selective apparatus capable of performing the same sort of
function in choosing the proper hundreds group.
In the next movement of his dial the calling subscriber will send five
impulses. This will cause the last chosen selective switch to move its
selective fingers opposite a group of terminals representing the ends of
a group of trunks each leading to a switch that is capable of making
connection with any one of the lines in the fifth hundred of the ninth
thousand. Again during the pause by the subscriber, the switch that
chose this group of trunks will start automatically to pick out and
connect with the first idle one of them, and will thus extend the line
to a selective switch that is capable of reaching the desired line,
since it has access to all of the lines in the chosen hundred. The third
movement of the dial sends six impulses and this causes this last chosen
switch to move opposite the sixth group of ten terminals, so that there
has now been chosen the nine hundred and fifty-sixth group of ten lines.
The final movement of the dial sends seven impulses and the last
mentioned switch connects with the seventh line terminal in the group of
ten previously chosen and the connection is complete, assuming that the
called line was not already engaged. If it had been found busy, the
final switch would have been prevented from connecting with it by the
electrical condition of certain of its contacts and the busy signal
would have been transmitted back to the calling subscriber.
_Fundamental Idea._ This idea of subdividing the subscribers' lines in
an automatic exchange, of providing different groups of trunks so
arranged as to afford by combination a number of possible parallel paths
between any two lines, of having the calling subscriber select, by the
manipulation of his instrument, the proper group of trunks any one of
which might be used to establish the connection he desires, and of
having the central-office apparatus act automatically to choose and
connect with an idle one in this chosen group, should be firmly grasped.
It appears, as we have said, in every successful automatic system
capable of serving more than one small group of lines, and until it was
evolved automatic telephony was not a success.
_Testing._ As each trunk is chosen and connected with, conditions are
established, by means not unlike the busy test in multiple manual
switchboards, which will guard that trunk and its associated apparatus
against appropriation by any other line or apparatus as long as it is
held in use. Likewise, as soon as any subscriber's line is put into use,
either by virtue of a call being originated on it, or by virtue of its
being connected with as a called line, conditions are automatically
established which guard it against being connected with any other line
as long as it is busy. These guarding conditions of both trunks and
lines, as in the manual board, are established by making certain
contacts, associated with the trunks or lines, assume a certain
electrical condition when busy that is different from their electrical
condition when idle; but unlike the manual switchboard this different
electrical condition does not act to cause a click in any one's ear, but
rather to energize or de-energize certain electromagnets which will
establish or fail to establish the connection according to whether it is
proper or improper to do so.
_Local and Inter-Office Trunks._ The groups of trunks that are used in
building up connections between subscribers' lines may be local to the
central office, or they may extend between different offices. The action
of the two kinds of trunks, local or inter-office, is broadly the same.
CHAPTER XXIX
THE AUTOMATIC ELECTRIC COMPANY'S SYSTEM
Almost wherever automatic telephony is to be found--and its use is
extensive and rapidly growing--the so-called Strowger system is
employed. It is so named because it is the outgrowth of the work of
Almon B. Strowger, an early inventor in the automatic telephone art.
That the system should bear the name of Strowger, however, gives too
great prominence to his work and too little to that of the engineers of
the Automatic Electric Company under the leadership of Alexander E.
Keith.
=Principles of Selecting Switch.= The underlying features of this
automatic system have already been referred to in the abstract. A better
grasp of its principles may, however, be had by considering a concrete
example of its most important piece of apparatus--the selecting switch.
The bare skeleton of such a switch, sufficient only to illustrate the
salient point in its mode of operation, is shown in Fig. 380. The
essential elements of this are a vertical shaft capable of both
longitudinal and rotary motion; a pawl and ratchet mechanism actuated by
a magnet for moving the shaft vertically a step at a time; another pawl
and ratchet mechanism actuated by another magnet for rotating the shaft
a step at a time; an arm carrying wiper contacts on its outer end,
mounted on and moving with the shaft; and a bank of contacts arranged on
the inner surface of a section of a cylinder adapted to be engaged by
the wiper contacts on this movable arm.
These various elements are indicated in the merest outline and with much
distortion in Fig. 380, which is intended to illustrate the principles
of operation rather than the details as they actually are in the system.
In the upper left-hand corner of this figure, the magnet shown will, if
energized by impulses of current, attract and release its armature and,
in doing so, cause the pawl controlled by this magnet to move the
vertical shaft of the switch up a step at a time, as many steps as
there are impulses of current. The vertical movement of this shaft will
carry the wiper arm, attached to the lower end of the shaft, up the same
number of steps and, in doing so, will bring the contacts of this wiper
arm opposite, but not engaging, the corresponding row of stationary
contacts in the semi-cylindrical bank. Likewise, through the ratchet
cylinder on the intermediate portion of the shaft, the magnet shown at
the right-hand portion of this figure will, when energized by a
succession of electrical impulses, rotate the shaft a step at a time, as
many steps as there are impulses. This will thus cause the contacts of
the wiper arm to move over the successive contacts in the row opposite
to which the wiper had been carried in its vertical movement.
[Illustration: Fig. 380. Principles of Automatic Switch]
At the lower left-hand corner of this figure, there is shown a pair of
keys either one of which, when operated, will complete the circuit of
the magnet to which it is connected, this circuit including a common
battery. In a certain rough way this pair of key switches in the lower
left-hand corner of the drawing may be taken as representing the
call-transmitting apparatus at the subscriber's station, and the two
wires extending therefrom may be taken as representing the line wires
connecting that subscriber's station to the central office; but the
student must avoid interpreting them as actual representations of the
subscriber's station calling apparatus or the subscriber's line since
their counterparts are not to be found in the system as it really
exists. Here again accuracy has been sacrificed for ease in setting
forth a feature of operation.
Still referring to Fig. 380, it will be seen that the bank contacts
consist of ten rows, each having ten pairs of contacts. Assume again,
for the sake of simplicity, that the exchange under consideration has
one hundred subscribers and that each pair of bank contacts represents
the terminals of one subscriber's line. Assume further that the key
switches in the lower left-hand corner of the figure are being
manipulated by a subscriber at that station and that he wishes to obtain
a connection with line No. 67. By pressing and releasing the left-hand
key six times, he will cause six separate impulses of current to flow
through the upper left-hand magnet and this will cause the switch shaft
to move up six steps and bring the wiper arm opposite the sixth row of
bank contacts. If he now presses and releases his right-hand key seven
times, he will, through the action of the right-hand magnet, rotate the
shaft seven steps, thus bringing the wipers into contact with the
seventh contact of the sixth row and thus into contact with the desired
line. As the wiper contacts on the switch arm form the terminals of the
calling subscriber's line, it will be apparent that the calling
subscriber is now connected through his switch with the line of
subscriber No. 67.
As stated, each of the pairs of bank contacts are connected with the
line of a subscriber; the line, Fig. 380, is shown so connected to the
forty-first pair of contacts, that is to the first contact in the fourth
row. The selecting switch shown in Fig. 380 would be for the sole use of
the subscriber on the line No. 41. Each of the other subscribers would
have a similar switch for his own exclusive use. Since any of the
switches must be capable of reaching line No. 67, for instance, when
moved _up_ six rows and _around_ seven, it follows that the
sixty-seventh pair of contacts in each bank of the entire one hundred
switches must also be connected together and to line No. 67. The same
is, of course, true of all the contacts corresponding to any other
number. Multiple connections are thus involved between the corresponding
contacts of the banks, in much the same way as in the corresponding
jacks in the multiple of a manual switchboard. As a result of this
multiple connection of the bank contacts, any subscriber may move the
wiper arm of his selecting switch into connection with the line of any
other subscriber.
_The "Up-and-Around" Movement._ The elemental idea to be grasped by the
discussion so far, is the so-called "up-and-around" method of action of
the selecting switches employed in this system. This preliminary
discussion may be carried a step further by saying that the arrangement
is such that when a subscriber presses both his keys and grounds both of
the limbs of his line, such a condition is brought about as will cause
all holding pawls to be withdrawn from the shaft, and thus allow it to
return to its normal position with respect to both its vertical and
rotary movements. No attempt has been made in Fig. 380 to show how this
is accomplished.
=Function of Line Switch.= Such a system as has been briefly outlined in
the foregoing would require a separate selecting switch for each
subscriber's line and would be limited to use in exchanges having not
more than one hundred lines. In the modern system of the Automatic
Electric Company, the requirement that each subscriber shall have a
selective switch, individual to his own line, has been eliminated by
introducing what is called an _individual line switch_ by means of which
any one of a group of subscribers' lines, making a call, automatically
appropriates one of a smaller group of selecting switches and makes it
its own only while the connection exists.
=Subdivision of Subscribers' Lines.= The limitation as to the size of
the exchange has been overcome, without increasing the number of bank
contacts in any selecting switch, by dividing the subscribers' lines
into groups of one hundred and causing selecting switches automatically
to extend the calling subscriber's line first into a group of groups
corresponding, for instance, to the thousand containing the called
subscriber's line, and then into the particular group containing the
line, and lastly, to connect with the individual line in that group.
=Underlying Feature of Trunking System.= It will be remembered that in
the chapter on fundamental principles of automatic systems, it was
stated that the subscriber, by means of the signal transmitter at his
station, was made to govern the action of the central-office apparatus
in the selection of a proper group of trunks; and the group being
selected, the central-office apparatus was made to act automatically to
pick out and connect with the first idle trunk of such group. This
selection by the subscriber of a group followed by the automatic
selection from among that group forms the basis of the trunking system.
It is impossible, by means of any simple diagram, to show a complete
scheme of trunking employed, but Fig. 381 will give a fundamental
conception of it. This figure shows how a single calling line, indicated
at the bottom, may find access into any particular line in an office
having a capacity for ten thousand.
=Names of Selecting Switches.= Selecting switches of the "up-and-around"
type are the means by which the calling line selects and connects with
the trunk lines required in building up the connection, and finally
selects and connects with the line of the called subscriber. Where such
a switch is employed for the purpose of selecting a _trunk_, it is
called a selector switch. It is a _first selector_ when it serves to
pick out a major group of lines, _i. e._, a group containing a
particular thousand lines or, in a multi-office system, a group
represented by a complete central office. It is a _second selector_ when
it serves to make the next subdivision of groups; a _third selector_ if
further subdivision of groups is necessary; and finally it is _a
connector_ when it is employed to pick out and connect with the
_particular line in the final group of one hundred lines_ to which the
connection has been brought by the selectors. In a single office of
10,000-line capacity, therefore, we would have first and second
selectors and connectors, the first selectors picking out the thousands,
the second selectors the hundreds, and the connectors the individual
line. In a multi-office system we may have first, second, and third
selectors and connectors, the first selector picking out the office, the
second selector the thousands in that office, the third selector the
hundreds, and the connector the individual lines.
=The Line Switch.= In addition to the selectors and connectors there are
line switches, which are comparatively simple, one individual to each
line. Each of these has the function, purely automatic, of always
connecting a line, as soon as a call is originated on it, to some one of
a smaller group of first selectors available to that line. This idea may
be better grasped when it is understood that, in the earlier systems of
the Automatic Electric Company, there was a first selector permanently
associated with each line. By the addition of the comparatively simple
line switch, a saving of about ninety per cent of the first selectors
was effected, since the number of first selectors was thereby reduced
from a number equal to the number of lines in a group to a number equal
to the number of simultaneous connections resulting from calls
originating in that group. In other words, by the line switch, the
number of first selectors is determined by the traffic rather than by
the number of lines.
=Scheme of Trunking.= With this understanding as to the names and
broader functions of the things involved, Fig. 381 may now be
understood. The line switch of the single line, as indicated here, has
only the power of selection among three trunks, but it is to be
understood that in actual practice, it would have access to a greater
number, usually ten. So, also, throughout this diagram we have shown the
apparatus and trunks arranged in groups of three instead of in groups of
ten, only the first three thousands groups being indicated and the first
three hundreds groups in each thousand. Again only three levels instead
of ten are indicated for each selecting switch, it being understood that
in the diagram the various levels are represented by concentric arcs of
circles, and the trunk contacts by dots on these arcs.
_Line-Switch Action._ When the subscriber, whose line is shown at the
bottom of the figure, begins to make a call, the line switch acts to
connect his line with one of the first selector trunks available to it.
This selection is entirely preliminary and, except to start it, is in no
way under the control of the calling subscriber. The calling line now
has under its control a first selector which, for the time being,
becomes individual to it. Let it be assumed that the line switch found
the first of the first selector trunks already appropriated by some
other switch, but that the second one of these trunks was found idle.
This trunk being appropriated by the line switch places the center one
of the first selectors shown under the control of the subscriber's line.
This first selector then acts in response to the first set of selective
impulses sent out by his signal transmitter.
[Illustration: DEAN HARMONIC CONVERTER Dry Cell Type for Magneto
Exchange. _The Dean Electric Co._]
[Illustration: Fig. 381. Scheme of Trunking]
_First Selector Action._ We will assume that the calling subscriber
desires to connect with No. 3213. The first movement of the subscriber's
signal transmitter will send, therefore, three impulses over the line.
These impulses will act on the vertical magnet of the first selector
switch to move it up three steps. On this "level" of the contact bank of
this switch all of the contacts will represent second selector trunks
leading to the _third_ thousand group. The other ends of these trunks
will terminate in the wipers and also in the controlling magnets of
second selectors serving this thousand. This function on the part of the
first selector controlled by the act of the subscriber will have thus
selected a _group_ of trunks leading to the _third_ thousand, but the
subscriber has nothing to do with which one of the trunks of this group
will actually be used. Immediately following the vertical movement of
the first selector switch the rotary movement of this switch will start
and will continue until the wipers of that switch have found contacts of
an idle trunk leading to a second selector. Assuming that the first
trunk was the one found idle, the first selector wipers would pause on
the first pair of contacts in the third level of its bank, and the trunk
chosen may be seen leading from that contact off to the group of second
selectors belonging to the third thousand. For clearness, the chosen
trunks in this assumed connection are shown heavier than the others.
_Second Selector Action._ The next movement of the dial by the
subscriber in establishing his desired connection will send two
impulses, it being desired to choose the _second_ hundred in the _third_
thousand. The first selector will have become inoperative before this
second series of impulses is sent and, therefore, only the second
selector will respond. Its vertical magnet acting under the influence of
these two impulses will step up its wiper contacts opposite the second
row of bank contacts, and the subscriber will thus have chosen the
_group_ of trunks leading to the _second_ hundred in the _third_
thousand. Here, again, the automatic operation of picking out the first
idle one of this chosen group of trunks will take place without the
volition of the subscriber, and it will be assumed that the first two
trunks on this level of the second selector were found already engaged
and that the third was therefore chosen. The connection continues, as
indicated by heavy lines in Fig. 381, to the third one of the connectors
in the _second_ hundred of the _third_ thousand. Any one of these
connectors would have accomplished the purpose but this is assumed to be
the first one found idle by the second selector.
_Connector Action._ The third movement of the subscriber's dial will
send but one impulse, this corresponding to the _first_ group of ten in
the _second_ hundred in the _third_ thousand. This impulse will move the
connector shaft up to the first level of bank contacts; and from now on
the action of the connector differs radically from that of the
selectors. The connector is not searching for an idle trunk in the group
but for a particular line and, therefore, having chosen the group of ten
lines in the desired hundred, the connector switch waits for further
guidance from the subscriber. This comes in the form of the final set of
impulses sent by the subscriber's signal transmitter which, in this
case, will be three in number, corresponding to the final digit in the
number of the called subscriber. This series of impulses will control
the rotary movement of the connector wipers which will move along the
first level and stop on the third one. The process is seen to be one of
successive selection, first of a large group, then of a smaller, again
of a smaller, and finally of an individual.
If the line is found not busy, the connection between the two
subscribers is complete and the called subscriber's bell will be rung.
If it is found busy, however, the connector will refuse to connect and
will drop back to its normal position, sending a busy signal back to the
calling subscriber. The details of ringing and the busy-back operation
may only be understood by a discussion of drawings, subsequently to be
referred to.
=Two-Wire and Three-Wire Systems.= In most of the systems of the
Automatic Electric Company in use today the impulses by which the
subscriber controls the central-office apparatus flow over one side of
the line or the other and return by ground. The metallic circuit is used
for talking and for ringing the called subscriber's bell, while ground
return circuits, on one side of the line or the other, are used for
sending all the switch controlling impulses.
Recently this company has perfected a system wherein no ground is
required at the subscriber's station and no ground return path is used
for any purpose between the subscriber and the central office.
This later system is known as the "two-wire" system, and in
contra-distinction to it, the earlier and most used system has been
referred to as the "three-wire." It is not meant by this that the line
circuits actually have three wires but that each line employs three
conductors, the two wires of the line and the earth. The three-wire
system will be referred to and described in detail, and from it the
principles of the two-wire system will be readily understood.
[Illustration: Fig. 382. Automatic Wall Set]
[Illustration: Fig. 383. Automatic Desk Stand]
=Subscriber's Station Apparatus.= The detailed operation of the
three-wire system may be best understood by considering the subscriber's
station apparatus first. The general appearance of the wall set is shown
in Fig. 382, and of the desk set in Fig. 383. These instruments embody
the usual talking and call-receiving apparatus of a common-battery
telephone and in addition to this, the signal transmitter, which is the
thing especially to be considered now. The diagrammatic illustration of
the signal transmitter and of the relation that its parts bear to the
other elements of the telephone set is shown in Fig. 384. It has already
been stated that the subscriber manipulates the signal transmitter by
rotating the dial on the face of the instrument. A clearer idea of this
dial and of the finger stop for it may be obtained from Figs. 382 and
383.
[Illustration: Fig. 384. Circuits of Telephone Set]
_Operation._ To make a call for a given number the subscriber removes
his receiver from its hook, then places his forefinger in the hole
opposite the number corresponding to the first digit of the desired
number. By means of the grip thus secured, he rotates the dial until its
movement is stopped by the impact of the finger against the stop. The
dial is then released and in its return movement it sends the number of
impulses corresponding to the first digit in the called number. A
similar movement is made for each digit.
In Fig. 384 is given a phantom view of the dial, in order to show more
clearly the relation of the mechanical parts and contacts controlled by
it. For a correct idea of its mechanical action it must be understood
that the shaft _1_, the lever _2_, and the interrupter segment _3_ are
all rigidly fastened to the dial and move with it. A coiled spring
always tends to move the dial and these associated parts back to their
normal positions when released by the subscriber, and a centrifugal
governor, not shown, limits the speed of the return movement.
The subscriber's hook switch is mechanically interlocked with the dial
so as to prevent the dial being moved from its normal position until the
hook is in its raised position. This interlocking function involves also
the pivoted dog _4_. Normally the lower end of this dog lies in the path
of the pin _5_ carried on the lever _2_, and thus the shaft, dial, and
segment are prevented from any considerable movement when the receiver
is on the hook. However, when the receiver is removed from its hook, the
upwardly projecting arm from the hook engages a projection on the dog
_4_ and moves the dog out of the path of the pin _5_. Thus the dial is
free to be rotated by the subscriber. The pin _6_ is mounted in a
stationary position and serves to limit the backward movement of the
dial by the lever _2_ striking against it.
Ground Springs:--Five groups of contact springs must be considered, some
of which are controlled wholly by the position of the switch hook,
others jointly by the position of the switch hook and the dial, others
by the movement of the dial itself, and still others by the pressure of
the subscriber's finger on a button. The first of these groups consists
of the springs _7_ and _8_, the function of which is to control the
continuity of the ground connection at the subscriber's station. The
arrangement of these two springs is such that the ground connection will
be broken until the subscriber's receiver is removed from its hook. As
soon as the receiver is raised, these springs come together in an
obvious manner, the dog _4_ being lifted out of the way by the action of
the hook. The ledge on the lower portion of the spring _7_ serves as a
rest for the insulated arm of the dog _4_ to prevent this dog, which is
spring actuated, from returning and locking the dial until after the
receiver has been hung up.
Bell and Transmitter Springs:--The second group is that embracing the
springs _9_, _10_, _11_, and _12_. The springs _10_ and _11_ are
controlled by the lower projection from the switch hook, the spring
_11_ engaging the spring _12_ only when the hook is down. The spring
_10_ engages the spring _9_ only when the hook lever is up and not then
unless the dial is in its normal position. While the hook is raised,
therefore, the springs _9_ and _10_ break contact whenever the dial is
moved and make contact again when it returns to its normal position. The
springs _11_ and _12_ control the circuit through the subscriber's bell
while the springs _9_ and _10_ control the continuity of the circuit
from one side of the line to the other so as to isolate the limbs from
each other while the signal transmitter is sending its impulses to the
central office.
Impulse Springs:--The third group embraces springs _13_, _14_, and _15_
and these are the ones by which the central-office switches are
controlled in building up a connection.
Something of the prevailing nomenclature which has grown up about the
automatic system may be introduced at this point. The movements of the
selecting switches at the central office are referred to as _vertical_
and _rotary_ for obvious reasons. On account of this the magnet which
causes the vertical movement is referred to as the _vertical magnet_ and
that which accomplishes the _rotary_ movement as the _rotary magnet_. It
happens that in all cases the selecting impulses sent by the
subscriber's station, corresponding respectively to the number of digits
in the called subscriber's number, are sent over one side of the line
and in nearly all cases these selecting impulses actuate the vertical
movements of the selecting switches. For this reason the particular limb
of the line over which the selecting impulses are sent is called the
_vertical limb_. The other limb of the line is the one over which the
single impulse is sent after each group of selecting impulses, and it is
this impulse in every case which causes the selector switch to start
rotating in its hunt for an idle trunk. This side of the line is,
therefore, called _rotary_. For the same reasons the impulses over the
vertical side of the line are called _vertical impulses_ and those over
the rotary side, _rotary impulses_. The naming of the limbs of the line
and of the current impulses _vertical_ and _rotary_ may appear odd but
it is, to say the least, convenient and expressive.
Coming back to the functions of the third group of springs, _13_, _14_,
and _15_, _15_ may be called the _vertical spring_ since it sends
vertical impulses; _13_, the _rotary spring_ since it sends rotary
impulses; and _14_, the _ground spring_ since, when the hook is up, it
is connected with the ground.
On the segment _3_ there are ten projections or cams _16_ which, when
the dial is moved, engage a projection of the spring _15_. When the dial
is being pulled by the subscriber's finger, these cams engage the spring
_15_ in such a way as to move it away from the ground spring and no
electrical contact is made. On the return of the dial, however, these
cams engage the projection on the spring _15_ in the opposite way and
the passing of each cam forces this vertical spring into engagement with
the ground spring. It will readily be seen, therefore, by a
consideration of the spacing of these cams on the segment and the finger
holes in the dial that the number of cams which pass the vertical spring
_15_ will correspond to the number on the hole used by the subscriber in
moving the dial.
Near the upper right-hand corner of the segment _3_, as shown in Fig.
384, there is another projection or cam _17_, the function of which is
to engage the rotary spring _13_ and press it into contact with the
ground spring. Thus, the first thing that happens in the movement of the
dial is for the projection _17_ to ride over the hump on the rotary
spring and press the contact once into engagement with the ground
spring; and likewise, the last thing that happens on the return movement
of the dial is for the rotary spring to be connected once to the ground
spring after the last vertical impulse has been sent.
If both the rotary and vertical sides of the line are connected with the
live side of the central-office battery, it follows that every contact
between the vertical and the ground spring or between the rotary and the
ground spring will allow an impulse of current to flow over the vertical
or the rotary side of the line.
We may summarize the action of these impulse springs by saying that
whenever the dial is moved from its normal position, there is, at the
beginning of this movement, a single rotary impulse over the rotary side
of the line; and that while the dial returns, there is a series of
vertical impulses over the vertical side of the line; and just before
the dial reaches its normal position, after the sending of the last
vertical impulse, there is another impulse over the rotary side of the
line.
The mechanical arrangements of the interrupter segment _3_ and its
associated parts have been greatly distorted in Fig. 384 in order to
make clear their mode of operation. This drawing has been worked out
with great care, with this in mind, at a sacrifice of accuracy in regard
to the actual structural details.
Ringing Springs:--The fourth group of springs in the subscriber's
telephone is the ringing group and embraces the springs _18_, _19_, and
_20_. The springs _19_ and _20_ are normally closed and maintain the
continuity of the talking circuit. When, however, the button attached to
the spring _19_--which button may be seen projecting from the instrument
shown in Fig. 382, and from the base of the one shown in Fig. 383--is
pressed, the continuity of the talking circuit is interrupted and the
vertical side of the line is connected with the ground. It is by this
operation, after the connection has been made with the desired
subscriber's line, that the central-office apparatus acts to send
ringing current out on that line.
Release Springs:--The fifth set of springs is the one shown at the
left-hand side of Fig. 384, embracing springs _21_, _22_, and _23_. The
long curved spring _21_ is engaged by the projecting lug on the switch
hook when it rises so as to press this spring away from the other two.
On the return movement of the hook, however, this spring is pressed to
the left so as to bring all three of them into contact, and this, it
will be seen, grounds both limbs of the line at the subscriber's
station. This combination cannot be effected by any of the other springs
at any stage of their operation, and it is the one which results in the
energization of such a combination of relays and magnets at the central
office as will release all parts involved in the connection and allow
them to return to their normal positions ready for another call.
_Salient Points._ If the following things are borne in mind about the
operation of the subscriber's station apparatus, an understanding of the
central-office operations will be facilitated. First, the selective
impulses always flow over the vertical side of the line; they are always
preceded and always followed by a single impulse over the rotary side of
the line. The ringing button grounds the vertical side of the line and
the release springs ground both sides of the line simultaneously.
=The Line Switch.= The first thing to be considered in connection with
the central-office apparatus is the line switch. This, it will be
remembered, is the device introduced into each subscriber's line at the
central office for the purpose of effecting a reduction of the number of
first selectors required at the central office, and also for bringing
about certain important functional results in connection with trunking
between central and sub-offices. The function of the line switch in
connection with the subscriber's line, however, is purely that of
reducing the number of first selectors.
The line switches of one hundred lines are all associated to form a
single unit of apparatus, which, besides the individual line switches,
includes certain other apparatus common to those lines. Such a group of
one hundred line switches and associated common apparatus is called a
_line-switch unit_, or frequently, a _Keith unit_. Confusion is likely
to arise in the mind of the reader between the individual line switch
and the line-switch unit, and to avoid this we will refer to the piece
of apparatus individual to the line as the line switch, and to the
complete unit formed of one hundred of these devices as a line-switch
unit.
_Line and Trunk Contacts._ Each line switch has its own bank of contacts
arranged in the arc of a circle, and in this same arc are also placed
the contacts of each of the ten individual trunks which it is possible
for that line to appropriate. The contacts individual to the
subscriber's line in the line switch are all multipled together, the
arrangement being such that if a wedge or plunger is inserted at any
point, the line contacts will be squeezed out of their normal position
so as to engage the contacts of the trunk corresponding to the
particular position in the arc at which the wedge or plunger is
inserted. A small plunger individual to each line is so arranged that it
may be thrust in between the contact springs in the line-switch bank in
such manner as to connect any one of the trunks with the line terminals
represented in that row, the particular trunk so connected depending on
the portion of the arc toward which the plunger is pointed at the time
it is thrust in the contacts.
These banks of lines and trunk contacts are horizontally arranged, and
piled in vertical columns of twenty-five line switches each. The ten
trunk contacts are multipled vertically through the line-switch banks,
so that the same ten trunks are available to each of the twenty-five
lines. We thus have, in effect, an old style, Western Union, cross-bar
switchboard, the line contacts being represented in horizontal rows and
the trunk contacts in vertical rows, the connection between any line and
any trunk being completed by inserting a plunger at the point of
intersection of the horizontal and the vertical rows corresponding to
that line and trunk.
_Trunk Selection._ The plungers by which the lines and trunks are
connected are, as has been said, individual to the line, and all of the
twenty-five plungers in a vertical row are mounted in such manner as to
be normally held in the same vertical plane, and this vertical plane is
made to oscillate back and forth by an oscillating shaft so as always
_to point the plungers toward a vertical row of trunk contacts that
represent a trunk that is not in use at the time_. The to-and-fro
movement of this oscillating shaft, called the _master bar_, is
controlled by a master switch and the function of this master switch is
always to keep the plungers pointed toward the row of contacts of an
idle trunk. The thrusting movement of the individual plungers into the
contact bank is controlled by magnets individual to the line and under
control of the subscriber in initiating a call. As soon as the plunger
of a line has been thus thrust into the contact bank so as to connect
the terminals of that line with a given trunk, the plunger is no longer
controlled by the master bar and remains stationary. The master bar then
at once moves all of the other plungers that are not in use so that they
will point to the terminals of another trunk that is not in use. The
plungers of all the line switches in a group of twenty-five are,
therefore, subject to the oscillating movements of the master bar when
the line is not connected to a first selector trunk. As soon as a call
is originated on a line, the corresponding plunger is forced into the
bank and is held stationary in maintaining the connection to a first
selector trunk, and all of the other plungers not so engaged, move on so
as to be ready to engage another idle trunk.
_Trunk Ratio._ The assignment of ten trunks to twenty-five lines would
be a greater ratio of trunks than ordinary traffic conditions require.
This ratio of trunks to lines is, however, readily varied by multipling
the trunk contacts of several twenty-five line groups together. Thus,
ten trunks may be made available to one hundred subscribers' lines by
multipling the trunks of four twenty-five line switch groups together.
In this case the four master bars corresponding to the four groups of
twenty-five line switches are all mechanically connected together so as
to move in unison under the control of a single master switch. If more
than ten and less than twenty-one trunks are assigned to one hundred
lines, then each set of ten trunks is multipled to the trunk contacts of
fifty line switches, the two master bars of these switches being
connected together and controlled by a common master switch.
_Structure of Line Switch._ The details of the parts of a line switch
that are individual to the line are shown in Fig. 385, the line and
trunk contact bank being shown in the lower portion of this figure and
also in a separate view in the detached figure at the right. A detailed
group of several such line switches with the oscillating master bar is
shown in Fig. 386. This figure shows quite clearly the relative
arrangement of the line and trunk contact banks, the plungers for each
bank, and the master bar.
[Illustration: Fig. 385. Line Switch]
In practice, four groups of twenty-five line switches each are mounted
on a single framework and the group of one hundred line switches,
together with certain other portions of the apparatus that will be
referred to later, form a line-switch unit. A front view of such a unit
is shown in Fig. 387. In order to give access to all portions of the
wiring and apparatus, the framework supporting each column of fifty line
switches is hinged so as to open up the interior of the device as a
whole. A line-switch unit thus opened out is shown in Fig. 388.
[Illustration: Fig. 386. Portion of Line-Switch Unit]
_Circuit Operation._ The mode of operation of the line switch may be
best understood in connection with Fig. 389, which shows in a schematic
way the parts of a line switch that are individual to a subscriber's
line, and also those that are common to a group of fifty or one hundred
lines. Those portions of Fig. 389 which are individual to the line are
shown below the dotted line extending across the page. The task of
understanding the line switch will be made somewhat easier if Figs. 385
and 389 are considered together. The individual parts of the line switch
are shown in the same relation to each other in these two figures with
the exception that the bank of line and trunk springs in the lower
right-hand corner of Fig. 389 have been turned around edgewise so as to
make an understanding of their circuit connections possible.
[Illustration: Fig. 387. Line-Switch Unit]
[Illustration: Fig. 388. Line-Switch Unit]
[Illustration: Fig. 389. Circuits of Line-Switch Unit]
The vertical and rotary sides of the subscriber's line are shown
entering at the lower left-hand corner of this figure, and they pass to
the springs of the contact bank. Immediately adjacent to these springs
are the trunk contacts from which the vertical and the rotary limbs of
the first selector trunk proceed. The plunger is indicated at _1_, it
being in the form of a wheel of insulating material. It is carried on
the rod _2_ pivoted on a lever _3_, which, in turn, is pivoted at _4_ in
a stationary portion of the framework. A spring _5_, secured to the
underside of the lever _3_ and projecting to the left beyond the pivot
_4_ of this lever, serves always to press the right-hand portion of the
lever _3_ forward in such direction as to tend to thrust it into the
contact bank. The plunger is normally held out of the contact bank by
means of the latch _6_ carried on the armature _7_ of the trip magnet.
When the trip magnet is energized it pulls the armature _7_ to the left
and thus releases the plunger and allows it to enter the contact bank.
[Illustration: POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE Mercury Arc
Rectifier Panel and Transformer at Right.]
The master bar is shown at _8_, and a feather on this bar engages a
notch in the segment attached to the rear end of the plunger rod _2_.
This master bar is common to all of the plunger rods and by its
oscillatory movement, under the influence of the master switch, it
always keeps all of the idle plunger bars pointed toward the contacts of
an idle trunk. As soon, however, as the trip magnet is operated to cause
the insertion of a plunger into the contact bank, the feather on the
master bar is disengaged by the notch in the segment of the plunger rod,
and the plunger rod is, therefore, no longer subject to the oscillating
movement of the master bar.
When the release magnet is energized, it attracts its armature _9_ and
this lifts the armature _7_ of the trip magnet so that the latch _6_
rides on top of the left-hand end of the lever _3_. Then, when the
release magnet is de-energized, the spring _5_, which was put under
tension by the latch, moves the entire structure of levers back to its
normal position, withdrawing the plunger from the bank of contacts. The
notch on the edge of the segment of the plunger rod, when thus released,
will probably not strike the feather on the master bar, and the plunger
rod will thus not come under the control of the master bar until the
master bar has moved, in its oscillation, so that the feather registers
with the notch, after which this bar will move with all the others.
If, while the plunger is waiting to be picked up by the master bar, the
same subscriber should call again, his line will be connected with the
same trunk as before. There is no danger in this, however, that the
trunk will be found busy, because the master bar will not have occupied
a position which would make it possible for any of the lines to
appropriate this trunk during the intervening time.
_Master Switch._ Associated with each master bar there is a master
switch which determines the position in which the master bar shall stop
in order that the idle plungers may be pointed always to the contacts of
an idle trunk. The arm _10_ of this switch is attached to the master bar
and oscillates with it and serves to connect the segment _11_
successively with the contacts _12_, which are connected respectively to
the third, or release wire of each first selector trunk. In the figure
the arm _10_ is shown resting on the sixth contact of the switch and
this sixth contact is connected to a spring _13_ in the line-switch
contact bank that has not yet been referred to. As soon as the plunger
is inserted into the contact bank, the spring _14_ will be pressed into
engagement with the spring _13_, and this spring _14_ is connected with
the live side of the battery through the release magnet winding.
The contact strip _11_ on the master switch is thus connected through
the release magnet to the battery and from this current flows through
the left-hand winding of the master-switch relay. This energizes this
relay and causes the closure of the circuit of the locking magnet which
magnet unlocks the master bar to permit its further rotation. The
unlocking of the master bar brings the spring _15_ into engagement with
_16_ and thus energizes the master magnet, the armature of which
vibrates back and forth after the manner of an electric-bell armature,
and steps the wheel _17_ around. The wheel _17_ is mechanically
connected to the master bar so that each complete revolution of the
wheel will cause one complete oscillation of the master bar. The master
bar will thus be moved so as to cause all the idle plungers to sweep
through an arc and this movement will stop as soon as the master-switch
arm _10_ connects the arc _11_ with one of the contacts _12_ that is not
connected to the live side of the battery through the springs _13_ and
_14_ of some other line switch. It is by this means that the plungers of
the line switches are always kept pointing at the contacts of an idle
trunk. The way in which this feature has been worked out must demand
admiration and accounts for the marvelous quickness of this line switch.
The fact that the plungers are pointed in the right direction before the
time comes for their use, leaves only the simple thrusting motion of the
plunger to accomplish the desired connection immediately upon the
initiation of a call by the subscriber.
_Locking Segment._ It will be understood that the locking segment _18_
and the master-switch contact finger _10_ are both rigidly connected
with the master bar _8_ and move with it, the locking segment _18_
serving always to determine accurately the angular position at which the
master bar and the master-switch arm are brought to rest.
_Bridge Cut-Off._ One important feature of automatic switching,
particularly as exemplified in the system of the Automatic Electric
Company, is the disconnection, after its use, of each operating magnet
of each piece of apparatus involved in making a connection. Since these
operating magnets are always bridged across the line at the time of
their operation and then cut off after they have performed their
function, this feature may be referred to as the _bridge cut-off_.
_Guarding Functions._ Still another feature of importance is the means
for guarding a line or a piece of apparatus that has already been
appropriated or made busy, so that it will not be appropriated or
connected with for use in some other connection. For this latter purpose
contacts and wires are associated with each piece of apparatus, which
are multipled to similar contacts on other pieces of apparatus in much
the same way and for a similar purpose that the test thimbles in a
multiple switchboard are multipled together. Such wires and contacts in
the Automatic Electric Company's apparatus are called _private wires_
and _contacts_.
The bridge cut-off and guarding functions are provided for in the line
switch by a bridge cut-off relay shown in Fig. 389 and also in Fig. 385,
it being the upper one of the individual line relays in each of those
figures. This bridge cut-off relay is operated as soon as the plunger of
the line is thrust into the bank; the contacts _19_ and _20_, closed by
the plunger, serving to complete the circuit of this relay. To make
clear the bridge cut-off feature it will be noted that the trip magnet
of a line switch is connected in a circuit traced from the rotary side
of the line through the contacts _21_ and _22_ of the bridge cut-off
relay, thence through the coil of the trip magnet to the common wire
leading to the spring _23_ of the master-bar locking device and thence
to the live side of the battery. Obviously, therefore, as soon as the
bridge cut-off relay operates, the trip magnet becomes inoperative and
can cause no further action of the line switch because its circuit is
broken between the springs _21_ and _22_.
The private or guarding feature is taken care of by the action of the
plunger in closing contacts _19_ and _20_, since the private wire
leading to the bridge cut-off relay is, as has already been stated,
connected to ground when these contacts are closed. This private wire
leads off and is multipled to the private contacts on all the connectors
that have the ability to reach this line, and the fact that this wire
is grounded by the line switch as soon as it becomes busy, establishes
such conditions at all of the connectors that they will refuse to
connect with this line as long as it is busy, in a way that will be
pointed out later on.
_Relation of Line Switch and Connectors._ The vertical and rotary wires
of the subscriber's line are shown leading off to the connector banks at
the left-hand side of Fig. 389, and one side of this connection passes
through the contacts _24_ and _25_ of the bridge cut-off relay on the
line switch. It is through this path that a connection from some other
line through a connector to this line is established and it is seen that
this path is held open until the bridge cut-off relay of the line switch
is operated. For such a connection to this line the bridge cut-off relay
of the line switch is operated over the private wire leading from the
connector, and the operation of the bridge cut-off relay at this time
serves to render inoperative the line switch, so that it will not
perform its usual functions should the called subscriber start to make a
call after his line had been seized.
_Summary of Line-Switch Operation._ To summarize the operation of a line
switch when a call is originated on its line, the first movement of the
calling subscriber's dial will ground the rotary side of the line and
operate the trip magnet. This will cause the plunger to be inserted into
the bank, and thus extend the line to the first selector trunk through
the closing of the right-hand set of springs shown in the lower
right-hand corner of Fig. 389. The insertion of the plunger will also
connect the battery through the left-hand winding of the master-switch
relay and, by the sequence of operations which follows, cause the master
bar to move all of the idle plungers so as to again point them to an
idle trunk. The closure of contacts _19_ and _20_ by the plunger causes
the operation of the bridge cut-off relay which opens the circuit of the
trip magnet, rendering it inoperative; and also establishes ground
potential on all the private wire contacts of that line in the banks of
the connectors, so as to guard the line and its associated apparatus
against intrusion by others. The line is cut through, therefore, to a
first selector and all of the line-switch apparatus is completely cut
off from the talking circuit.
It must be remembered that all of the actions of the line switch, which
it has taken so long to describe, occur practically instantaneously and
as a result of the first part of the first movement of the subscriber's
dial. The line switch has done its work and "gone out of business"
before the selective impulses of the first digit begin to take place.
=Selecting Switches.= The first selector is now in control of the
calling subscriber. The circuits and elements of the first selector
switch are shown in Fig. 390. The general mechanical structure of the
first selectors, second selectors, and connectors, is the same and may
be referred to briefly here. Fig. 391 shows a rear view of a first
selector; Fig. 392, a side view of a second selector; and Fig. 393, a
front view of a connector. The arrangement of the vertical and rotary
magnets, of the selector shafts, and of the contact banks are identical
in all three of these pieces of apparatus and all these switches work on
the "up-and-around principle" referred to in connection with Fig. 380.
It is thought that with the general structure shown in Figs. 391, 392,
and 393 in mind, the actual operation may be understood much more
readily from Fig. 390.
Four magnets--the vertical, the rotary, the private, and the
release--produce the switching movements of the machine. These magnets
are controlled by various combinations brought upon the circuits by
three relays--the vertical, the rotary, and the back release. The fourth
relay shown, called the _off-normal_, is purely for signaling purposes,
as will be described.
_Side Switch._ Another important element of the selecting switches is
the so-called side switch which might better be called a pilot
switch--but we are not responsible for its name. This side switch has
for its function the changing of the control of the subscriber's line to
successive portions of the selector mechanism, rendering inoperative
those portions that have already performed their functions and that,
therefore, are no longer needed. This switch may be seen best in Fig.
392 just above the upper bank of contacts. It is shown in Fig. 390
greatly distorted mechanically so as to better illustrate its electrical
functions.
[Illustration: Fig. 390. Circuits of First Selector]
The contact levers _1_, _2_, _3_, and _4_ of the side switch are carried
upon the arm _5_ which is pivoted at _6_. All of these contact levers,
therefore, move about _6_ as an axis. The side switch has three
positions and it is shown, in Fig. 390, in the first one of these. When
the private magnet armature is attracted and released once, the
escapement carried by it permits the spring _7_ to move the arm _5_ so
as to bring the wipers of the side switch into its second position; the
second pulling up and release of the private magnet armature will cause
the movement of the side switch wipers into the third position. It is to
be noted that the escapement which releases the side switch arm may be
moved either by the private or by the rotary magnet, since the armature
of the latter has a finger which engages the private magnet armature.
[Illustration: Fig. 391. Rear View of First Selector]
_Functions of Side Switch._ The functions of the side switch may be
briefly outlined in connection with the first selector, as an example.
In the first position it extends the control of the subscriber's signal
transmitter through the first selector trunk and line relays to the
vertical and private magnets so that these magnets will be responsive to
the selecting impulses corresponding to the first digit. In its second
position it brings about such a condition of affairs that the rotary
magnet will be brought into play and automatically move the wipers over
the bank contacts in search of an idle trunk. In its third position,
both the vertical and rotary relays are cut off and the line is cut
straight through to the second selector trunk, and only those parts of
the first selector apparatus are left in an operative state which have
to do with the private or guarding circuits and with the release.
Similar functions are performed by the side switch in connection with
the other selecting switches.
[Illustration: Fig. 392. Side View of Second Selector]
_Release Mechanism._ Another one of the features of the switch that
needs to be considered before a detailed understanding of its operation
may be had, is the mechanical relation of the holding and the release
dog. This dog is shown at _8_ and, in the language of the art, is called
the _double dog_. As will be seen, it has two retaining fingers, one
adapted to engage the vertical ratchet and the other, the rotary ratchet
on the selector shaft. This double dog is pivoted at _9_ and is
interlinked in a peculiar way with the armature of the vertical magnet,
the armature of the release magnet, and the arm of the side switch. The
function of this double dog is to hold the shaft in whatever vertical
position it is moved by the vertical magnet and then, when the rotary
magnet begins to operate, to hold the shaft in its proper angular
position. It will be noted that the fixed dog _10_ is ineffective when
the shaft is in its normal angular position. But as soon as the shaft is
rotated, this fixed dog _10_ becomes the real holding pawl so far as the
vertical movement is concerned. The double dog _8_ is normally held out
of engagement with the vertical and the rotary ratchets by virtue of the
link connection, shown at _11_, between the release magnet armature and
the rear end of the double dog. On the previous release of the switch
the attraction of the release magnet armature permitted the link _11_ to
hook over the end of the dog _8_ and thus, on its return movement, to
pull this dog out of engagement with its ratchets. This movement also
resulted in pushing on the link _12_ which is pivoted to the side switch
arm _5_, and thus the return movement of the release magnet is made to
restore the side switch to its normal position. In order that the double
dog may be made effective when it is required, and in order that the
side switch may be free to move under the influence of the private
magnet, the double dog is released from its connection with the release
magnet armature by the first movement of the vertical magnet in a manner
which is clear from the drawing.
=First Selector Operation.= In discussing the details of operation of
the various selectors it will be found convenient to divide the
discussion according to the position of the side switch. This will bring
about a logical arrangement because it is really the side switch which
determines by its position the sequence of operation.
[Illustration: Fig. 393. Front View of Connector]
_First Position of Side Switch._ This is the position shown in Fig. 390,
and is the normal position. The vertical and the rotary lines extending
from the calling subscriber are continued by the levers _1_ and _2_ of
the side switch through the vertical and the rotary relay coils,
respectively, to the live side of battery. The lever _4_ of the side
switch in this position connects to ground the circuit leading from the
line switch through the release trunk, and the winding of the off-normal
relay. This winding is thus put in series with the release magnet of
the line switch, but on account of high resistance of the off-normal
relay no operation of the release magnet is caused. This will, however,
permit such current to flow through the release circuit as will energize
the sensitive off-normal relay and cause it to attract its armature and
light the off-normal lamp. If this lamp remains lighted more than a
brief period of time, it will attract notice and will indicate that the
corresponding selector has been appropriated by a line switch and that
for some reason the selector has gone no further. This lamp, therefore,
is an aid in preventing the continuance of this abnormal condition.
The first thing that happens after the line switch has connected the
calling subscriber with the first selector is a succession of impulses
over the vertical side of the line, this being the set of impulses
corresponding in number to the thousands digit or to the office, if
there is more than one. It will be understood that here we are
considering a single office of ten-thousand-line capacity or
thereabouts, and that, therefore, this first set of impulses corresponds
to the thousands digit in the called subscriber's line. Each one of
these impulses will flow from the battery through the vertical relay and
each movement of this relay armature will close the circuit of the
vertical magnet and cause the shaft of the selector to be stepped up to
the proper level. Immediately following the first series of selecting
impulses from the subscriber's station, a single impulse follows over
the rotary side of the line. This gives the rotary relay armature one
impulse and this in turn closes the circuit of the private magnet once.
The single movement of the private magnet armature allows the escapement
finger on the arm _5_ to move one step and this brings the side switch
contacts into the second position.
_Second Position of Side Switch._ In this position lever _4_ of the side
switch places a ground on the wire leading through the rotary magnet to
a source of interrupted battery current. The impulses which thus flow
through the rotary magnet occur at a frequency dependent upon the
battery interrupter and this is at a rate of approximately fifteen
impulses per second. The rotary magnet will step the selector shaft
rapidly around until something occurs to stop these impulses. This
something is the finding by the private wiper of an ungrounded private
contact in the bank, since all of the contacts corresponding to busy
trunks are grounded, as will be explained.
The action of the private magnet enters into this operation in the
following way: A circuit may be traced from the battery through the
private magnet to the third side switch wiper when in its second
position, thence through the back release relay to the private wiper. If
the wiper is at the time on the private bank contact of a busy trunk, it
will find that contact grounded and the private magnet will be
energized. The energizing of this magnet will not, however, cause the
release of the side switch. It must be energized and de-energized. The
private magnet armature will, therefore, be operated by the finger of
the rotary magnet armature on the first rotary step. The private magnet
will be energized and hold its armature operated if the private wiper
finds a ground on the first bank contact and will stay energized as long
as the private wiper is passing over private contacts of busy trunks.
Its armature will not be allowed to fall back during the passage of the
wiper from one trunk to another, because during that interval the finger
of the rotary magnet will hold it operated. As soon, however, as the
private wiper reaches the private bank contact of an idle trunk, no
ground will be found and the circuit of the private magnet will be left
open. When the impulse through the rotary magnet ceases, the private
magnet armature will fall back and the side switch will be released to
its third position.
_Third Position of Side Switch._ The first thing to be noted in this
position is that the calling line is cut straight through to the second
selector trunk, the connection being clean with no magnets bridged
across or tapped off. The third wiper of the side switch, when in its
third position, is grounded and this connects the release wire of the
second selector trunk, on which the switch wipers rest, through the
private wiper, the winding of the back release magnet, and the third
wiper of the side switch to ground. This establishes a path for the
subsequent release current through the back release magnet; and, of
equal importance, it places a ground on the private bank contact of that
trunk so that the private wiper of any other switch will be prevented
from stopping on the contacts of this trunk in the same manner that the
wiper of this switch was prevented from stopping on other trunks that
were already in use.
The fourth lever on the side switch, when in its third position, serves
merely to close the circuit of the rotary off-normal lamp. This lamp is
for the purpose of calling attention to any first selector switch that
has been brought into connection with some second selector trunk and
which, for some reason, has failed in its release. These off-normal
lamps are so arranged that they may be switched off manually to avoid
burning them during the hours of heaviest traffic. At night they afford
a ready means of testing for switches that have been left off-normal,
since the manual switches controlling these lamps may then be closed,
and any lamps which burn will show that the switches corresponding to
them are off-normal. Simple tests then suffice to show whether they are
properly or improperly in their off-normal position.
_Release of the First Selector._ As will be shown later, the normal way
of releasing the switches is from the connector back over the release
wire. It is sufficient to say at this point that when the proper time
for release comes, an impulse of current will come back over the second
selector trunk release wire through the private wiper, to the back
release relay magnet, and thence to ground through the third wiper of
the side switch which is in its third position. It may be asked why the
back release magnet was not energized during the previous operations
described, when current passed through it. The reason for this is that
in those previous operations the private magnet was always included in
series in the circuit and on account of the high resistance of the
private magnet, sufficient current did not pass through the back release
magnet to energize it.
When the back release relay is energized, it closes the circuit of the
release magnet and thus, through the link _11_, draws the double dog
away from its engagement with the shaft ratchets and at the same time,
through the link _12_, restores the side switch to its normal position.
Whenever the release magnet is operated it acts as a relay to close a
pair of contacts associated with it and thus to momentarily ground the
release wire of the first selector trunk extending back to the line
switch. Referring to Fig. 389, it will be seen that this path leads
through the contacts _13_ and _14_ and the release magnet to the
battery. It is by this means that the line switch is released, the
release impulse being relayed back from the first selector.
=Second Selector Operation.= For the purpose of considering the action
of the second selector, we will go back to the point where the first
selector had connected with a second selector trunk and where its side
switch had moved into its third position. In this condition, it will be
remembered, the trunk line was cut through to a second selector trunk
and all first selector apparatus cleared from the talking circuit.
The second selector chosen is one corresponding to the thousands group
as determined by the first digit of the called subscriber's number. The
circuits of a second selector are shown in Fig. 394 and it must be borne
in mind that the mechanical arrangements for producing the vertical and
the rotary movement of the shaft and for operating the side switch are
practically the same as those of the first selector. As in the first
selector, the sequence of operation is controlled by the successive
positions of the side switch, the first position permitting the
selection of the hundreds corresponding to the vertical impulses, the
second position allowing the selector to search for an idle trunk in
that hundred, and the third position cutting the trunk through and
clearing the circuit of obstructing apparatus.
_First Position of Side Switch._ The first thing that happens when the
subscriber begins to move his dial in the transmission of the second
series of selecting impulses is the sending of a preliminary impulse
over the rotary side of the line. This, in the case of the second
selector, energizes the rotary relay which, in turn, energizes the
private magnet; but the private magnet in the case of the second
selector can do nothing toward the release of the side switch because
the projection _5'_, on the side switch arm _5_, meets a projection on
the rear of the selector shaft which thus prevents the movement of the
side switch arm _5_ until the selector shaft has been moved out of its
normal position.
Immediately after the establishment of the connection to the selector,
the second set of selecting impulses comes in over the vertical wire
from the subscriber's station. These impulses, corresponding in number
to the hundreds digit, will energize the vertical relay and cause it, in
turn, to energize the vertical magnet, stepping up the selector shaft to
the level corresponding to the hundred sought. The single rotary
impulse, which follows just before the subscriber's dial reaches its
normal position, will energize the rotary relay of the second selector.
This, in turn, energizes the private magnet which makes a single
movement of its armature and allows the escapement finger on the side
switch arm to move one step and bring the side switch contacts into the
second position.
[Illustration: Fig. 394. Circuits of Second Selector]
_Second Position of Side Switch._ No detailed discussion of this is
necessary, since, with the side switch in its second position, the
actions which occur in causing the wipers of the second selector to seek
and connect with an idle trunk line, are exactly the same as in the case
of the first selector. When the second selector wipers finally reach a
resting place on the bank contacts, the private magnet armature,
operated during the hunting process, is released and the side switch is
thus shifted into the third position.
_Third Position of Side Switch._ The moving of the side switch into its
final position brings about the same state of affairs with respect to
the second selector that already exists with respect to the first
selector. The trunk line is cut straight through and all bridge circuits
or by-paths from it are cut off. The same guarding conditions are
established to prevent other lines or other pieces of apparatus from
making connections that will interfere with the one being established,
and the same provisions are made for working the back release when the
proper impulse comes from the connector, and for passing this back
release impulse on to the first selector in the same way that the first
selector passes it on to the line switch. The line of the calling
subscriber has now been extended to a connector, and that connector is
one of a group--usually ten--which alone has the ability to reach the
particular hundred lines containing the line of the desired subscriber.
The selection has, therefore, been narrowed down from one in ten
thousand to one in one hundred.
=The Connector=--_Its Functions._ It has already been stated that the
connector is of the same general type of apparatus as the first and the
second selectors. Unlike the first and the second selectors, however,
the connector is required to make a double selection under the guidance
of the subscriber. The first selector makes a single selection of a
group under the guidance of the subscriber and then an automatic
selection in that group not controlled by the subscriber. So it is with
the second selector. The connector, however, makes a selection of a
group of ten under the guidance of the subscriber and then, again under
the guidance of the subscriber, it picks out a particular one of that
group.
The connector also has other functions in relation to the ringing of
the called subscriber and the giving of a busy signal to the calling
subscriber in case the line wanted is found busy. It has still other
functions in that the talking current, which is finally supplied to
connected subscribers, is supplied through paths furnished by it.
_Location of the Connectors._ Connectors are the only ones of the
selecting switches that are in any sense individual to the subscribers'
lines. None of them is individual to a subscriber's line, but it may be
said that a group of ten connectors is individual to a group of one
hundred subscribers' lines. Since each group of one hundred lines has a
group of connectors of its own and since each one hundred lines also has
a line-switch unit of its own, and since the lines of this group must be
multipled through the bank contacts of the connectors of this individual
group and through the bank contacts of the line switches of this
particular unit, it follows that on account of the wiring problems
involved there is good reason for mounting the connectors in close
proximity to the line switches representing the same group of lines.
Some help in the grasping of this thought may result if it be remembered
that the line switch is, so to speak, the point of entry of a call and
that the connector is the point of exit, and, in order to reduce the
amount of wiring and to economize space, the point of exit and the point
of entry are made as close together as possible.
The relative locations and grouping of the line switches and connectors
are clearly shown in Fig. 395, which is a rear view of the same
line-switch unit that was illustrated in Figs. 387 and 388.
[Illustration: GAS ENGINE AND POWER BOARD Citizens' Telephone Co.,
Racine, Wis. _The Dean Electric Co._]
=Operation of the Connector.= The circuits of the connector are shown in
Fig. 396. In addition to the features that have been pointed out in the
first and the second selectors, all of which are to be found, with some
modifications, perhaps, in the connector, there must be considered the
features in the connector of busy-signal operation, of ringing the
called subscriber, of battery supply to both subscribers, and of the
trunk release operation. These may be best understood by tracing through
the operations of the connector from the time it is picked up by a
second selector until the connection is finally completed, or until the
busy signal has been given in case completion was found impossible. As
in the first and the second selectors, the sequence of operations is
determined by the position of the side switch.
[Illustration: Fig. 395. Connector Side of Line-Switch Unit]
[Illustration: Fig. 396. Circuits of Connector]
_First Position of Side Switch._ The connector in a ten-thousand-line
system is the recipient of the impulses resulting from the third and
fourth movements of the subscriber's dial. Considering the third
movement of the subscriber's dial, the first impulse resulting from it
comes over the rotary side of the line and results in the rotary relay
attracting its armature once. This results in a single impulse through
the private magnet which, however, does nothing because the projection
_5'_ strikes against a projection on the selector shaft. These two
projections interfere only when the selector shaft is in its normal
position. Then follows the series of impulses from the subscriber's
station corresponding to the tens digit in the called subscriber's
number. These pass over the vertical side of the line and through the
vertical relay, energizing that relay a corresponding number of times.
The vertical magnet, as in the case of the first and the second
selectors, is included in the circuit controlled by the vertical relay
and this results in the connector shaft being stepped up to the level
corresponding to the particular tens group containing the called
subscriber's number. It will be noted that the impulses from the
vertical side of the line, which cause this selection, pass through one
winding _13_ of the calling battery supply relay. This relay is operated
by these vertical selecting impulses, but in this position of the side
switch the closure of its local circuits accomplishes nothing.
Immediately after the tens group of selecting impulses over the vertical
side of the line, there follows a single rotary impulse from the
subscriber's station which, as in the case of the first and the second
selectors, energizes the rotary relay and causes it to give one impulse
to the private magnet. This impulse is now able, since the shaft has
moved from its normal position, to release the side switch arm one
notch, and the side switch, therefore, moves into its second position.
_Second Position of Side Switch._ It is principally in this second
position of the side switch that the connector selecting function
differs from that of the first and the second selector. There is no
trunk to be hunted, but rather the rotary movement of the connector
wipers must be made in response to the impulses, from the subscriber's
station, which correspond to the units digit in the selected number. The
first impulse corresponding to the fourth movement of the subscriber's
dial is a rotary one, and, as usual, it passes through the rotary relay
winding and this, in turn, gives an impulse to the private magnet. The
private magnet at this time has already released the side switch arm to
its second position, but it is unable to release it further because of a
feather on the wiper shaft--which projects just far enough to engage the
lug _5'_, when the shaft is in its normal angular position--thus
preventing the side switch arm from moving farther than its second
position.
Then follows over the vertical side of the line the last set of
selecting impulses corresponding to the units digit. This, as before,
energizes the vertical relay, but in the second position of the side
switch, it is to be noted, that the vertical relay no longer controls
the vertical magnet; the side switch has shifted the control of the
vertical relay to the rotary magnet. The rotary magnet is, therefore,
energized a number of times corresponding to the last digit in the
called number and the wipers of the connectors are thus brought to the
contacts of the line sought--their final goal. At this point many things
may happen, and the things that do happen depend on whether the called
subscriber's line is idle or busy.
Called-Line Busy:--It will first be assumed that the called line is
busy. The testing operation at the connectors occurs in the second
position of the side switch. If the called line is busy, it will be
either because it is connected to by some other connector or because it
has itself made a call. In the former case the private contacts of that
line in the banks of all the connectors serving that hundreds group of
lines will be grounded through the private wiper of some other
connector. That this is so, may be seen by tracing the circuit from the
private wiper on the shaft to the third side switch wiper which is
grounded in the third position; the other connector that has already
engaged the line will, of course, have its side switch in its final, or
third position. Again, if the line called is busy, because a call has
already been made from this line to some other line, the private
contacts on the connectors corresponding to the line will be grounded,
as will be seen by tracing from the private bank contacts, which are
shown in Fig. 396, through the private wire to the line switch, which is
shown in Fig. 389, and from thence to ground through the springs _19_
and _20_, which are brought together when the line switch is operated.
In any event, therefore, the determining condition of a busy line is
that its private bank contacts on all connectors of its group shall be
grounded. Under the present assumed condition, therefore, the connector
wipers, which have been brought to the bank contacts of the desired
line, will find a ground at the private bank contact. The connector
shaft stops for an instant on the contacts of this busy line and
immediately there follows over the rotary side of the line the
inevitable single rotary impulse. This energizes the rotary relay and
this, as usual, energizes the private magnet. Remembering now that the
connector side switch is in its second position and that the private
wiper of the connector has found a ground, we may trace back from the
private wiper through the third side switch wiper to its second contact;
thence through the contact springs _14_ and _15_, closed by the private
magnet; thence through the release magnet; thence through the contact
springs _16_ and _17_ of the calling battery supply relay to the live
side of the battery. This calling battery supply relay will, at this
time, have its core energized because the coil _18_ is in series with
the rotary relay coil which, as just stated, was energized by the last
rotary impulse. This series of operations has led to the energizing of
the release magnet, and, as a result, the double dog of the connector is
pulled out of the connector shaft ratchets and the shaft and the side
switch are restored to their normal position.
Busy-Back Signal:--The connector has dropped back to normal in all
respects. The calling subscriber, not knowing this, presses his ringing
button. This grounds the vertical side of the line at his station and
operates the vertical relay at the connector. This steps the shaft of
the connector up one step and causes the closure of the contacts _19_
and _20_ at the top of the connector shaft. This establishes a
connection to a circuit carrying periodically interrupted battery
current on which an inductive hum is placed. This circuit may be traced
from this source through the springs _20_ and _19_ to the first wiper of
the side switch, thence through the normally closed contacts of the
ringing relay to the rotary side of the line, and the varying potential
to which this path is subjected produces an inductive flow back to the
calling subscriber's telephone, and gives him the necessary signal which
consists of a hum or buzzing noise with which all users of automatic
systems soon become familiar.
Release on Busy Connection:--The connector, since its last release, has
been stepped up one notch and must again be released. When the
subscriber hangs up his receiver after receiving the busy signal, he
grounds both sides of his line momentarily by the action of the springs
_21_, _22_, and _23_ of Fig. 384. This operates the rotary and the
vertical relays on the connector simultaneously and brings together for
the first time the springs _21_ and _22_ of Fig. 396. This establishes a
connection from the battery through the springs _16_ and _17_ on the
calling battery supply relay, thence through the release magnet of the
connector, thence through the springs _22_ and _21_ of the vertical and
the rotary relay, thence through the release trunk back to the second
selector. From here the circuit passes through the private wiper of that
selector and the back release relay to ground through the third side
switch wiper which is in the third position. Considering this circuit in
respect to its action on the connector it is obvious that it energizes
the release magnet on the connector which restores the connector to
normal as before. At the second selector this circuit passed through the
back release relay, which closed a circuit through the release magnet
and through the back release relay contacts, thence back over the second
selector release trunk to the back release relay of the first selector,
and through the third wiper of the side switch on that selector to
ground, since that side switch also is in its third position. The
current through this circuit energizes the release magnet of the second
selector and restores it to its normal position and also energizes the
back release relay of the first selector. This, in turn, closes the
circuit from the battery through the release magnet of the first
selector and contacts of the back release relay to ground. This works
the release magnet of the first selector and restores that selector to
normal. The contacts on the first selector release magnet, shown in Fig.
390, are closed by the action of the release magnet and this closes the
path from ground back through the first selector release wire, and
through the contacts _13_ and _14_ of the line switch, through the line
switch release magnet to battery, and this restores the line switch to
normal.
The reason for the term _back release_ will now be apparent. The release
operation at the connector is relayed back to the second selector; that
of the second selector back to the first selector; and that of the first
selector back to the line switch. Until this plan was adopted, the
release magnet of each selector and connector involved in a connection
was left bridged across the talking circuit so as to be available for
release; and it sometimes occurred that a first selector would be
released before a second selector or connector, which latter switches
would thus be left off-normal until rescued by an attendant. The back
release plan makes it impossible for the connection necessary for the
release of a switch to be torn down until the release is actually
accomplished.
Called Line Found Idle:--It will be remembered that, before the
digression necessary to trace through the operations occurring upon the
finding of a busy line, the connector wipers had been brought, by the
influence of the calling subscriber's impulses, into engagement with the
contacts of the desired line; that the connector side switch was in its
second position; and that the final rotary impulse following the last
series of selecting impulses had not been sent. The condition now to be
assumed is that the called subscriber's line is free and the private
wiper, therefore, has found and rests on an ungrounded private bank
contact. The final rotary impulse which immediately follows will operate
the rotary relay and this, in turn, will operate the private magnet.
This happened under the assumed condition that the line was busy, but in
that case the release magnet was also operated at the same time and
restored all conditions to normal. Under the present condition the
operation of the private magnet will perform its usual function and move
the side switch of the connector into its third position.
_Third Position of Side Switch._ When the side switch of the connector
moves to its third position, it, as usual, cuts the talking circuit
straight through from the vertical and the rotary sides of the trunk
leading from the previous selector to the outgoing terminal of the
subscriber's line, which may be traced upon Fig. 396 back through the
line switch, shown in Fig. 389. Several things are to be noted about the
talking circuit so established: First, the inclusion of the condensers
in the vertical and the rotary sides of the connector circuit. The
purpose of this will be referred to later. Second, the inclusion in this
circuit at the connector of a pair of normally closed contacts in the
ringing relay. It may be said in passing that the ringing relay
corresponds exactly in function to a ringing key in a manual
switchboard. Third, the talking circuit leading from the connector to
the called subscriber's line passes on one side through the springs _24_
and _25_ of the bridge cut-off relay of the line switch, which is shown
in Fig. 389. These springs are normally open and would prevent the
completion of the talking circuit but for the fact that the bridge
cut-off relay of the line switch is energized over the private wire
leading to the connector bank and then through the connector wiper to
the third side switch wiper which, at this time, is in its third
position. The talking circuit is thus complete. The operation of this
bridge cut-off relay on the line switch has not only completed the
talking circuit but it has also opened the circuit of the trip magnet of
the line switch so as to prevent the operation of the trip magnet by the
subscriber on that line in case he should attempt to make a call during
the interval between the time when his line was connected with by the
connector and the time when he answers the call.
The third wiper of the connector side switch when moved into its third
position, puts the ground on all of the private bank contacts of the
line chosen and thus guards that line against connection by others, as
already described. It also operates the bridge cut-off relay of the line
switch as just mentioned.
The fourth wiper of the side switch, when moved into its third position,
establishes such a connection as will place the ringing relay under the
control of the vertical relay. This may be seen by tracing from ground
to the vertical relay springs _23_ and _24_, thence through the normally
closed upper pair of contacts on the private magnet, thence through the
fourth wiper on the side switch to its third contact, thence through the
ringing relay magnet, and through the springs _16_ and _17_ of the
calling battery supply relay and to battery. The calling battery supply
relay winding being in series with the vertical relay winding, the two
operate together and close the two normally open points in the ringing
relay circuit. This ringing relay acts as an ordinary ringing key and
connects the generator to the called subscriber's line in an obvious
manner, at the same time opening the talking circuit back of the ringing
relay in order to prevent the ringing current chattering the relays in
the circuit back of it. All that remains now is for the called
subscriber to respond. When he does he closes the metallic circuit of
the line through his talking apparatus.
_Battery Supply to Connected Subscriber._ Throughout the whole process
of building up a connection, it will be remembered that both sides of
the calling line are connected through the respective vertical and
rotary relays involved in building up the connection with the live side
of the battery. At the time when the connection is finally established
and the called subscriber rung, both sides of the calling line are
connected through various relay windings to the live side of the
battery. Such a condition leaves both sides of the line at the same
potential and, therefore, there is no tendency for current to flow
through the calling subscriber's talking apparatus, even though it is
connected across the circuit of the line. It remains, therefore, to be
seen how these conditions are so changed after the building up of a
connection as to supply the calling subscriber with talking current.
The calling subscriber can get no current until the called subscriber
responds. When the connection is first made with the called subscriber's
line, battery connection to his line is made from the live side of
battery through the normally closed contacts of the calling battery
supply relay, thence through the winding _25_ of the called battery
supply relay to the vertical side of the called line. The grounded side
of the battery is connected to the rotary side of his line through the
third wiper of the connector and the coil _26_ of the called battery
supply relay. As a result, this subscriber receives proper talking
current through the coils _25_ and _26_, and this relay is operated by
the flow of this current. The operation of this called battery supply
relay merely shifts the connection of the rotary side of the calling
subscriber's line from its normal battery connection, to ground, and
thus the battery is placed straight across the calling subscriber's line
so as to supply talking current. This supply circuit to the calling
subscriber may be traced from the live side of the battery through the
winding _13_ of the calling battery supply relay and the winding of the
vertical relay to the vertical side of the line, and from the grounded
side of battery through the third side switch wiper in its third
position to the now closed pair of contacts in the called battery supply
relay through the coil _18_ of the calling battery supply relay and the
coil of the rotary relay to the rotary side of the line.
It will be noted that the system of battery supply is that of the
standard condenser and retardation coil scheme largely employed in
manual practice; and that aside from the coils through which the battery
current is supplied to the connected subscribers, there are no taps
from, or bridges across, the two sides of the talking circuit.
=Release after Conversation.= It remains now only to secure the
disconnection of the subscribers after they are through talking. When
the calling subscriber hangs up, the whole disconnection is brought
about, all of the apparatus, including connector, selectors, and line
switch, returning to normal. This is done by the back release system and
is accomplished in almost the same way as has already been described in
connection with the disconnect after an unsuccessful call. There is this
difference, however: after an unsuccessful call when the line called for
was found busy, the release was made while the connector side switch was
in its normal position. In the present case, the release must be made
with the connector side switch in its third position and with the
talking battery bridged across the metallic circuit rather than
connected between each limb of the line and ground. It must be
remembered that the calling battery supply relay, while traversed by
current during the conversation, is not magnetically energized because,
with the current flowing through the metallic circuit of the line, the
two windings exert a differential effect. As soon, however, as the
calling subscriber hangs up his receiver, this differential action
ceases, due to the grounding of both sides of the line at the
subscriber's station. This relay, therefore, operates and cuts off
battery from the called battery supply relay and this, in turn, releases
its armature and thus changes the connection of the rotary side of the
calling line from ground to live side of the battery. The normal
condition of the battery connection now being restored, both the
vertical and the rotary relays at the connector become operated, due to
the ground on both sides of the line at the subscriber's station, and
this, as we have seen, is the condition which brings about the operation
of the connector release magnet, and the relaying back of the disconnect
impulse successively through the selectors to the line switch.
=Multi-Office System.= In exchanges involving more than one office, the
same general principles and mode of operation already outlined apply. If
the total number of subscribers in the multi-office exchange is to be
less than ten thousand, then four digit numbers suffice, and the first
movement of the dial may be made to select the office into which the
connection is to go, the subscribers' lines being so numbered with
respect to the offices that each office will contain only certain
thousands. The choosing of the thousand by the calling subscriber,
therefore, takes care in itself of the choice of offices. Where,
however, a multi-office exchange is to provide for connections among a
greater number of lines than ten thousand and less than one hundred
thousand, then it will take five movements of the dial to make the
selection--the five movements corresponding either to the five digits in
a number or to the name of an office, as indicated on the dial, and the
four digits of a smaller number. The lines may all carry five digit
numbers or, what is considered better practice, may be designated by an
office name followed by a four digit number. In this latter case the
numbers of the subscribers' lines will in each case be contained in one
or more of the tens of thousands groups, no number having more than four
digits. And the first movement of the dial, whether the name or number
plan be adopted, will select an office; or, looking at it another way,
will select a group of ten thousand and this being done, the next four
successive movements of the dial will select the numbers in that ten
thousand in just the some way that has been already described.
Certain difficulties arise, however, in multi-office working due to the
fact that the three-wire trunks between offices would in most cases be
objectionable. As long as the trunks extend between the various groups
of apparatus in the same office, it is cheaper to provide three wires
for each of them than it is to make any additional complication in the
apparatus. Where the trunking is done between offices, however, the
system may be so modified as to work over two wire inter-office trunks.
_The Trunk Repeater._ The purpose of the trunk repeater is to enable the
inter-office trunking to be done over two wires. It may be said that the
trunk repeater is a device placed in the outgoing trunk circuit at the
office in which a call originates, which will do over the two wires of
the trunk leading from it to the distant office just the same thing that
the subscriber's signal transmitter does over the two wires of the
subscriber's lines. It has certain other functions in regard to feeding
the battery for talking purposes back to the calling subscriber's line,
taking the place in this respect of the calling battery feed relay in
the connector in a single office exchange.
[Illustration: Fig. 397. Circuits of Trunk Repeater]
The circuits of a trunk repeater are shown in Fig. 397. In considering
it, it must be understood that the three wires entering the figure at
the left are the vertical, rotary, and release wires of a second
selector trunk leading from the first selector banks in the same office.
The two wires leading from the right of the figure are those extending
to the distant office, and terminate there in second selectors. The
vertical and the rotary sides of this trunk as shown at the left will
receive the impulses from the subscriber's station coming through the
line switch and the first selector, as usual. The vertical impulses will
pass through the winding of the vertical relay and through the winding
_1_ of the calling battery supply relay and thence to battery, the same
as on a connector. These impulses will work the armatures of both of
these relays in unison. The movements of the vertical relay armature in
response to these impulses will cause corresponding impulses to flow
over a circuit which may be traced from ground, through the springs _3_
and _2_ of the vertical relay, the springs _4_ and _5_ of the bridged
relay _6_ and thence to the vertical side of the trunk and to the
distant office, where it passes into a second selector and through its
vertical relay to battery. Thus the vertical impulses are passed on over
the two-wire trunk to the second selector at the distant office. It
becomes necessary, however, to prevent these impulses from passing back
through the winding of the bridge relay _6_ and this is done by means of
the sluggish relay _7_. This relay receives local battery impulses in
unison with those sent over the trunk by the vertical relay, these being
supplied from the battery at the local office through the contacts _8_
and _9_ of the calling battery supply relay, which works in unison with
the vertical relay. These rapidly recurring impulses are too fast for
the sluggish relay _7_ to follow. And this relay merely pulls up its
armature and cuts off both sides of the trunk leading back to the first
selector. The rotary impulses are repeated to the rotary side of the
two-wire trunk in a similar way.
Considering now the operation of the trunk repeater in the reverse
direction, the action of the bridging relay _6_ is of vital importance.
Normally both sides of trunk line are connected to the live side of the
battery and, therefore, there is no difference of potential between them
and no tendency to operate the bridged relay. When the connection has
been fully established to the subscriber at the distant office, and that
subscriber has responded, the action of his battery supply relay will,
as before stated, change the connection of the rotary side of the line
from battery to ground, and thus bridge the battery at the distant
exchange across the trunk. This action will pull up the bridged relay
_6_ at the trunk repeater and will perform exactly the same function
with respect to the connection of the battery with the calling
subscriber's line. In other words, it will change the connection of the
rotary side of the calling line from battery to ground, thus
establishing the necessary difference in potential to give the calling
subscriber the necessary current for transmission purposes. The
disconnect feature is about the same as already described. When the
calling subscriber hangs up his receiver both the vertical and rotary
relays of the trunk repeater operate, which places the ground on both
sides of the two-wire trunk to the distant office, which is the
condition for releasing all of the apparatus there.
For the purpose of convenience the simplified diagram of Fig. 398 has
been prepared, which shows the complete connection from a calling
subscriber to a called subscriber in a multi-office exchange, wherein
the first movement of the dial is employed to establish the connection
to the proper office and the four succeeding movements to make a
selection among ten thousand lines in that office. This circuit,
therefore, employs at the first office the line switch, the first
selector, and the trunk repeater; and at the second office the second
selector, third selector, connector, and line switch.
The third selector is omitted from Fig. 398, but this will cause no
confusion, since it is exactly like the second selector. The circuits
shown are exactly like those previously described but in drawing them
the main idea has been to simplify the connections to the greatest
possible extent at a sacrifice in the clearness with which the
mechanical inter-relation of parts is shown. No correct understanding of
the circuits of an automatic system is possible without a clear idea of
the mechanical functions performed by the different parts, and,
therefore, we have described what are apparently the more complex
circuit drawings first. It is believed that the student, in attempting
to gain an understanding of this marvel of mechanical and electrical
intricacy, will find his task less burdensome if he will refer freely to
both the simplified circuit drawing of Fig. 398 and the more complex
ones preceding it. By doing so he will often be enabled to clear up a
doubtful circuit point from the simpler diagram and a doubtful
mechanical point from those diagrams which represent more clearly the
mechanical relation of parts.
[Illustration: Fig. 398. Connection between a Calling and a Called
Subscriber in an Automatic System]
=Automatic Sub-Offices.= Obviously, the system of trunking employed in
automatic exchanges lends itself with great facility to the subdivision
of an exchange into a large number of comparatively small office
districts and the establishment of branch offices or sub-offices at the
centers of these districts.
The trunking between large offices has already been described. An
attractive feature of the automatic system is the establishment of
so-called sub-stations or sub-offices. Where there is, in an outlying
district, a distinct group of subscribers whose lines may readily be
centered at a common point within that district and where the number of
such subscribers and lines is insufficient to establish a fully equipped
office, it is possible to establish a so-called sub-station or
sub-office connected with the main office of that district by trunk
lines. At this sub-office there are placed only line switches and
connectors. When a call is originated on one of these sub-office lines,
the line switch acts instantly to connect that line with one of the
trunks leading to the main office of that district, at which this trunk
terminates in a first selector. From there on, the connection is the
same as that in a system in which no sub-offices are employed. Calls
coming into this sub-office over trunk lines from the main office are
received on the connectors at the sub-office and the connection is made
with the sub-office line by the connector in the usual manner. This
arrangement, it is seen, amounts merely to a stretching of the connector
trunks for a given group of lines so that they will reach out from a
main office to a sub-office, it being more economical to lengthen the
smaller number of trunks and by so doing to decrease in length the
larger number of subscribers' lines.
=The Rotary Connector.= For certain purposes it becomes desirable in
automatic work to employ a special form of connector which will have in
itself a certain ability to make automatic selection of one of a group
of previously chosen trunks in much the same manner as the first and
second selectors automatically choose the first idle one of a group of
trunks.
Such a use is demanded in private branch-exchange working where a given
business establishment, for instance, has a plurality of lines
connecting its own private switchboard with the central office. The
directory number of all these lines is, for convenience, made the same,
and it is important, therefore, that when a person attempts to make a
connection with this establishment, he will not fail to get his
connection simply because the first one of these lines happens to be
busy. For such use a given horizontal row of connector terminals or a
part of such a row is assigned to the lines leading to the private
branch exchange and the connector is so modified as to have a certain
"discretionary" power of its own. As a result, when the common number of
all these lines is called, the connector will choose the first one, if
it is not already engaged by some other connector, but if it is, it will
pass on to the next, and so on until an idle one is found. It is only
when the connector has hunted through the entire group of lines and
found them all busy that it will refuse to connect and will give the
busy signal to the calling subscriber.
=Party Lines.= The description of this system as given above has been
confined entirely to direct line working; however, party lines may be
and are frequently employed.
The circuits and apparatus used with direct lines are, with slight
modifications, applicable to use with party lines.
The harmonic method of ringing is employed and the stations are so
arranged with respect to the connectors that those requiring the same
frequency for ringing the bells are in groups served by the same set of
connectors.
[Illustration: POWER MACHINERY Citizens' Telephone Company, Racine, Wis.
_The Dean Electric Co._]
The party lines are operated on the principle commonly known in manual
practice as the jack per station arrangement. Each party line will,
therefore, have sets of terminals appearing in separate hundreds; the
connectors associated with each of these hundreds being so arranged as
to impress the proper frequency of ringing current on the line.
From the subscribers' standpoint the operation is the same as for direct
lines, as the particular hundreds digit of a number serves to select one
of a group of connectors capable of connecting the proper ringing
current to the line.
To avoid confusion, which would be caused by a subscriber on a party
line attempting to make a call when the line is already in use by some
other subscriber, the subscribers' stations are so arranged that when
the line is in use all other stations on the line are locked out.
[Illustration: Fig. 399. Wall Set for Two-Wire System]
=The Two-Wire Automatic System.= The two-wire system that has recently
been introduced by the Automatic Electric Company brings about the very
important result of accomplishing all of the automatic switching over
metallic circuit lines without the use of ground or common returns. The
system is thus relieved of the disturbing influences to which the
three-wire system is sometimes subjected, due to differences in earth
potential between various portions of the system, which may add to or
subtract from the battery potential and alter the net potential
available between two distant points. The introduction of this system
has also made possible certain other incidental features of advantage,
one of which is a great simplification and reduction in size of the
subscriber's station signal-transmitting apparatus.
With the doing away of the ground as a return circuit, it becomes
impossible to send vertical impulses over one side of the line and to
follow them by single rotary impulses over the other side of the line.
Yet it becomes necessary to distinguish between the pure selective
impulses and those impulses which dictate a change of function at the
central office. The plan has, therefore, been adopted of accomplishing
the selection in each case by short and rapidly recurring impulses and
of accomplishing those functions formerly brought about by the single
impulse over the rotary side of the line by a pause between the
respective series of selective impulses. This is accomplished at the
central office by replacing the vertical and the rotary relays of the
three-wire system by a quick-acting and a sluggish relay, respectively;
the quick-acting relay performing the functions previously carried out
by the vertical relay, and the sluggish relay acting only during the
pauses between the successive series of quick impulses to do the things
formerly done by the rotary relay. This has resulted in a delightful
simplification of subscriber's apparatus, since it is now necessary only
to provide a device which will connect the two sides of the line
together the required number of times in quick succession and then allow
a pause with the circuit closed while the subscriber is getting ready to
transmit another set of impulses corresponding to another digit. The
calling device has no mechanical function co-acting with any of the
other parts of the telephone and may be considered as a separate
mechanical device electrically connected with the line. The transmitting
device is not much larger than a large watch and a good idea of it may
be had from Fig. 399, which shows the latest wall set, and Fig. 400,
which shows the latest desk set of the Automatic Electric Company. We
regret the fact that this company has made the request that the complete
details of their two-wire system be not published at this time.
[Illustration: Fig. 400. Desk Stand for Two-Wire System]
CHAPTER XXX
THE LORIMER AUTOMATIC SYSTEM
The Lorimer automatic telephone system has not been commercially used in
this country but is in commercial operation in a few places in Canada.
It is interesting from several points of view. It was invented, built,
and installed by the Lorimer Brothers--Hoyt, George William, and
Egbert--of Brantford, Ontario. These young men without previous
telephonic training and, according to their statements, without ever
having seen the inside of a telephone office, conceived and developed
this system and put it in practical operation. With the struggles and
efforts of these young men in accomplishing this feat we have some
familiarity, and it impresses us as one of the most remarkable inventive
achievements that has come to our attention, regardless of whatever the
merits or demerits of the system may be.
The Lorimer system is interesting also from the fact that, in most
cases, it represents the mechanical rather than the electrical way of
doing things. The switches are power driven and electrically controlled
rather than electrically driven and electrically controlled, as in the
system of the Automatic Electric Company.
The subscriber's station apparatus consists of the usual receiver,
speech transmitter, call bell, and hook switch, and in addition a signal
transmitter arranged to be manipulated by the subscriber so as to
control the operation of the central-office apparatus in connecting with
any desired line in the system.
The central-office apparatus is designed throughout upon the principle
of switching by means of power-driven switches which are under the
control of the signal transmitters of the calling subscriber's station.
The switches employed in making a connection are all so arranged with
respect to constantly rotating shafts that the movable member of such
switches may be connected to the shafts by means of electromagnets
controlled directly or indirectly by relays, which, in turn, are brought
under the control of the signal transmitters.
The circuits are so designed in many instances that the changes
necessary for the different steps are brought about by the movement of
the switches themselves, thus permitting the use of circuits which are
rather simple. The switches employed are all of a rotary type; the
co-ordinate selection, which is accomplished in the Automatic Electric
Company's system by a vertical and rotary movement, being brought about
in this system by the independent rotation of two switches.
=Subscriber's Station Equipment.= A subscriber's desk-stand set, except
the call bell, is shown in Fig. 401, and a wall set complete in Fig.
402. In both of these illustrations may be seen the familiar
transmitter, receiver, and hook switch, and in the wall set, the call
bell. The portion of these telephone sets which is unfamiliar at present
is the part which is enclosed in the enlarged base of the desk stand and
the protruding device below the speech transmitter in the wall set--the
signal transmitter referred to earlier in the chapter. The small push
button and small plate through which the number may be seen directly
below the transmitter in Fig. 402, are for the purpose of registering
calls.
[Illustration: Fig. 401. Lorimer Automatic Desk Stand]
The signal transmitter is a device whose function is to record
mechanically the number of the subscriber's station with which
connection is desired, and to transmit that record to the central office
by a system of electrical impulses over the line conductors. Instead of
operating by its own initiative, the signal transmitter is adapted to
respond to central-office control in transmitting electrically the
number which has been recorded mechanically upon it.
The signal transmitter shown removed from the base of the desk stand at
the left in Fig. 403 comprises in part four sets of contact pins having
ten pins in each set, one set for each of the digits of a four-digit
number. There are also several additional contact pins for signaling and
auxiliary controlling purposes. All of these contact pins are arranged
upon the circumference of a circle and a movable brush mounted upon a
shaft at the center of the circle is adapted to be rotated by a clock
spring and to make contact with each of the pins successively. The call
is started, after the number desired has been set on the dial, by giving
the crank at the right of the signal transmitter a complete turn and
thus winding the spring. The shaft carrying the signal transmitter brush
carries also an escapement wheel, the pallet of which is directly
controlled by an electromagnet.
[Illustration: Fig. 402. Lorimer Automatic Wall Set]
The four dials with the numerals printed on them are attached to four
levers, respectively, and are moved by their levers opposite windows,
near the top of the casing. Through each of these windows a single
numeral may be seen on the corresponding one of the dials. The dials may
be adjusted so that the four numerals seen will read from left to right
to correspond to the number of the line with which connection is
desired.
The setting of the dials so that the number desired shows at the small
circular opening results in connecting the earth or a common return
conductor to one pin of each set of ten pins, the pin grounded in each
set depending upon the numerical value of the digit for which the dial
is set.
The circle of contact pins is set in an insulating disk, the signal
transmitting brush operates upon the pins on one side of the disk, and
electrical fingers attached to the dials operate upon the pins on the
other side of the disk. The escapement wheel is a single toothed disk
attached directly to the shaft which carries the signal brush and its
pallet is attached rigidly to the magnet armature.
[Illustration: Fig. 403. Desk Stand with Signal Transmitter Removed]
Once a call has been turned in, the entire subscriber's station
equipment is locked beyond power of the subscriber to tamper with it in
any way, rendering it impossible either to defeat the call which has
been started or to prevent the subscriber's station as a whole from
returning completely to normal position and thus restoring itself for
regular service. The key shown just below the signal transmitter in the
case of the desk stand, and at the right in the wall set, is for the
purpose of operating a relay at the central office which, in turn,
connects ringing current to the line of the subscriber with which
connection has been made, and thus actuates the call bell.
As the number set up at the signal transmitter remains in full view
until reset for some other number, it is easily checked by inspection
and also lessens the labor involved in making a second call for the same
line, which is frequently necessary when the line is found busy the
first time called.
=Central-Office Apparatus.= The subscriber's lines are divided into
groups of one hundred lines each at the central office, each group being
served by a single unit of central-office apparatus. In a
central-office unit there is "sectional apparatus" which appears but
once for the unit of one hundred lines; "divisional apparatus" which
appears a number of times for each unit, depending upon the traffic; and
"line apparatus" which appears one hundred times for each unit or once
for each line.
The sectional apparatus comprises devices whose duties are, first, to
detect a calling line, and second, to assign to the calling line a set
of idle divisional apparatus which serves to perform the necessary
switching functions and complete the connection.
The sets of divisional apparatus, or, as called in this system,
"divisions," are common to a section and are employed in a manner
similar to the connecting cords of a manual switchboard. The number of
these divisions provided for each section is, therefore, determined by
the number of simultaneous connections resulting from calls originating
in the section. It has been the custom in building this apparatus to
provide each section with seven divisions or connective elements.
The line apparatus comprises one relay, having a single winding, and two
pairs of contacts operated by its armature. This device is substantially
the well known cut-off relay almost universally employed in
common-battery systems. The fixed multiple contacts of the lines in the
switching banks of the connecting apparatus are considered as pertaining
to the various pieces of apparatus on which they are found rather than
to their respective lines. A good idea may be obtained of the
arrangement of the sectional and divisional apparatus by referring to
Fig. 404, which is one unit of a thousand-line equipment. The apparatus
in the vertical row at the extreme left of the illustration is the
sectional apparatus, while the remaining seven vertical rows of
apparatus are the divisions.
_The Section._ The sectional apparatus for each unit consists of three
separate devices called for convenience a _decimal indicator_, a
_division starter_, and a _decimal-register controller_. All of these
devices are normally motionless when idle. The energization of the
decimal indicator, in response to the inauguration of a call at a
subscriber's station, results immediately in an action of the division
starter which starts a division to connect with the line calling. It
results also in the starting of the decimal-register controller, the
remaining unit of sectional apparatus.
It is thus seen that upon the starting of a call by a subscriber, all
of the sectional apparatus belonging to his one hundred lines
immediately becomes active, the division starter acting to start a
division, the decimal indicator becoming energized to indicate the tens
group in which the call has appeared, and the decimal-register
controller becoming active to adjust the decimal register of the
division assigned by the division starter. The division starter having
assigned a division for the exclusive use of this particular call,
passes to a position from which it may start a similar idle division
when the next call is received. The decimal register controller makes
its half revolution for the call and comes to rest, awaiting a
subsequent call, and the decimal indicator continues energized but only
momentarily, since it is released by the action of the cut-off relay
when the call is taken in charge by the divisional connective devices.
Calls may follow each other rapidly, the connective devices being
entirely independent of each other after having been assigned to the
respective calling lines. As has been described, the decimal indicator
starts the division starter and the decimal-register controller in quick
succession. The division starter, shown at the extreme bottom of the
left-hand row of Fig. 404, is a cylinder switch of the same general type
as used throughout this system. In it the terminals of a switch in each
division appear as fixed contact points in a circle over which move the
brushes of the division starter.
The decimal-register controller has the duties of transmitting to the
divisional apparatus a series of current impulses corresponding in
number to the numerical value of the tens digit of the calling line.
This is effected by providing before a movable brush ten contacts from
which the brush may receive current. These contacts are normally not
connected to battery, so that the brush in passing over them does not
receive current from them; however, when the brush has reached the
contact corresponding in number to the tens digit of the calling line, a
relay associated with the decimal-register controller charges the
contacts with the potential of the main battery, and each of the
remaining contacts passed over by the brush sends a current impulse to a
device designed to indicate on the division selected for the call the
tens digit of the calling line.
_The Connective Division._ The connective division, seven of which are
shown in Fig. 404, is an assemblage of switches comprising, as a whole,
a set suitable for a complete connection from calling to called
subscriber. Each connective division in the unit illustrated is
completely equipped to care for a called number of three digits, _i.
e._, each division will connect its calling line with any one of one
thousand lines which may be called. By a system of interconnecting
between divisions, each division may be equipped with interconnecting
apparatus so as to make it possible to complete a call with any one of
ten thousand lines. Each connecting division of a ten-thousand-line
exchange comprises six major switches. Of the six major switches, one is
termed a _secondary connector_, another an _interconnector_, and the
four remaining are termed the _primary portion_ of the division.
[Illustration: Fig. 404. Unit of Switching Apparatus]
Before taking up the operation of the switches, the mechanical nature of
the switches themselves will be described. The switches are built with a
contact bank cylindrical in form and with internal movable brushes
traveling in a rotary manner in circular paths upon horizontal rows of
contacts fixed in the cylindrical banks. For driving these brushes a
constantly rotating main power-driven shaft is provided. Between each
shaft and the rotating brushes of each major switch is an electric
clutch, which, by the movement of an armature, causes the brushes of the
switch to partake of the motion of the shaft and by the return of the
armature to come again to rest. The motion of the brushes of the major
switches, or cylinder switches, as they are frequently called because of
their form, is constantly in the same direction. They have a normal
position upon a set of the cylinder contacts. They leave their normal
position and take any predetermined position as controlled by the
magnets of the clutch, and, having served the transient purpose, they
return to their normal position by traversing the remainder of their
complete revolution and stopping in their position of rest or idleness.
The mechanical construction of each of the cylinder switches is such
that it may disengage its clutch and bring its brushes to rest only with
the brushes in some one of a number of predetermined positions. The
locations of the brushes in these positions of rest, or "stop"
positions, as they are called, may differ with the different cylinder
switches, according to the nature of the duty required of the switch,
and the total number of stop positions also may vary. The primary and
secondary connectors, the interconnector selectors, and the
interconnectors each have eleven stop positions; the rotary switch has
eight stop positions; the signal-transmitter controller has but two.
In the six cylinder switches making up a connective division and
required for any conversation, in a ten-thousand-line exchange some of
the switches are set to positions which are determined by the control of
the calling subscriber and represent by their selective positions the
value of some digit of the calling or called subscriber's number. Others
are switches controlling the call in its progress and controlling the
switches responsive to the call. These latter switches take positions
independent of the numbers.
In addition to the major switches, there are upon each division four
minor switches termed _registers_. Each consists of an arc of fixed
contacts accompanied by a set of brushes which sweep over the contacts.
Instead of being driven by an electromagnet, the register brushes are
placed under tension of a spring which tends at all times to draw them
forward. They are then restrained by an escapement device similar to a
pallet escapement in a clock, the pallet being controlled by the
register's magnets. When a series of impulses are received by the
register magnets, the pallet is actuated a corresponding number of times
and the register brushes are permitted to move forward under tension of
their powerful propelling spring. Each register is associated with a
major switch, and the register brushes are engaged by a cam upon the
associated major switch, and are restored to normal position against the
tension of their propelling spring, the force of restoration being
obtained from the main shaft.
The electrical clutches which connect and disconnect the movable brushes
of the major switches from the main driving shaft are controlled in all
instances by circuits local to the central office. In some instances
these circuits include relay contacts and are controlled by a relay. In
other instances they are formed solely through switch contacts. In all
cases the control, when from a distance, is received upon relays
suitable for being controlled by the small currents which are adapted to
flow over long lines. In all instances the power for moving a brush is
derived from the main shaft and only the control of the movement is
derived from electromagnets, relays, or other electric sources. In many
instances the clutch circuit is closed through contacts of its own
switch and, therefore, may be closed only when its switch is in some
predetermined position. All of the switches are mechanically powerful
and designed particularly to sustain the wear of long-continued and
oft-repeated usage. This is true also of the moving parts which carry
the brushes and of the journals sustaining those parts.
_The Switches of the Connective Division._ The six major switches of the
connecting division are as follows:
The Primary Connector:--The function of this switch is to connect the
conductors of the calling line with the switching devices of the
connective division. Associated with this switch is a register termed
the _decimal register_. The one hundred lines of the section are
terminated in fixed multiple contacts in the cylinder switch of the
primary connector. The calling line is selected and connected with by
adjusting the decimal register to a position corresponding to the
calling line's tens digit and adjusting the brushes of the cylinder
switch to a position corresponding to the calling line's unit digit.
The Rotary Switch:--This is a master switch, or pilot switch,
consisting of a cylinder switch without register. Its duty is the
control of other switches and the completion of circuits formed in part
through other switches. It is the pilot switch and the switch of
initiative and control for the entire connective division.
Signal-Transmitter Controller:--The primary function of this switch is
the generation of signaling impulses of two classes. Impulses of the
first class pass over central-office circuits only and are effective
upon magnets of the divers major and minor switches; impulses of the
second class pass over a line conductor of the calling line and are
effective upon the signal transmitter at the subscriber's station. The
impulses sent out over the line to the subscriber's station cause the
brush to pass over the contacts and thereby indicate the numerical
values of the various digits set by the dials. This switch also enters
in an important manner into the circuits involved in the testing of the
called line for the busy condition. It is controlled by the rotary
switch.
Interconnector Selector:--In an exchange using four digits in the
numbers, the register of the interconnector selector is adjusted in each
call to a position corresponding to the numerical value of the thousands
digit of the called number. The cylinder switch then acts to select an
idle trunk. The switch is controlled by the rotary switch in connection
with the signal transmitter controller.
Interconnector:--This switch is similar to the interconnector selector
in design and in function. It is a cylinder switch with register. The
register is adjusted in each call to a position corresponding to the
numerical value of the hundreds digit of the number called and the
cylinder switch then operates to select an idle trunk. The switch is
controlled by the rotary switch in connection with the signal
transmitter controller.
Secondary Connector:--This switch contains in its cylinder bank of
contacts the multiple points of one hundred subscribers' lines and its
function is to connect the conductors of the called line to the
conductors of the connective division. This is accomplished by adjusting
the register to correspond to the value of the tens digit of the line
desired and by adjusting the cylinder brushes to correspond to the value
of the units digit of the line. The switch is controlled by the rotary
switch in connection with the signal-transmitter controller.
=Operation.= A brief description of the progress of a call from its
institution to the complete connection and subsequent disconnection
begins with the adjustment of the dial indicators of the telephone set
and the turning of the crank of the signal transmitter one revolution.
This act, performed by the calling subscriber, connects one of the line
conductors to earth. Immediately the decimal indicator associated with
the section in which the calling line terminates is energized and starts
the division starter. The division starter instantly starts the rotary
switch of an idle division. The rotary switch now starts the
decimal-register controller and connects to it the decimal register of
the primary connector of the division selected.
All of the above acts in the central office occur practically
simultaneously. The impulses generated by the controller are effective
upon the decimal register of the started division and, therefore, adjust
that register to a position corresponding to the tens value of the
calling line.
The rotary switch now disconnects the tens register and starts the
cylinder brushes of the primary connector which automatically stop when
they encounter the calling line. At this instant the cut-off relay of
the line is energized and the decimal indicator is released. The call
now is clear of all sectional apparatus and another call may come
through immediately, being assigned in charge of another idle division.
The total time in which any call is in charge of the sectional
apparatus, _i. e._, the total time from the grounding of the line
conductor at the sub-station until the line has been connected with by
the primary connector of some division of that section and the sectional
apparatus has been released by the operation of the cut-off relay,
approximates two-fifths of a second.
The next operation initiated by the rotary switch is the starting of the
signal-transmitter controller of the connective division, which, in
turn, adjusts the register of the interconnector selector to a position
corresponding to the thousands digit of the number of the called line as
indicated by the signal transmitter at the calling station. This selects
an interconnector serving the lines of the selected thousand.
This initial selection being completed the rotary switch readjusts the
circuits of the connective division in such manner that in the further
progress of the signal-transmitter controller, its impulses will be
effective upon the register of the selected interconnector. In this
manner, the register of the interconnector, which may be upon the same
connective division as the rotary switch handling the call, or which may
be the interconnector of some other division, as determined by the
number of the called subscriber, is adjusted to a position corresponding
to the second or hundreds digit of the number called. The cylinder
switch of the interconnector then selects and appropriates an idle trunk
extending to a secondary connector upon some connective division serving
the hundred selected.
The rotary switch again shifts the circuits of the connective division
in such manner that the signal-transmitter controller is effective upon
the secondary connector, both register and cylinder, and adjusts the
register and cylinder, respectively, with their brushes in contact with
the tens and units digits, respectively, of the number of the called
line.
The conductors of the called line now are connected through the
secondary connector, the interconnector, and the interconnector selector
to the rotary switch; the conductors of the calling line are connected
through the primary connector to the rotary switch; thus completely
connecting the lines except at the rotary switch. To effect the
connecting together of the two lines, both rotary switch and
signal-transmitter controller must pass forward into their next
positions, the connection when thus effected being made through
conductors containing a repeating coil and main battery connection for
supplying talking current to the two lines and containing also ringing
and supervisory relays.
The called line is tested to determine if busy during the short interval
in which the rotary switch takes a short step to connect the calling and
the called lines. In this step of the rotary switch the busy-test relay
is connected to the guard wire or busy-test wire of the called line, and
if that line be busy, the relay interferes with the control exercised by
the rotary switch upon the signal-transmitter controller, and the
controller is prevented from taking the step required to connect the
line. Thus, when a busy line is encountered, the final step of the
rotary switch is taken to set up the conversation conditions, but the
signal-transmitter controller does not take its final step; by this
failure of the signal-transmitter controller due to the action of the
busy-test relay, the calling line is not connected to the called line
but is connected to a busy-back tone generator instead.
Whether the line encountered be busy or idle, the connective division
remains in its condition as then adjusted until the subscriber hangs his
receiver upon the hook switch to obtain disconnection. The ringing of
the bell of the called station is done directly by the calling
subscriber in pressing the ringing key.
The disconnection is effected, when the receiver of the calling line is
hung up, by the supervisory relay in the central office, whose winding
is included in the line circuit, and whose contacts act directly to
start the rotary switch. In disconnecting, the rotary switch starts the
primary and the secondary connectors and thus instantly releases both
the calling and the called lines. Thereafter the rotary switch in
passing from position to position restores switch after switch of the
connective division to normal and finally itself returns to normal in
preparation for its assignment to service in answering a subsequent
call.
CHAPTER XXXI
THE AUTOMANUAL SYSTEM
Two systems of telephony are now in common use in this country--the
manual system and the automatic. With the growth of the automatic, and
the gradually ripening conviction, which is now fully matured in the
minds of most telephone engineers, that automatic switching is
practical, there has been a growing tendency toward doing automatically
many of the things that had previously been done manually. One of the
results of this tendency has been the production of the _automanual_
system, the invention of Edward E. Clement, an engineer and patent
attorney, of Washington, D. C. In connection with Mr. Clement's name, as
inventor, must be mentioned that of Charles H. North, whose excellent
work as a designer and manufacturer has contributed much toward the
present excellence of this highly interesting system.
=Characteristics of System.= The name "automanual" is coined from the
two words, automatic and manual, and is intended to suggest the idea
that the system partakes in part of the features of the automatic system
and in part of those of the manual system.
We regret that neither space nor the professional relation which we have
had with the development of this system will permit us to make public an
extended and detailed description of its apparatus and circuits. Only
the general features of the system may, therefore, be dealt with.
[Illustration: POWER APPARATUS FOR COMMON-BATTERY MANUAL OFFICE OF
MEDIUM SIZE]
The underlying idea of the automanual system is to relieve the
subscriber of all work in connection with the building up of his
connection, except the asking for it; to complicate the subscriber's
station equipment in no way, it being left the same as in the
common-battery manual system; to do away with manual apparatus, such as
jacks, cords and plugs, at the central office, and to substitute for it
automatic switching apparatus which will be guided in its movements,
not by the subscriber, but by a very much smaller number of operators
than would be necessary to manipulate a manual switchboard.
=General Features of Operation.= A broad view of the operation of the
system is this. The subscriber desiring to make a call takes down his
receiver, and this causes a lamp to light in front of an operator. The
operator presses a button and is in telephonic communication with the
subscriber. Receiving the number desired, the operator sets it up on a
keyboard in just about the same way that a typist will set up the
letters of a short word on a typewriting machine. The setting up of the
number on the keyboard being accomplished, the proper condition of
control of the associated automatic apparatus at the central office is
established and the operator has no further connection with the call.
The automatic switching apparatus guided by the conditions set up on the
operator's keyboard proceeds to make the proper selection of trunks and
to establish the proper connections through them to build up a talking
circuit between the calling subscriber and the called and to ring the
called subscriber's bell, or, if his line is found busy, the apparatus
refuses to connect with it and sends a busy signal back to the calling
subscriber. The operator performs no work in disconnecting the
subscribers, that being automatically taken care of when they hang up
their receivers at the close of the conversation.
From the foregoing it will be seen that there is this fundamental
difference between the automatic and the automanual--the automatic
system dispenses entirely with the central-office operator for all
ordinary switching functions; the automanual employs operators but
attempts to so facilitate their work that they may handle very many more
calls than would be possible in a manual system, and at the same time
secures the advantages of secrecy which the automatic system secures to
its subscribers.
=Subscriber's Apparatus.= One of the main points in the controversy
concerning automatic _versus_ manual systems is whether or not it is
desirable to have the subscriber ask for his connection or to have him
make certain simple movements with his fingers which will lead to his
securing it. The developers of the automanual system have taken the
position that the most desirable way, so far as the subscriber is
concerned, is to let him ask for it. It is probable that this point
will not be a deciding one in the choice of future systems, since it
already seems to be proven that the subscribers in automatic systems are
willing to go through the necessary movements to mechanically set up the
call. The advantage which the automanual system shares with the manual,
however, in the greater simplicity of its subscriber's station
apparatus, cannot be gainsaid.
[Illustration: Fig. 405. Operators' Key Tables]
[Illustration: Fig. 406. Top View of Key Table]
=Operator's Equipment.= The general form of the operator's equipment is
shown in Fig. 405. A closer view of the top of one of the key tables is
shown in Fig. 406. As will be seen, the equipment on each operator's
position consists of three separate sets of push-button keys closely
resembling in external appearance the keys of a typewriter or adding
machine. Immediately above each set of keys are the signal lamps
belonging to that set.
The operator's keys are arranged in strips of ten, placed _across_
rather than _lengthwise_ on the key shelf. One of these strips is shown
in Fig. 407. There are as many strips of keys in each set as there are
digits in the subscribers' numbers, _i. e._, three in a system having a
capacity of less than one thousand; four in a system of less than ten
thousand; and so on. In addition to the number keys of each set is a
partial row of keys, including what is called a _starting key_ and also
keys for making the party-line selection.
[Illustration: Fig. 407. Strip of Selecting Keys]
[Illustration: Fig. 408. Wiring of Key Shelf]
The simplicity of the operator's key equipment is one of its attractive
features. Fig. 408 shows one of the key shelves opened so as to expose
to view all of the apparatus and wiring that is placed before the
operator. The reason for providing more than one key set on each
operator's position is, that after a call has been set up on one key
set, a few seconds is required before the automatic apparatus controlled
by the key set can do its work and release the key set ready for another
call. The provision of more than one key set makes it possible for the
operator to start setting up another call on another key set without
waiting for the first to be released by the automatic apparatus.
[Illustration: Fig. 409. Switch Room of Automanual Central Office]
=Automatic Switching Equipment.= A general view of the arrangement of
automatic switches in an exchange established by the North Electric
Company at Ashtabula, Ohio, is shown in Fig. 409. The desk in the
foreground is that of the wire chief. This automatic apparatus consists
largely of relays and automatic selecting switches. The switches are of
the step-by-step type, having vertical and rotary movements, and an idea
of one of them, minus its contact banks, is given in Fig. 410. The
control of the automatic switches by the operator's key sets is through
the medium of a power-driven, impulse-sending machine. From this machine
impulses are taken corresponding to the numbers of the keys depressed.
[Illustration: Fig. 410. Selecting Switch]
=Automatic Distribution of Calls.= A feature of great interest in this
system is the manner in which the incoming calls are distributed among
the operators. From each key set an operator's trunk is extended to what
is called a secondary selector switch, through which it may be connected
to a primary selector trunk and calling line. When a subscriber calls by
taking down his receiver, his line relay pulls up and causes a primary
selector switch to connect his line with an idle local trunk or link
circuit, at the same time starting up a secondary selector switch which
immediately connects the primary trunk and the calling line to an
operator's idle key set. If an operator is at the time engaged in
setting up a call on a key set, or if that key set is still acting to
control the sending of impulses to the automatic switches, it may be
said to be busy, and it is not selected by this preliminary selecting
apparatus in response to an incoming call. As soon, however, as the
necessary impulses have been taken from the key set by the automatic
apparatus, that key set is released and is again ready to receive a
call. In this way the calls come before each operator only as that
operator is able and ready to receive them.
=Setting up a Connection.= As soon as the key-set lamp lights, in
response to such an incoming call, the operator presses a listening
button, receives the number from the subscriber, and depresses the
corresponding number buttons on that key set, thereby determining the
numbers in each of the series of impulses to be sent to the selector and
the connector switches to make the desired connection. The operator
repeats this number to the calling subscriber as she sets it up, and
then presses the starting button, whereupon her work is done so far as
that call is concerned. If, upon repeating the call to the subscriber,
the operator finds that she is in error, she may change the number set
up at any time before she has pressed the starting button.
=Building up a Connection.= The keys so set up determine the number of
impulses that will be transmitted by the impulse-sending machine to the
selector and the connector switches. These switches, impelled by these
impulses, establish the connection if the line called for is not already
connected to. If a party-line station is called for, the proper station
on it will be selectively rung as determined by the party-line key
depressed by the operator. If the line is found busy, the connector
switch refuses to make the connection and places a busy-back signal on
the calling line.
=Speed in Handling Calls.= This necessarily brief outline gives an idea
only of the more striking features of the automanual system. A study of
the rapidity with which calls may be handled in actual practice shows
remarkable results as compared with manual methods of operating. The
operators set up the number keys corresponding to a called number with
the same rapidity that the keys of a typewriter are pressed in spelling
a word. In fact, even greater speed is possible, since it is noticed
that the operators frequently will depress all of the keys of a number
at once, as by a single striking movement of the fingers. The rapidity
with which this is done defies accurate timing by a stop watch in the
hands of an expert. It is practically true, therefore, that the time
consumed by the operator in handling any one call is that which is taken
in getting the number from the subscriber and in repeating it back to
him.
TABLE XI
Total Time Consumed by Operator in Handling Calls on Automanual System
+-----------------------------------------------------------------+
| First 100 Calls |
+-----------------------------------------------------------------+
|Longest Individual Period 12.40 seconds |
|Average five longest Individual Periods 7.44 seconds |
|Average ten longest Individual Periods 6.34 seconds |
|Shortest Individual Period 1.60 seconds |
|Average five shortest Individual Periods 1.92 seconds |
|Average ten shortest Individual Periods 1.96 seconds |
|Average Entire 100 Calls 3.396 seconds |
|Hourly Rate at which calls were being handled 1060 |
+-----------------------------------------------------------------+
+-----------------------------------------------------------------+
| Second 100 Calls |
+-----------------------------------------------------------------+
|Longest Individual Period 7.60 seconds |
|Average five longest Individual Periods 5.52 seconds |
|Average ten longest Individual Periods 5.34 seconds |
|Shortest Individual Period 2.00 seconds |
|Average five shortest Individual Periods 2.04 seconds |
|Average ten shortest Individual Periods 2.18 seconds |
|Average Entire 100 Calls 3.374 seconds |
|Hourly Rate at which calls were being handled 1067 |
+-----------------------------------------------------------------+
+-----------------------------------------------------------------+
| Third 100 Calls |
+-----------------------------------------------------------------+
|Longest Individual Period 5.40 seconds |
|Average five longest Individual Periods 5.32 seconds |
|Average ten longest Individual Periods 4.44 seconds |
|Shortest Individual Period 1.60 seconds |
|Average five shortest Individual Periods 1.65 seconds |
|Average ten shortest Individual Periods 1.80 seconds |
|Average Entire 100 Calls 3.160 seconds |
|Hourly Rate at which calls were being handled 1139 |
+-----------------------------------------------------------------+
Owing to the difficulty of securing accurate traffic data by means of a
stop watch, an automatic, electrical timing device, capable of
registering seconds and hundredths of a second, has been used in
studying the performance of this system in regular operation at
Ashtabula Harbor. The operators were not informed that the records were
being taken, and the data tabulated represents the work of two operators
in handling regular subscribers' calls. The figures in Table XI are
given by C. H. North as representing the total time consumed by the
operator from the time her line lamp was lighted until her work in
connection with the call was finished, and it included, therefore, the
pressing of the listening button, the receiving of the number from the
subscriber, repeating it back to him, setting up the connection on the
keys, and pressing the starting key.
It will be seen that the average time for each 100 calls is quite
uniform and is slightly over three seconds. The considerable variation
in the individual calls, ranging from a maximum of 12.40 seconds down to
a minimum of 1.60 seconds, is due almost entirely to the difference
between the subscribers in the speed with which they can give their
numbers. These figures indicate that, in each of the tests, calls were
being handled at the rate of more than one thousand per hour by each
operator.
The test of the subscriber's waiting time, _i. e._, the time that he
waited for the operator to answer, for one hundred calls made without
the knowledge of the operator, showed the results as given in Table XII,
in which a split second stop watch was used in making the observations.
TABLE XII
Subscribers' Waiting Time
+----------------------------------------------------------+
|Number of Calls Tested 100 |
|Longest Individual Period 5.20 seconds |
|Average 5 Longest Individual Periods 4.64 seconds |
|Average 10 Longest Individual Periods 3.80 seconds |
|Shortest Individual Period 1.00 seconds |
|Average 5 Shortest Individual Periods 1.28 seconds |
|Average 10 Shortest Individual Periods 1.34 seconds |
|Average Entire 100 Calls 2.07 seconds |
+----------------------------------------------------------+
The length of time which the subscriber has to wait before receiving an
answer from the operator is, of course, one of the factors that enters
into the giving of good telephone service, and the times shown by this
test are considerably shorter than ordinarily maintained in manual
practice. The waiting time of the subscriber is not, of course, a part
of the time that is consumed by the operator, and the real economy so
far as the operator's time is concerned is shown in the tests recorded
in Table XI.
CHAPTER XXXII
POWER PLANTS
The power plant is an organization of devices to furnish to a telephone
system the several kinds of current, at proper pressures, for the
performance of the several general electrical tasks within the exchange.
=Kinds of Currents Employed.= Sources of both direct and alternating
current are required and a single exchange may employ these for one or
more of the following purposes:
_Direct Current._ Current which flows always in one direction whether
steady or varying, is referred to as direct current, and may be required
for transmitters, for relays, for line, supervisory, and auxiliary
signals, for busy tests, for automatic switches, for call registers, for
telegraphy, and in the form of pulsating current for the ringing of
biased bells.
_Alternating Current._ Sources of alternating current are required for
the ringing of bells, for busy-back and other automatic signals to
subscribers, for howler signals to attract the attention of subscribers
who have left their receivers off their hooks, and for signaling over
composite lines.
=Types of Power Plants.= Clearly the requirements for current supply
differ greatly for magneto and common-battery systems. There is,
however, no great difference between the power plants required for the
automatic and the manual common-battery systems.
In the simplest form of telephone system--two magneto telephones on a
private line--the power plant at each station consists of two elements:
one, the magneto generator, which is a translating device for turning
hand power into alternating current for ringing the bell of the distant
station; and the other, a primary battery which furnishes current to
energize the transmitter. In such a system, therefore, each telephone
has its own power plant. The term power plant, however, as commonly
employed in telephone work, refers more particularly to the organization
of devices at the central office for furnishing the required kinds of
current, and it is to power plants in this sense that this chapter is
devoted.
_Magneto Systems._ If magneto lines be connected to a switchboard, the
current for throwing the drop at the switchboard is furnished by the
subscriber's generator, and the current for energizing the subscriber's
transmitter is furnished by the local battery at his station; but
sources of current must be provided for enabling the central-office
operator to signal or talk to the subscribers. These are about the only
needs for which current must be furnished in an ordinary magneto central
office. If a multiple board is employed, direct current is also needed
for the purpose of the busy test and also for operating the drop
restoring circuits, if the electrical method of restoring the drops is
employed.
_Common-Battery Systems._ In common-battery systems the requirements are
very much more extensive. The subscribers' telephones have no power
plants of their own, but are provided with a common source of direct
current located at the central office for supplying the talking current,
and for operating the central-office signals, and the operators are
provided with one or more common sources of alternating or pulsating
current for ringing the subscribers' bells. Common-battery equipment
requires the use of currents of different kinds for a greater number of
auxiliary purposes than does magneto equipment. These facts make the
power plant in a common-battery office much more important than in a
magneto office.
=Operators' Transmitter Supply.= In a small magneto exchange, the
transmitter current may be had from primary batteries, a separate
battery being employed for each operator's set. When there are more than
three or four operators, however, it is usual, even in magneto offices,
to obtain the transmitter current from a common storage battery. A
storage battery has the fortunate quality of very low internal
resistance, therefore a number of operators' transmitters may be
actuated by one source without introducing cross-talk. In other words, a
storage battery is a current-furnishing device of good regulation, the
variation of consumption in one circuit leading from it causing slight
variation in the currents of other circuits leading from it. If this
were not so, cross-talk would exist between the telephones of the
operators' positions connected to the same battery. This regulating
quality enables the multiple feeding of telephone circuits to be carried
further than the mere supplying of operators' sets and is the quality
which makes possible the successful use of a storage battery as the
single source of transmitter current for common-battery central-office
equipment.
In furnishing a plurality of operators' transmitters from a common
battery, the importance of low resistance and inductance in the portion
of the path that is common to all of the circuits must not be
overlooked. Not only is a battery of extremely low resistance required,
but also conductors leading from it that are common to two or more of
the circuits should be of very low resistance and consequently large in
cross-section and as short as possible. In common-battery offices there
is obviously no need of employing a separate battery for the operators'
transmitters, since they may readily be supplied from the common storage
battery which supplies direct current to the subscribers' lines.
=Ringing-Current Supply.= _Magneto Generators._ As a central-office
equipment is required to ring many subscribers' bells, only the small
ones find it convenient to ring them by means of hand-operated magneto
generators. Small magneto switchboards are usually equipped so that each
operator is provided with a hand-generator, but even where such is the
case some source of ringing current not manually operated is desirable.
In larger switchboards the hand generators are entirely dispensed with.
The magneto generator may be driven by a belt from any convenient
constantly moving pulley, and the early telephone exchanges were often
equipped with such generators having better bearings and more current
capacity than those in magneto telephones. These were adapted to be run
constantly from some source of power, delivering ringing current to the
operators' keyboards at from 16 to 20 cycles per second.
_Pole Changers._ Vibrating pole changers were also used in the early
exchanges, but passed out of use, partly because of poor design, but
more because of the absence of good forms of primary batteries for
vibrating them and for furnishing the direct currents to be transformed
into alternating line current for ringing the bells. The pole changer
was redesigned after the beginning of the great spread of telephony in
the United States in 1893. Today it is firmly established as an element
of good telephone practice. Fig. 411 illustrates the principle upon
which one of the well-known pole changers--the Warner--operates. In
this _1_ is an electromagnet supplied by a constant-current battery _2_
to keep the vibratory system continually in motion. This motor magnet
and its battery work in a local circuit and cause vibration in exactly
the same manner as the armature of an ordinary electric door bell is
caused to vibrate. The battery from which the ringing current is derived
is indicated at _3_, and the poles of this are connected, respectively,
to the vibrating contacts _4_ and _5_. These contacts are merely the
moving members of a pole changing switch, and a study of the action will
readily show that when these moving parts engage the right-hand
contacts, current will flow to the line supposed to be connected to the
terminals _6_ and _7_ in one direction, while, when these parts engage
the left-hand contacts, current will flow to the line in the reverse
direction. The circuit of the condenser shown is controlled by the
armature of the relay _8_.
The winding of this relay is put directly in the circuit of the main
battery _3_, so that whenever current is drawn from this battery to ring
a distant bell, this relay will be operated and will bridge the
condenser across the circuit of the line. The purpose of the condenser
is to make the impulses flowing from the pole changer less abrupt, and
the reason for having its bridged circuit normally broken is to prevent
a waste of current from the battery _3_, due to the energy which would
otherwise be consumed by the condenser if it were left permanently
across the line.
[Illustration: Fig. 411. Warner Pole Changer]
[Illustration: Fig. 412. Pole Changers for Harmonic Ringing]
Pole changers for ringing bells of harmonic party lines are required to
produce alternating currents of practically constant frequencies. The
ideal arrangement is to cause the direct currents from a storage battery
to be alternated by means of the pole changers, and then transformed
into higher voltages required for ringing purposes, the transformer
also serving to smooth the current wave, making it more suitable for
ringing purposes. In Fig. 412 such an arrangement, adapted to develop
currents for harmonic ringing on party lines, is shown. The regular
common battery of the central office is indicated at _1_, _2_ being an
auxiliary battery of dry cells, the purpose of which will be presently
referred to. At the right of the battery _1_ there is shown the calling
plug with its associated party-line ringing keys adapted to impress the
several frequencies on the subscribers' lines. The method by which the
current from the main storage battery passes through the motor magnets
of the several vibrators, and by which the primary currents through the
transformers are made to alternate at the respective frequencies of
these vibrators, will be obvious from the drawing. It is also clear that
the secondary currents developed in these transformers are led to the
several ringing keys so as to be available for connection with the
subscribers' lines at the will of the operator. The condensers are
bridged across the primary windings of the transformers for the purpose
of aiding in smoothing out the current waves. The use of the auxiliary
battery _2_ and the retardation coil _3_ in the main supply lead is for
the purpose of preventing the pulsating currents drawn from the main
battery _1_ from making the battery "noisy." These two batteries have
like poles connected to the supply lead, and the auxiliary battery
furnishes no current to the system except when the electromotive force
of the impulse flowing from the main battery is choked down by the
impedance coil and the deficiency is then momentarily supplied for each
wave by the auxiliary battery. This is the method developed by the Dean
Electric Company for preventing the pole-changer system from causing
disturbances on lines supplied from the same main battery.
[Illustration: Fig. 413. Multi-Cyclic Generator Set]
_Ringing Dynamos._ Alternating and pulsating currents for ringing
purposes are also largely furnished from alternating-current dynamos
similar to those used in commercial power and lighting work, but
specially designed to produce ringing currents of proper frequency and
voltage. These are usually driven by electric motors deriving their
current either from the commercial supply mains or from the
central-office battery. In large exchanges harmonic ringers are usually
operated by alternating-current generators driven by motors, a separate
dynamo being provided to furnish the current of each frequency. Fig. 413
shows a set of four such generators directly connected to a common
motor. As no source of commercial power for driving such generators is
absolutely uniform, and since the frequency of the ringing current must
remain very close to a constant predetermined rate, some means must be
employed for holding the generators at a constant speed of revolution,
and this is done by means of a governor shown at the right-hand end of
the shaft in Fig. 413. The principle of this governor is shown in Fig.
414. A weighted spring acts, by centrifugal force, to make a contact
against an adjustable screw, when the speed of the shaft rises a
predetermined amount. This spring and its contact are connected to two
collector rings _1_ and _2_ on the motor shaft, and connection is made
with these by the brushes _3_ and _4_. The closing of the governor
contact serves, therefore, merely to short-circuit the resistance _5_,
which is normally included in the shunt field of the motor. This
governor is based on the principle that weakening the field increases
the speed. It acts to insert the resistance in series with the field
winding when the speed falls, and this, in turn, results in restoring
the speed to normal.
[Illustration: Fig. 414. Governor for Harmonic Ringing Generators]
=Auxiliary Signaling Currents.= Alternating currents, such as those
employed for busy signals to subscribers in automatic systems, those for
causing loud tones in receivers which have been left off the hook
switch, and those for producing loud tones in calling receivers
connected to composite lines, all need to be of much higher frequency
than alternating current for ringing bells. The simplest way of
producing such tones is by means of an interrupter like that of a
vibrating bell; but this is not the most reliable way and it is usual to
produce busy or "busy-back" currents by rotating commutators to
interrupt a steady current at the required rate. As the usual busy-back
signal is a series of recurrent tones about one-half second long,
interspersed with periods of silence, the rapidly commuted direct
current is required to be further commuted at a slow rate, and this is
conveniently done by associating a high-speed commutator with a
low-speed one. Such an arrangement may be seen at the left-hand end of
the multicyclic alternating machine shown in Fig. 413. This commuting
device is usually associated with the ringing machine because that is
the one thing about a central office that is available for imparting
continuous rotary motion.
=Primary Sources.= Most telephone power plants consume commercial
electric power and deliver special electric current. Usually some
translating device, such as a motor-generator or a mercury-arc
rectifier, is employed to transform the commercial current into the
specialized current required for the immediate uses of the exchange.
_Charging from Direct-Current Mains._ In some cases commercial direct
current is used to charge the storage batteries without the intervention
of the translating devices, resistances being used in series with the
battery to regulate the amount of current. Commercial direct current
usually is available at pressures from 110 volts and upward, while
telephone power plants contain storage batteries rarely of pressures
higher than 50 volts. To charge a 50-volt storage battery direct from
110-volt mains results in the loss of about half the energy purchased,
this lost energy being set free in the form of heat generated in the
resistance devices. Notwithstanding this, it is sometimes economical to
charge directly from the commercial direct-current power mains, but only
in small offices where the total amount of current consumed is not large
and where the greatest simplicity in equipment is desirable. It is
better, however, in nearly all cases, to convert the purchased power
from the received voltage to the required voltage by some form of
translating device, such as a rotary converter or a mercury-arc
rectifier.
_Rotary Converters._ Broadly speaking, a rotary converter consists of a
motor adapted to the voltage and kind of current received, mechanically
coupled to a generator adapted to produce current of the required kind
and voltage. The harmonic ringing machine shown in Fig. 413 is an
example of this, this particular one being adapted to receive direct
current at ordinary commercial pressure and to deliver four different
alternating currents of suitable pressures and frequencies. It is to be
understood, however, that the conversion may be from direct current to
direct current, from alternating to direct, or from direct to
alternating. Such a device where the motor is a separate and distinct
machine from the generator or generators is called a _motor-generator_.
It is usual to connect the motors and the generators together directly
by a coupling having some flexibility, as shown in Fig. 413, so as to
prevent undue friction in the bearings.
[Illustration: THE POWER AND WIRE CHIEF'S ROOM OF THE EXCHANGE AT WEBB
CITY, MISSOURI]
As an alternative to the converting device made up of a motor coupled to
a generator, both motor and generator windings may be combined on the
same core and rotate within the same field. Such a rotary converter has
been called a _dynamotor_. As a rule the dynamotor is only suitable for
small power-plant work. It has the following objectionable features:
(_a_) It is difficult to regulate its output, since the same field
serves for both the motor and the dynamo windings. For this reason its
main use is as a ringing machine where the regulation of the output is
not an important factor. (_b_) Furthermore, the fact that the motor and
dynamo armature windings are on the same core makes it difficult to
guard against breakdowns of the insulation between the two windings,
especially when the driving current is of high voltage.
_Charging Dynamos._ The dynamo for charging the storage battery is, of
course, a direct-current machine and may be a part of a motor generator
or it may derive its power from some other than an electric motor, such
as a gas or steam engine. It should be able to develop a voltage
slightly above that of the voltage of the storage battery when at its
maximum charge, so as always to be able to deliver current to the
charging battery regardless of the state of charge. A 30-volt generator,
for example, can charge eleven cells in series economically; a 60-volt
generator can charge twenty-five cells in series economically.
Battery-charging generators are controlled as to their output by varying
a resistance in series with their fields. Such machines are usually
shunt-wound. Sometimes they are compound-wound, but compounding is less
important in telephone generators than in some other uses. A feature of
great importance in the design of charging generators is smoothness of
current. If it were possible to design generators to produce absolutely
even or smooth current, the storage battery would not be such an
essential feature to common-battery exchanges, because then the
generator might deliver its current directly to the bus bars of the
office without any storage-battery connection and without causing noise
on the lines. Such generators have been built in small units. Even if
these smooth current generators were commercially developed to a degree
to produce absolutely no noise on the lines, the storage battery would
still be used, since its action as a reservoir for electrical energy is
important. It not only dispenses with the necessity of running the
generators continuously, but it also affords a safeguard against
breakdowns which is one of its important uses.
The ability to carry the load of a central office directly on the
charging generator without the use of a storage battery is of no
importance except in an emergency which takes the storage battery wholly
out of service. Since the beginning of common-battery working such
emergencies have happened a negligible number of times. Far more
communities have lacked telephone service because of accidents beyond
human control than because of storage-battery failures.
In power plants serving large offices, the demand upon the storage
battery is great enough to require large plate areas in each cell. The
internal resistance, therefore, is small and considerable fluctuations
may exist in the charging current without their being heard in the
talking circuits. The amount of noise to be heard depends also on the
type of charging generator. Increasing the number of armature coils and
commutator segments increases the smoothness of the charging current.
The shape of the generator pole pieces is also a factor in securing such
smoothness.
If, with a given machine and storage battery, the talking circuits are
disturbed by the charging current, relief may be obtained by inserting a
large impedance in the charging circuit. This impedance requires to be
of low resistance, because whatever heat is developed in it is lost
energy. This means that the best conditions exist when the resistance is
low and the inductance large. These conditions are satisfied by using in
the impedance coil many turns of large wire and an ample iron core.
Dynamotors are not generally suitable for charging purposes. Not only is
the difficulty in regulating their output a disadvantage, but the fact
that the primary and secondary windings are so closely associated on the
armature core makes them carry into the charging current, not only the
commutator noises of the generator end, but of the motor end as well.
_Mercury-Arc Rectifiers._ In common-battery offices serving a few
hundred lines, and where the commercial supply is alternating current,
it is good practice to transform it into direct-battery charging current
by means of a mercury-arc rectifier. It is a device broadly similar to
the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no
moving parts and operates at high efficiency without introducing noises
into the telephone lines. It requires little care and has good length of
life.
[Illustration: Fig. 415. Mercury-Arc Rectifier Circuits]
The circuit of a mercury-arc rectifier charging outfit is shown in Fig.
415. The mercury-arc rectifier proper consists of a glass bulb
containing vacuum and a small amount of mercury. When its terminals are
connected, as indicated--the two anodes across an alternating-current
source and the cathode with a circuit that is to be supplied with direct
current--this device has the peculiarity of action that current will
flow alternately from the two anodes always to the cathode and never
from it. The cathode, therefore, becomes a source of positive potential
and, as such, is used in charging the storage battery through the series
reactance coil and the compensating reactances, as indicated. The line
transformer shown at the upper portion of Fig. 415, is the one for
converting the high-potential alternating current to the comparatively
low-potential current required for the action of the rectifier. The
transformer below this has a one-to-one ratio, and is called the
insulating transformer. Its purpose is to safeguard the telephone
apparatus and circuits against abnormal potentials from the line, and
also to prevent the ground, which is commonly placed on the neutral wire
of transformers on commercial lighting circuits, from interfering with
the ground that is commonly placed on the positive pole of the
central-office battery.
=Provision Against Breakdown.= In order to provide against breakdown of
service, a well-designed telephone power plant should have available
more than one primary source of power and more than one charging unit
and ringing unit.
_Duplicate Primary Sources._ In large cities where the commercial power
service is highly developed and a breakdown of the generating station is
practically impossible, it is customary to depend on that service alone.
In order to insure against loss of power due to an accident to portions
of the distributing system, it is the common custom to run two entirely
separate power leads into the office, coming, if possible, from
different parts of the system so that a breakdown on one section will
not deprive the telephone exchange of primary power. In smaller places
where the commercial service is not so reliable, it is usual to provide,
in addition to the commercial electric-power service, an independent
source of power in the form of a gas or steam engine. This may be run as
a regular source, the commercial service being employed as an emergency
or _vice versâ_, as economy may dictate. In providing a gas engine for
driving charging dynamos, it is important to obtain one having as good
regulation as possible, in order to obtain a charging current of
practically constant voltage.
_Duplicate Charging Machines._ The storage batteries of telephone
exchanges are usually provided of sufficient capacity to supply the
direct-current needs of the office for twenty-four hours after a full
charge has been given them. This in itself is a strong safeguard against
breakdown. In addition to this the charging machines should be in
duplicate, so that a burnt-out armature or other damage to one of the
charging units will not disable the plant.
_Duplicate Ringing Machines._ It is equally important that the ringing
machines, whether of the rotary or vibrating type, be in duplicate. For
large exchanges the ringing machines are usually dynamos, and it is not
unusual to have one of these driven from the commercial power mains and
the other from the storage battery. With this arrangement complete
failure of all sources of primary power would still leave the exchange
operative as long as sufficient charge remains in the storage battery.
_Capacity of Power Units._ In designing telephone switchboards it is the
common practice to so design the frameworks that the space for multiple
jacks is in excess of that required for the original installation. In a
like manner, the power plant is also designed with a view of being
readily increased in capacity to an amount sufficient to provide current
for the ultimate number of subscribers' lines for which the switchboard
is designed. The motor generators, or whatever means are provided for
charging the storage batteries, are usually installed of sufficient size
to care for the ultimate requirements of the office. The ringing
machines are also provided for the ultimate equipment. However, in the
case of the storage battery, it is common practice to provide the
battery tanks of sufficient size to care for the ultimate capacity,
while the plates are installed for a capacity only slightly in excess of
that required for the original installation. As the equipment of
subscribers' lines is increased, additional plates may, therefore, be
added to the cells without replacing the storage battery as a whole, and
without making extraordinary provisions to prevent the interruption of
service. It is also customary to provide charging and supply leads from
the storage battery of carrying capacity sufficient for the ultimate
requirements of the office.
=Storage Battery.= The storage battery is the power plant element which
has made common-battery systems possible. The common-battery system is
the element which has made the present wide development of telephony
possible.
A storage-battery cell is an electro-chemical device in which a chemical
state is changed by the passage of current through the cell, this state
tending to revert when a current is allowed to flow in the opposite
direction. A storage cell consists of two conductors in a solution, the
nature and the relation of these three elements being such that when a
direct current is made to pass from one conductor to the other through
the solution, the compelled chemical change is proportional to the
product of the current and its duration. When the two conductors are
joined by a path over which current may flow, a current does flow in the
opposite direction to that which charged the cell.
All storage batteries so far in extensive use in telephone systems are
composed of lead plates in a solution of sulphuric acid in water called
the _electrolyte_. In charging, the current tends to oxidize the lead of
one plate and de-oxidize the other. In discharging, the tendency is
toward equilibrium.
The containers, employed in telephone work, for the plates and
electrolyte are either of glass or wood with a lead lining, the glass
jars being used for the smaller sized plates of small capacity cells,
while the lead-lined wooden tanks are employed with the larger capacity
cells. The potential of a cell is slightly over two volts and is
independent of the shape or size of the plates for a given type of
battery. The storage capacity of a cell is determined by the size and
the number of plates. Therefore, by increasing the number of plates and
the areas of their surfaces, the ampere-hour capacity of the cell is
correspondingly increased. The desired potential of the battery is
obtained by connecting the proper number of cells in series.
Storage-battery cells used in telephone work vary from 2 plates having
an area of 12 square inches each, to cells having over 50 plates, each
plate having an area of 240 square inches. The ampere-hour capacity of
these batteries varies from 6 ampere hours to 4,000 ampere hours,
respectively, when used at an average 8-hour discharge rate. In Fig. 416
is illustrated a storage cell employing a glass container and having
fifteen plates. Each plate is 11 inches high and 10-1/2 inches wide,
with an area, therefore, of 115.5 square inches. Such a cell has a
normal capacity of 560 ampere hours. The type illustrated is one made by
the Electric Storage Battery Company of Philadelphia, Pa.[A]
[Illustration: Fig. 416. Storage Cell]
_Installation._ In installing the glass jars it is customary to place
them in trays partially filled with sand. They are, however, at times
installed on insulators so designed as to prevent moisture from causing
leakage between the cells. The cells using wooden tanks are placed on
glass or porcelain insulators, and the tanks are placed with enough
clearance between them to prevent the lead lining of adjacent tanks from
being in contact and thereby short-circuiting the cells. After the
positive and the negative plates have been installed in the tanks, their
respective terminals are connected to bus bars, these bus bars being,
for the small types of battery, lead-covered clamping bolts, while in
the larger types reinforced lead bus bars are employed, to which the
plates are securely joined by a process called lead burning. This
process consists in melting a portion of the bus bar and the terminal
lug of the plate by a flame of very high temperature, thus fusing each
individual plate to the proper bus bar. The plates of adjacent cells are
connected to the same bus bar, thus eliminating the necessity of any
other connection between the cells.
_Initial Charge._ As soon as the plates have been installed in the tanks
and welded to the bus bars, the cell should be filled with electrolyte
having a specific gravity of 1.180 to 1.190 to one-half inch above the
tops of the plates and then the charge should be immediately started at
about the normal rate. In the case of a battery consisting of cells of
large capacity, it is customary to place the electrolyte in the cells as
nearly simultaneously as possible rather than to completely fill the
cells in consecutive order. When the electrolyte is placed in the cells
simultaneously, the charge is started at a very much reduced rate before
the cells are completely filled, the rate being increased as the cells
are filled, the normal rate of charge being reached when the cells are
completely filled. Readings should be taken hourly of the specific
gravity and temperature of the electrolyte, voltage of the cells, and
amperage of charging current. A record or log should be kept of the
specific gravity and voltage of each of the cells of the battery
regularly during the life of the battery and it is well to commence this
record with the initial charge.
The initial charge should be maintained for at least ten hours after the
time when the voltage and specific gravity have reached a maximum. If
for any reason it is impractical to continue the initial charge
uninterrupted, the first period of charging should be at least from
twelve to fifteen hours. However, every effort should be made to have
the initial charge continuous, as an interruption tends to increase the
time necessary for the initial charge, and if the time be too long
between the periods of the initial charge, the efficiency and capacity
of the cells are liable to be affected. In case of a large battery,
precaution should be taken to insure that the ventilation is
exceptionally good, because if it is not good the temperature is liable
to increase considerably and thereby cause an undue amount of
evaporation from the cells.
The object of the temperature readings taken during the charge is to
enable corrections to be made to the specific gravity readings as
obtained by the hydrometer, in order that the correct specific gravity
may be ascertained. This correction is made by adding .001 specific
gravity for each three degrees in temperature above 70° Fahrenheit, or
subtracting the same amount for each three degrees below 70° Fahrenheit.
At the time the cells begin to gas they should be gone over carefully to
see that they gas evenly, and also to detect and remedy early in the
charging period any defects which may exist. If there is any doubt in
regard to the time at which the cells reach a maximum voltage and
specific gravity, the charge should be continued sufficiently long
before the last ten hours of the charge are commenced to eliminate any
such doubt, as in many cases poor efficiency and low capacity of a cell
later in its life may be traced to an insufficient initial charge.
_Operation._ After the battery has been put in commission the periodic
charges should be carefully watched, as excessive charging causes
disintegration and decreases the life and capacity of the battery;
while, on the other hand, undercharging will result in sulphating of the
plates and decrease of capacity, and, if the undercharge be great, will
result in a disintegration of the plates. It is, therefore, essential
that the battery be charged regularly and at the rate specified for the
particular battery in question. In order to minimize the chance of
either continuously overcharging or undercharging the battery, the
charges are divided into two classes, namely, regular charges and
overcharges. The regular charges are the periodic charges for the
purpose of restoring the capacity of the battery after discharge. The
overcharges, which should occur once a week or once in every two weeks,
according to the use of the battery, are for the purpose of insuring
that all cells have received their proper charge, for reducing such
sulphating as may have occurred on cells undercharged, and for keeping
the plates, in general, in a healthy condition. The specific gravity of
the electrolyte, the voltage of the battery, and the amount of gasing
observed are all indications of the amount of charge which the battery
has received and should all be considered when practicable. Either the
specific gravity or voltage may be used as the routine method of
determining the proper charge, but, however, if the proper charge is
determined by the voltage readings, this should be frequently checked by
the specific gravity, and _vice versâ_.
During the charging and discharging of a battery the level of the
electrolyte in the cells will fall. As the portion of the electrolyte
which is evaporated is mainly water, the electrolyte may be readily
restored to its normal level by adding distilled water or carefully
collected rain water.
_Pilot Cell._ As the specific gravity of all the cells of a battery,
after having once been properly adjusted, will vary the same in all the
cells during use, it has been found satisfactory to use one cell,
commonly termed the pilot cell, for taking the regular specific gravity
readings and only reading the specific gravity of all the cells
occasionally or on the overcharge. This cell must be representative of
all the cells of the battery, and if the battery is so subdivided in use
that several sets of cells are liable to receive different usage, a
pilot cell should be selected for each group.
_Overcharge._ If the battery is charged daily, it should receive an
overcharge once a week, or if charged less frequently, an overcharge
should be given at least once every two weeks. In making an overcharge
this should be done at a constant rate and at a rate specified for the
battery. During the overcharge the voltage of the battery and the
specific gravity of the pilot cell should be taken every fifteen minutes
from the time the gasing begins. The charge should be continued until
five consecutive, specific-gravity readings are practically the same.
The voltage of the battery should not increase during the last hour of
the charge.
As the principal object of the overcharge is to insure that all of the
cells have received the proper charge, it must, therefore, be continued
long enough to not only properly charge the most efficient cells, but
also to properly charge those which are lower in efficiency. The longer
the interval between overcharges, the greater will be the variation
between the cells and, therefore, it is necessary to continue the
overcharge longer when the interval between overcharges is as great as
two weeks. Before the overcharge is made the cells should be carefully
inspected for short circuits and other abnormal conditions. These
inspections may best be made by submerging an electric lamp in the cell,
if the cell be of wood, or of allowing it to shine through from the
outside, if it be of glass. By this means any foreign material may be
readily detected and removed before serious damage is caused. In making
these inspections it must be borne in mind that whatever tools or
implements are used must be non-metallic and of some insulating
material.
_Regular Charge._ Regular charges are the periodic charges for restoring
the capacity of the battery, and should be made as frequently as the use
of the battery demands. The voltage of the cells is a good guide for
determining when the battery should be recharged. The voltage of a cell
should never be allowed to drop below 1.8 volts, and it is usually
considered better practice to recharge when the battery has reached 1.9
volts. If a battery is to remain idle for even a short time, it should
be left in a completely charged condition.
The regular charges for cells completely equipped with plates should be
continued until the specific gravity of the pilot cell has risen to five
points below the maximum attained on the preceding overcharge, or, if
only partially equipped with plates, until it has risen to three points
below the previous maximum. The voltage per cell at this time should be
from .05 volts to .1 volts below that obtained on the previous
overcharge. At this time all the cells should be gasing, but not as
freely as on an overcharge.
_Low Cells._ An unhealthy condition in a cell usually manifests itself
in one of the following ways: Falling off in specific gravity or voltage
relative to the rest of the cells, lack of gasing when charged, and
color of the plates, either noticeably lighter or darker than those of
other cells of the battery. When any of the above conditions are found
in a cell, the cell should receive immediate attention, as a delay may
mean serious trouble. The cell should be thoroughly inspected to
determine if a short-circuit exists, either caused by some foreign
substance, by an excess of sediment in the bottom of the tank, or by
portions of the plates themselves. If such a condition is found, the
cause should be immediately removed and, if the defect has been of short
duration, the next overcharge will probably restore it to normal
condition. If the defect has existed for some time, it is often
necessary to give the cell a separate charge. This may be done by
connecting it directly to the charging generator with temporary leads
and thus bring it back to its normal condition. It is sometimes found
necessary to replace the cell in order to restore the battery to its
normal condition.
_Sediment._ The cells of the battery should be carefully watched to
prevent the sediment which collects in the bottom of the jar or tank
during use from reaching the bottom of the plates, thereby causing short
circuits between them. When the sediment in the cell has reached within
one-half inch of the bottom of the plates, it should be removed at once.
With small cells using glass jars this can most easily be done directly
after an overcharge by carefully drawing off the electrolyte without
disturbing the sediment and then removing it from the jar. The plates
and electrolyte should be replaced in the jar as soon as convenient to
prevent the plates from becoming dry. If the plates are large and in
wooden tanks, the sediment can most easily be removed by means of a
scoop made especially for the purpose. The preferable time to clean the
tanks is just before an overcharge.
_Replacing Batteries._ There comes a time in the life of nearly every
central-office equipment when the storage battery must be completely
renewed. This is due to the fact that the life of even the best of
storage batteries is not as great as the life of the average switchboard
equipment. It may also be due to the necessity for greater capacity than
can be secured with the existing battery tanks, usually caused by
underestimating the traffic the office will be required to handle.
Again, it is sometimes necessary to make extensive alterations in an
existing battery, perhaps due to the necessity for changing its
location. To change a battery one cell at a time, keeping the others in
commission meanwhile, has often been done, but it is always expensive
and unsatisfactory and is likely to shorten the life of the battery, due
to improper and irregular forming of the plates during the initial
charge. The advent of the electric automobile industry has brought with
it a convenient means for overcoming this difficulty. Portable storage
cells for automobile use are available in almost every locality and may
often be rented at small cost. A sufficient number of such cells may be
temporarily installed, enough of them being placed in multiple to give
the necessary output. By floating a temporary battery so formed across
the charging mains and running the generators continuously, a temporary
source of current supply may be had at small expense for running the
exchange during the period required for alterations. Usually a time of
low traffic is chosen for making the changes, such as from Saturday
evening to Monday morning. Very large central-office batteries, serving
as many as 6,000 lines, have thus been taken out of service and replaced
without interfering with the traffic and with the use of but a
comparatively few portable cells. One precaution has to be observed in
such work, and that is not to subject the portable cells to too great an
overcharge, due to the great excess of generator over battery capacity.
This is easily avoided by watching the ammeters to see that the input is
not in too great excess of the output, and if necessary, by frequently
stopping the machines to avoid this.
=Power Switchboard.= The clearing-house of the telephone power plant is
the power board. In most cases, it carries switches, meters, and
protective devices.
_Switches._ The switches most essential are those for opening and
closing the motor and the generator circuits of the charging sets and
with these usually are associated the starting rheostats of the motors
and the field rheostats of the generators. The starting rheostats are
adapted to allow resistance to be removed from the motor armature
circuit, allowing the armature to gain speed and increase its
counter-electromotive force without overheating. The accepted type has
means for opening the driving circuit automatically in case its voltage
should fall, thus preventing a temporary interruption of driving current
from damaging the motor armature on its return to normal voltage.
[Illustration: Fig. 417. Power-Plant Circuits]
_Meters._ The meters usually are voltmeters and ammeters, the former
being adapted to read the several voltages of direct currents in the
power plant. An important one to be known is the voltage of the
generator before beginning a battery charge, so that the generator may
not be thrown on the storage battery while generating a voltage less
than that of the battery. If this were done, the battery would discharge
through the generator armature. The voltmeter enables the voltage of the
charging generator to be kept above that of the battery, as the latter
rises during charge. It enables the performance of several cells of the
battery to be observed. A convenient way is to connect the terminals of
the several cells to jacks on the power board and to terminate the
voltmeter in a plug.
The ammeter, with suitable connections, enables the battery-charge rate
to be kept normal and the battery discharge to be observed. In order to
economize power, it is best to charge the battery during the hours of
heavy load. The generator output then divides, the switchboard taking
what the load requires, the battery receiving the remainder.
In systems requiring the terminal voltage of the equipment to be kept
constant within close limits, either it is necessary to use two
batteries--never drawing current from a battery during charge--or to
provide means of compensating for the rise of voltage while the battery
is under charge. The latter is the more modern method and is done either
by using fewer cells when the voltage per cell is higher or by inserting
counter-electromotive force cells in the discharge leads, opposing the
discharge by more or fewer cells as the voltage of the battery is
higher or lower. In either method, switches on the power board enable
the insertion and removal of the necessary end cells or
counter-electromotive force cells.
_Protective Devices._ The protective devices required on a power board
are principally _circuit-breakers_ and _fuses_. Circuit-breakers are
adapted to open motor and generator circuits when their currents are too
great, too small, or in the wrong direction. Fuses are adapted to open
circuits when the currents in them are too great. The best type is that
in which the operation of the fuses sounds or shows an alarm, or both.
=Power-Plant Circuits.= The circuit arrangement of central-office power
plants is subject to wide variation according to conditions. The type of
telephone switchboard equipment, whether magneto or common-battery,
automatic or manual, will, of course, largely affect the circuit
arrangement of the power plant. Fig. 417 shows a typical example of good
practice in this respect for use with a common-battery manual
switchboard equipment. Besides showing the switches for handling the
various machines and the charge-and-discharge leads from the storage
battery, this diagram shows how current from the storage battery is
delivered to various parts of the central-office equipment.
[Footnote A: The instructions given later in this chapter are for
batteries of this make, although they are applicable in many respects to
all types commonly used in telephone work.]
CHAPTER XXXIII
HOUSING CENTRAL-OFFICE EQUIPMENT
=The Central-Office Building.= Proper arrangement of the central-office
equipment depends largely upon the design of the central-office
building. The problem involved should not be solved by the architect
alone. The most careful co-operation between the engineer and the
architect is necessary in order that the various parts of the telephonic
equipment may be properly related, and that the wires connecting them
with each other and with the outside lines be disposed of with due
regard to safety, economy, and convenience. So many factors enter into
the design of a central-office building that it is impossible to lay
down more than the most general rules. The attainment of an ideal is
often impossible, because of the fact that the building is usually in
congested districts, and its very shape and size must be governed by the
lot on which it is built, and by the immediate surroundings. Frequently,
also, the building must be used for other purposes than those of a
telephone office, so that the several purposes must be considered in its
design. Again, old buildings, designed for other purposes, must
sometimes be altered to meet the requirements of a telephone office, and
this is perhaps the most difficult problem of all.
The exterior of the building is a matter that may be largely decided by
the architect and owner after the general character of the building has
been determined. One important feature, however, and one that has been
overlooked in many cases that we know of, is to so arrange the building
that switchboard sections and other bulky portions of the apparatus,
which are necessarily assembled at the factory rather than on the site,
may be brought into the building without tearing down the walls.
_Fire Hazard._ The apparatus to be housed in a central-office building
often represents a cost running into the hundreds of thousands of
dollars; but whether of large or small first cost, it is evident that
its destruction might incur a very much greater loss than that
represented by its replacement value. In guarding the central-office
equipment against destruction by fire or other causes, the telephone
company is concerned to a very much greater extent than the mere cost of
the physical property; since it is guarding the thing which makes it
possible to do business. While the cost of the central office and its
contents may be small in comparison with the total investment in outside
plant and other portions of the equipment, it is yet true that these
larger portions of the investment become useless with the loss of the
central office.
There is another consideration, and that is the moral obligation of the
operating company to the public. A complete breakdown of telephone
service for any considerable period of time in a large city is in the
nature of a public calamity.
For these reasons the safeguarding of the central office against damage
by fire and water should be in all cases a feature of fundamental
importance, and should influence not only the character of the building
itself, but in many cases the choice of its location.
_Size of Building._ It goes without saying that the building must be
large enough to accommodate the switchboards and other apparatus that is
required to be installed. The requirement does not end here, however.
Telephone exchange systems have, with few exceptions, grown very much
faster than was expected when they were originally installed. Many
buildings have had to be abandoned because outgrown. In planning the
building, therefore, the engineer should always have in mind its
ultimate requirements. It is not always necessary that the building
shall be made large enough at the outset to take care of the ultimate
requirements, but where this is not done, the way should be left clear
for adding to it when necessity demands.
[Illustration: RINGING AND CHARGING MACHINES AND POWER BOARD Plaza
Office, New York Telephone Co.]
_Strength of Building._ The major portion of telephone central-office
apparatus, whether automatic or manual, is not of such weight as to
demand excessive strength in the floors and walls of buildings.
Exceptions to this may be found in the storage battery, in the power
machinery, especially where subject to vibration, and in certain cases
in the cable runs. After the ultimate size of the equipment has been
determined, the engineer and the architect should confer on this point,
particularly with reference to the heavier portions of the apparatus,
to make sure that adequate strength is provided. The approximate weights
of all parts of central-office equipments may readily be ascertained
from the manufacturers.
_Provision for Employes._ In manual offices particularly it has been
found to be not only humane, but economical to provide adequate quarters
for the employes, both in the operating rooms and places where they
actually perform their work, and in the places where they may assemble
for recreation and rest. The work of the telephone operator,
particularly in large cities, is of such a nature as often to demand
frequent periods of rest. This is true not only on account of the
nervous strain on the operator, but also on account of the necessity,
brought about by the demands of economy, for varying the number of
operators in accordance with the traffic load. These features accentuate
the demand for proper rooms where recreation, rest, and nourishment may
be had.
_Provision for Cable Runways._ In very small offices no special
structural provision need be made in the design of the building itself
for the entrance of the outside cables, and for the disposal of the
cables and wires leading between various portions of the apparatus. For
large offices, however, this must necessarily enter as an important
feature in the structure of the building itself. It is important that
the cables be arranged systematically and in such a way that they will
be protected against injury and at the same time be accessible either
for repairs or replacement, or for the addition of new cables to provide
for growth. Disorderly arrangement of the wires or cables results in
disorder indeed, with increased maintenance cost, uneconomical use of
space, inaccessibility, liability to injury, and general unsightliness.
The carrying of cables from the basement to the upper floors or between
floors elsewhere must be provided for in a way that will not be wasteful
of space, and arrangements must be made for supporting the cables in
their vertical runs. In the aggregate their weight may be great, and
furthermore each individual cable must be so supported that its sheath
will not be subject to undue strain. Another factor which must be
considered in vertical cable runs is the guarding against such runs
forming natural flues through which flames or heated gases would pass,
in the event of even an unimportant fire at their lower ends.
=Arrangement of Apparatus in Small Manual Offices.= Where a
common-battery multiple switchboard equipment is used, at least three
principal rooms should be provided--one for the multiple switchboard
proper; one for the terminal and power apparatus, including the
distributing frames, racks, and power machinery; and the third for the
storage battery. These should adjoin each other for purposes of
convenience and of economy in wiring.
[Illustration: Fig. 418. Typical Small Office Floor Plan]
_Floor Plans for Small Manual Offices._ As was pointed out, there are
several plans of disposing of the main and intermediate distributing
frames and the line and cut-off relay racks. The one most practiced is
to mount the relay rack alongside the main and intermediate distributing
frame in the terminal room. A typical floor plan of such an arrangement
for a small office, employing as a maximum five sections of multiple
switchboards, is shown in Fig. 418. This is an ideal arrangement well
adapted for a rectangular floor space and on that account may often be
put into effect. It should be noted that the switchboard grows from left
to right, and that alternative arrangements are shown for disposing of
those sections beyond the second. The cable turning section through
which the multiple and answering jacks are led to the terminal frames is
placed as close as possible to the terminal frames. This results in a
considerable saving in cable. An interesting feature of this floor plan
is the arrangement of unitary sections of main and intermediate frames
and relay racks, representing recent practice of the Western Electric
Company. The iron work of the three racks is built in sections and these
are structurally connected across so that the first section of the main
frame, the intermediate frame, and the relay rack form one unit, the
structural iron work which ties them together forming the runway for the
cables between them. But two of these units, including two sections of
each frame, are shown installed, the provision for growth being
indicated by dotted lines.
The battery room in this case provides for the disposal of the battery
cells in two tiers. This room is merely partitioned off from the
distributing or terminal room. Where this is done the partition walls
should be plastered on both sides so as to prevent, as far as possible,
the entrance of any battery fumes into the apparatus rooms.
The wire chief's desk, as will be noted, is located in such a position
as to give easy access from it not only to the distributing frames and
relay rack, but to the power apparatus as well.
_Combined Main and Intermediate Frames._ For use in small exchanges, the
Western Electric Company has recently put on the market a combined main
and intermediate distributing frame. This is constructed about the same
as an ordinary main frame, the protectors being on one side and the line
and intermediate frame terminals on the other. The lower half of the
terminals on each vertical bay is devoted to the outside line terminals
and the upper half is devoted to intermediate frame terminals. This
arrangement is indicated in the elevation in Fig. 419. With the use of
this combined main and intermediate frame, the floor plan of Fig. 418
may be modified, as shown in Fig. 420.
[Illustration: Fig. 419. Combined Main and Intermediate Frames]
[Illustration: Fig. 420. Small Office Floor Plan]
[Illustration: Fig. 421. Terminal Apparatus--Small Office]
In Fig. 421 is given an excellent idea of terminal-room apparatus
carried out in accordance with the more usual plan of employing separate
main and intermediate distributing frames. At the extreme right of this
figure the protector side of the main frame is shown. It will be
understood that the line cables terminate on the horizontal terminal
strips on the other side of this frame and are connected through the
horizontal and vertical runways of the frame to the protector terminals.
The intermediate frame is shown in the central portion of the figure,
the side toward the left containing the answering-jack terminals, and
the side toward the right the multiple jack terminals, these latter
being arranged horizontally. This horizontal and vertical arrangement of
the terminals on the main and intermediate distributing frames has been
the distinguishing feature between the Bell and Independent practice,
the Bell Companies adhering to the horizontal and vertical arrangement,
while the Independent Companies have employed the vertical arrangement
on both sides. We are informed that in the future the new smaller
installations of the Bell Companies will be made largely with the
vertical arrangement on both sides. At the left of Fig. 421 is shown the
relay rack in two sections of two bays each. This illustration also
gives a good idea of the common practice in disposing of the cables
between the frames in iron runways just below the ceiling of the
terminal room.
_Types of Line Circuits._ The design of the terminal-room floor plan
will depend largely on the arrangement of apparatus in the subscribers'
line circuits with respect to the distributing frames and relay racks.
The Bell practice in this respect has already been referred to and is
illustrated in Fig. 348. In this the line and cut-off relays are
permanently associated with the answering jacks and lamps, resulting in
the answering-jack equipment being subject to change with respect to the
multiple and the line through the jumpers of the intermediate frame. The
practice of the Kellogg Company, on the other hand, has been illustrated
in Fig. 353, and in this the line and cut-off relays are permanently
associated with the multiple and with the line, only the answering jacks
and lamps being subject to change through the jumper wires on the
intermediate frame. This latter arrangement has led to a very desirable
parallel arrangement of the two distributing frames and the relay rack.
These are made of equal length so as to correspond bay for bay, and are
placed side by side with only enough space between them for the passage
of workmen--the relay rack lying between the main and intermediate
frames. In this scheme all the multiple and answering-jack cables run
from the intermediate distributing frame, and the cabling between the
intermediate frame and the relay rack and between the relay rack and the
main frame is run straight across from one rack to the other. This
results in a great saving of cable within the terminal room, over that
arrangement wherein the cabling from one frame to another is necessarily
led along the length of the frame to its end and then passes through a
single runway to the end of the other frame.
=Large Manual Offices.= For purposes of illustrating the practice in
housing the apparatus in very large offices equipped with manual
switchboards, we have chosen the Chelsea office of the New York
Telephone Company as an excellent example of modern practice.
[Illustration: Fig. 422. Floor Plan, Operating Room, Chelsea Office, New
York City]
The ground plan of the building is U-shaped, in order to provide the
necessary light over the rather large floor areas. The plan of the
operating floor--the sixth floor of the building--is shown in Fig. 422.
As will be seen, this constitutes a single operating room, the _A_-board
being located in the right wing and the _B_-board in the left. The point
from which both boards grow is near the center of the front of the
building, the boards coming together at this point in a common cable
turning section. The disposal of the various desks for the manager,
chief operator, and monitors is indicated. Those switchboard sections
which are shown in full lines are the ones at present installed, the
provision for growth being indicated in dotted lines.
[Illustration: Fig. 423. Terminal Room and Operators' Quarters, Chelsea
Office, New York City]
The fifth floor is devoted to the terminal room and operators' quarters,
the terminal room occupying the left-hand wing and the major portion of
the front of the building, and the operators' quarters the right-hand
wing. The line and the trunk cables come up from the basement of the
building at the extreme left, being supported directly on the outside
wall of the building. Arriving at the fifth floor, they turn
horizontally and are led under a false flooring provided with trap
doors, to the protector side of the main frame. The disposal of the
cables between the various frames will be more readily understood by
reference to the following photographs.
A general view of a portion of the _A_-board of the Chelsea office is
shown in Fig. 424, this view being taken from a point in the left-hand
wing looking toward the front. In Fig. 425 is shown a closer view of a
smaller portion of the board. Fig. 426 gives an excellent idea of the
rear of this switchboard and of the disposal of the cables and wires.
The main mass of cables at the top are those of the multiple.
Immediately below these may be seen the outgoing trunk cables. The forms
of the answering-jack cables lie below these and are not so readily
seen, but the cables leading from these forms are led down to the runway
at the bottom of the sections, and thence along the length of the board
to the intermediate distributing frame on the floor below. The layer of
cables, supported on the iron rack immediately above the answering-jack
cable runway, shown at the extreme bottom of the view, are those
containing the wires leading from the repeating coils to the cord
circuits.
An interesting feature of this board is the provisions for protection
against injury by fire and water. On top of the boards throughout their
entire length there is laid a heavy tarpaulin curtain with straps
terminating in handles hanging down from its edges. These may be seen in
Fig. 426 and also in Fig. 425. The idea of this is that if the board is
exposed to a water hazard, as in the case of fire, the board may be
completely covered, front and rear, with this tarpaulin curtain, by
merely pulling the straps. The entire force--both operators and
repairmen--is drilled to assure the carrying out of this plan.
The rear of the boards is adapted to be enclosed by wooden curtains,
similar to those employed in roll-top desks. These are all raised in the
rear view of Fig. 426, the housing for the rolled-up curtain being shown
at the extreme top of the sections. In order to guard the multiple
cables and the multiple jacks against fire which might originate in the
cord-circuit wiring, a heavy asbestos partition is placed immediately
above the cord racks and is clearly shown in Fig. 426.
[Illustration: Fig. 424. Subscribers' Board. Chelsea Office, New York
City]
[Illustration: Fig. 425. Subscribers' Board. Chelsea Office, New York
City]
[Illustration: Fig. 426. Rear View Chelsea Switchboard]
[Illustration: Fig. 427. Terminal and Power Apparatus. Chelsea Office]
A view of the terminal and power room is shown in Fig. 427. In the upper
left-hand corner the cables may be seen in their passage downward from
the cable turning section between the _A_- and _B_-boards. The large
group of cables shown at the extreme left is the _A_-board multiple.
This passes down and then along the horizontal shelves of the
intermediate frame, which is the frame in the extreme left of this view.
The _B_-board multiple comes down through another opening in the floor,
and as is shown, after passing under the _A_-board multiple joins it in
the same vertical run from which it passes to the intermediate frame.
The cord-circuit cables lead down through the same opening as that
occupied by the _A_-board multiple and pass off to the right-hand one of
the racks shown, which contains the repeating coils. The cables leading
from the opening in the ceiling to the right-hand side of the
intermediate distributing frame are the answering-jack cables, and from
the terminals on this side of this frame other cables pass in smaller
groups to the relay terminals on the relay racks which lie between the
intermediate frame and the coil rack.
The power board is shown at the extreme right. The fuse panel at the
left of the power board contains in its lower portion fuses for the
battery supply leads to the operator's position and to private-branch
exchanges, and in its upper portion lamps and fuses for the ringing
generator circuits for the various operators' positions and also for
private-branch exchanges.
At the lower left-hand portion of this view is shown the battery
cabinet. It is the practice of the New York Telephone Company not to
employ separate battery rooms, but to locate its storage batteries
directly in the terminal room and to enclose them, as shown, in a wooden
cabinet with glass panels, which is ventilated by means of a lead pipe
extending to a flue in the wall.
One unit of charging machines, consisting of motor and generator, is
shown in the immediate foreground. A duplicate of this unit is employed
but is not shown in this view. The various ringing and message register
machines are shown beyond the charging machines. Three of these smaller
machines are for supplying ringing current and the remainder are for
supplying 30-volt direct current for operating the message registers.
One of the machines of each set is wound to run from the main storage
battery in case of a failure of the general lighting service from which
the current for operating is normally drawn.
[Illustration: Fig. 428. Terminal Apparatus. Chelsea Office]
[Illustration: Fig. 429. Floor Plan, Automatic Office, Lansing,
Michigan]
Another view of the terminal-room apparatus is given in Fig. 428. This
is taken from the point marked _B_ on the floor plan of Fig. 423. At the
right may be seen the message registers on which the calls of the
subscribers in this office are counted as a basis for the bills for
their service. At the extreme left is shown the private-line test board.
Through this board run all of the lines leased for private use, and also
all of the order wire or call lines passing through this office. The
purpose of such an arrangement is to facilitate the testing of such line
wires. At the right of this private-line test board is shown a
four-position wire chief's desk, upon which are provided facilities for
making all of the tests inside and outside.
[Illustration: Fig. 430. Line-Switch Units]
[Illustration: Fig. 431. Automatic Apparatus at Lansing Office]
The main frame is shown at the right of Fig. 428, just to the right of a
gallery from which a step-ladder leads. The left-hand side of this frame
is the line or protector side, but the portion toward the observer in
this picture is unequipped. These equipped protector strips carry 400
pairs of terminals each, and the consequent length of these strips makes
necessary the gallery shown, in order that all of them may be readily
accessible.
[Illustration: Fig. 432. Main Distributing Frame, Lansing Office]
[Illustration: Fig. 433. Line Switches]
[Illustration: POWER PLANT FOR AUTOMATIC SWITCHBOARD EQUIPMENT Bay
Cities Home Telephone Company, Berkeley, Cal.]
[Illustration: Fig. 434. Secondary Line Switches and First Selectors]
=Automatic Offices.= There is no great difference in the amount of floor
space required in central offices employing automatic and manual
equipment. Whatever difference there is, is likely to be in favor of the
automatic. The fact that no such rigid requirement exists in the
arrangement of automatic apparatus, as that which makes it necessary to
place the sections of a multiple board all in one row, makes it possible
to utilize the available space more economically with automatic than
with manual equipment.
[Illustration: Fig. 435. Second Selectors]
[Illustration: Fig. 436. Toll Distributing Frame and Harmonic
Converters]
In manual practice it is necessary to place the distributing frames and
power apparatus in a separate room from that containing the switchboard,
but in an automatic exchange no such necessity exists; in fact, so far
as the distributing-frame equipment is concerned, it is considered
desirable to have it located in the same room as the automatic switches.
The battery room in an automatic exchange should be entirely separate
from the operating room, since the fumes from the battery would be fatal
to the proper working of the automatic switches.
_Typical Automatic Office._ The floor-plan and views of a medium-sized
automatic office at Lansing, Michigan, have been chosen as representing
typical practice. The floor plan is shown in Fig. 429. The apparatus
indicated in full lines represents the present equipment, and that in
dotted lines the space that will be required by the expected future
equipment.
In Fig. 430 is shown a group of five line-switch units, representing a
total of five hundred lines. The length of such a unit is practically
fourteen feet and the breadth over all about twenty-two inches.
Fig. 431 shows a general view of this Lansing office, taken from a point
of view indicated at _A_ on the floor plan of Fig. 429. Fig. 432 shows
the main distributing frame, which is of ordinary type; Fig. 433 shows a
closer view of some of the primary line switches; Fig. 434 is a view of
the secondary line switches and first selectors, the latter being on the
right; Fig. 435 is a view of the frequency selectors and second
selectors, the former being used in connection with party-line work; and
Fig. 436 is a view of the toll distributing frame and harmonic
converters for party-line ringing.
A general view of the main switching room in the Grant Avenue office of
the Home Telephone Company of San Francisco is given in Fig. 437, this
being taken before the work of installation had been fully completed.
The present capacity of the equipment is 6,000 and the ultimate 12,000
lines. This office is one of a number of similar ones recently installed
for the Home Telephone Company in San Francisco, the combination of
which forms by far the largest automatic exchange yet installed. The
scope of the plans is such as to enable 125,000 subscribers to be served
without any change in the fundamental design, and by means merely of
addition in equipment and lines as demanded by the future subscriptions
for telephone service.
[Illustration: Fig. 437. Grant Avenue Office--San Francisco]
CHAPTER XXXIV
PRIVATE BRANCH EXCHANGES
=Definitions.= A telephone exchange devoted to the purely local uses of
a private establishment such as a store, factory, or business office, is
a private exchange. If, in addition to being used for such local
communication, it serves also for communication with the subscribers of
a city exchange, it becomes in effect a branch of the city exchange and,
therefore, a private branch exchange. The term "P. B. X." has become a
part of the telephone man's vocabulary as an abbreviation for private
branch exchange.
Private exchanges for purely local use require no separate treatment as
any of the types of switching equipments for interconnecting the lines
for communication, that have been or that will be described herein, may
be used. The problem becomes a special one, however, when communication
must also be had with the subscribers of a public exchange, since then
trunking is involved in which the conditions differ materially from
those encountered in trunking between the several offices in a
multi-office exchange.
For such communication one or more trunk lines are led from the private
branch office usually to the nearest central office of the public
exchange and such trunks are called private branch-exchange trunks. They
are the paths for communication between the private exchange and the
public exchange. For establishing the connections either between the
local lines themselves or between the local lines and the trunks, and
for performing other duties that will be referred to, one or more
private branch-exchange operators are employed at the switchboard of the
private establishment.
The private branch exchange may operate in conjunction with a manual or
an automatic public exchange, but whether manual or automatic, the
private exchange is usually manually operated, although it is quite
possible to make a private branch exchange that is wholly automatic and
will, therefore, involve no operator at all.
=Functions of the Private Branch-Exchange Operator.= It is possible, as
just stated, entirely to dispense with the private branch-exchange
operator so far as the mere connection and disconnection of the lines is
concerned. But the real function of the private branch-exchange operator
is a broader one than this and it is for this reason that even in
connection with automatic public exchanges, operators are desirable at
the private branches. The private branch-exchange operator is, as it
were, the doorkeeper of the telephone entrance to the private
establishment. She is the person first met by the public in entering
this telephone door. There is the same reason, therefore, why she should
be intelligent, courteous, and obliging as that the ordinary doorkeeper
should possess these characteristics.
As to incoming traffic to a private branch exchange, an intelligent
operator may do much toward directing the calls to the proper department
or person, even though the person calling may have little idea as to
whom he desires to reach. This saves the time of the person who makes
the call as well as that of the people at the private branch stations,
since it prevents their being unnecessarily called.
The functions of the private branch-exchange operator are no less
important with respect to outgoing calls. It is the duty of the operator
to obtain connections through the city exchange for the private branch
subscriber, who merely asks for a certain connection and hangs up his
receiver to await her call when she shall have obtained it. This saving
of time of busy people by having the branch-exchange operator make their
calls for them has one attending disadvantage, which is that the person
in the city exchange who is called does not, when he answers his
telephone, find the real party with whom he is to converse, but has to
wait until that party responds to the private branch operator's call.
This is akin to asking a person to call at one's office and then being
out when he gets there. This drawback is greatly accentuated where both
the parties that are to be involved in the connection are people high in
authority in certain establishments at private branch exchanges. Some
business houses have made the rule that the private branch operator
shall not connect with their lines until she has actually heard the
voice of the proper party at the other end. When two subscribers in two
different private branch exchanges where this rule is enforced, attempt
to get into communication with each other, the possibilities of trouble
are obvious.
All that may be said on this matter is that the person who calls another
by telephone should extend that person the same courtesies that he would
had he called him in person to his office; and that a person who is
called by telephone by another should meet him with the same
consideration as if he had received a personal call at his office or
home. The arbitrary ruling made by some corporations and persons, which
results always in the "other fellow's" doing the waiting, is not
ethically correct nor is it good policy.
=Private Branch Switchboards.= Private branch switchboards may be of
common-battery or magneto types regardless of whether they work in
conjunction with main office equipments having common-battery or magneto
equipments. Usually a magneto private branch exchange works in
conjunction with a magneto main office, but this is not always true.
There are cases where the private branch equipment of modern
common-battery type works in conjunction with main office equipment of
the magneto type; and in some of these cases the private branch exchange
has a much larger number of subscribers than the main office. This is
likely to be true in large summer resort hotels located in small and
otherwise unimportant rural districts. In one such case within our
knowledge the private branch exchange has a larger number of stations
than the total census population of the town, resulting in an apparent
telephone development considerably greater than one hundred per cent.
_Magneto Type._ Where both the private branch and the main office
equipments are of the magneto type, the private branch requirements are
met by a simple magneto switchboard of the requisite size, and the
trunking conditions are met by ring-down trunks extending to the main
office. In this case the supervision is that of the ordinary
clearing-out drop type, the operators working together as best they may.
_Common-Battery Type._ The cases where the private branch board is of
common-battery type and the main office of magneto type are
comparatively so few that they need not be treated here. Where they do
occur they demand special treatment because the main portion of the
traffic over the trunk lines to the city or town central office is
likely to be toll traffic through that office over long-distance lines.
The principal reason why the equipment of the town offices under such
conditions is magneto rather than common battery is that the traffic
conditions are those of short season and heavy toll, and common-battery
switching equipment at the main office has no especial advantages for
toll work.
[Illustration: Fig. 438. Desk Type, Private Branch Board]
For small private branch exchanges the desk type of switch board, shown
in Fig. 438, is largely used. The operator frequently has other work to
do and the desk is, therefore, a convenience. In larger private
exchanges, such as those requiring more than one operator, some form of
upright cabinet is employed, and if, as sometimes occurs, the branch
exchange is of such size as to demand a multiple board, then the general
form of the board does not differ materially from the standard types of
multiple board employed in regular central office work. The most common
private branch-exchange condition is that of a common-battery branch
working into a common-battery main office. In such the main point to be
considered is that of supervision of trunk-line connections.
_Cord Type._ For the larger sizes of branch exchange switchboards, the
switching apparatus is practically the same as that of ordinary manual
switchboards wherein the connections are made between the various lines
by means of pairs of cords and plugs. The private branch-exchange trunk
lines usually terminate on the private branch board in jacks but in some
cases plug-ended trunks are used.
[Illustration: Fig. 439. Key Type, Private Branch Board]
The line signals may consist in mechanical visual signals or in lamps,
the choice between these depending largely on the source of battery
supply at the branch exchange, a matter which will be considered later.
The trunk-line signals at the private branch board are usually ordinary
drops which are thrown when the main-exchange operator rings on the line
as she would on an ordinary subscriber's line. Frequently, however, lamp
signals are used for this purpose, being operated by locking relays
energized when the main-office operator rings or, in some cases,
operated at the time when the main-office operator plugs into the
trunk-line jack.
[Illustration: Fig. 440. Circuits, Key-Type Board]
_Key Type._ For small private branch-exchange switchboards, a type
employing no cords and plugs has come into great favor during recent
years. Instead of connecting the lines by jacks and plugs, they are
connected by means of keys closely resembling ordinary ringing and
listening keys. Such a switchboard is shown in Fig. 439, this having a
capacity of three trunks, seven local lines, and the equivalent of five
cord circuits. The drops associated with the three trunks may be seen in
the upper left-hand side of the face of the switchboard. Immediately
below these in three vertical rows are the keys which are used in
connecting the trunks with the "cord circuits" or connecting bus wires.
At the right of the drop associated with the trunks are seven visual
signals, these being the calling signals of the local lines. The seven
vertical rows of keys, immediately to the right of the three trunk-line
rows, are the line keys. The throwing of any one of these keys and of a
trunk-line key in the same horizontal row in the same direction will
connect a line with a trunk through the corresponding bus wires, leaving
one of the supervisory visual signals, shown at the extreme top of the
board, connected with the circuit. The keys in a single row at the right
are those by means of which the operator may bridge her talking set
across any of the "cord circuits." The circuits of this particular board
are shown in Fig. 440. This is equipped for common-battery working, the
battery feed wires being shown at the left.
=Supervision of Private Branch Connections.= At the main office where
common-battery equipment is used, the private branch trunks terminate
before the _A_-operators exactly in the same way as ordinary
subscribers' lines, _i. e._, each in an answering jack and lamp at one
position and in a multiple jack on each section. It goes without saying,
therefore, that the handling of a private branch call, either incoming
or outgoing, should be done by the _A_-operator in the same manner as a
call on an ordinary subscriber's line, and that the supervision of the
connection should impose no special duties on the _A_-operator.
There has been much discussion, and no final agreement, as to the proper
method of controlling the supervisory lamp at the main office of a cord
that is, at the time, connected to a private branch trunk. Three general
methods have been practiced:
The first method is to have the private branch subscriber directly
control the supervisory lamp at the main office without producing any
effect upon the private branch supervisory signal; this latter signal
being displayed only after the connection has been taken down at the
main office and in response to the withdrawal of the main office plug
from the private branch jack. This is good practice so far as the
main-office discipline is concerned but it results in a considerable
disadvantage to both the city and private branch subscribers in that it
is impossible for the private branch subscriber, when connected to the
other, to re-signal the private branch operator without the connection
being first taken down.
The second method is to have the private branch subscriber control both
the supervisory signal at the private branch board and at the main
board. This has the disadvantage of bringing both operators in on the
circuit when the private branch subscriber signals.
The third method, and one that seems best, is to place the supervisory
lamp of the private branch board alone under the control of the private
branch subscriber, so that he may attract the attention of the private
branch operator without disturbing the supervisory signal at the main
office. The supervisory signal at the main office in this case is
displayed only when the private branch operator takes down the
connection. This practice results in a method of operation at the main
office that involves no special action on the part of the _A_-operator.
She takes down the connection only when the main-office subscriber has
hung up his telephone and the private branch subscriber has disconnected
from the trunk.
Whatever method is employed, private branch disconnection is usually
slow, and for this reason many operating companies instruct the
_A_-operators to disconnect on the lighting of the supervisory lamp of
the city subscriber.
=With Automatic Offices.= Private branch exchanges most used in
connection with automatic offices employ manual switchboards, with the
cord circuits of which is associated a signal transmitting device by
which the operator instead of the subscriber may manipulate the
automatic apparatus of the public exchange by impulses sent over the
private branch-exchange trunk lines. The subscriber's equipment at the
private branch stations may be either automatic or manual. Frequently
the same private branch exchange will contain both kinds. With the
manual sub-station equipment the operation is exactly the same as in a
private branch of a manual exchange, except that the private branch
operator by means of her dial makes the central-office connection
instead of telling the main-office operator to do so for her. With
automatic sub-station equipment at the private branch the subscribers,
by removing their receivers from their hooks, call the attention of the
private branch operator, who may receive their orders and make the
desired central-office connection for them, or who may plug their lines
through to the central office and allow the subscribers to make the
connection themselves with their own dials.
In automatic equipment of the common-battery type, some change always
takes place in the calling line at the time the called subscriber
answers. In the three-wire system during the time of calling, both wires
are of the same polarity with respect to earth. At the time of the
answering of the called subscriber, the two wires assume different
polarities, one being positive to the other. Such a change is sufficient
for the actuation of devices local to the private exchange switchboard
and may be interpreted through the calling supervisory signal in such a
way as to allow it to glow during calling and not to glow after the
called subscriber has answered. In the two-wire automatic system a
similar change can be arranged for, with similar advantageous results.
_Secrecy._ In private exchanges operating in connection with automatic
central offices, the secret feature of individual lines may or may not
be carried into the private exchange equipment. Some patrons of
automatic exchanges set a high value on the absence of any operator in a
connection and transact business over such lines which they would not
transact at all over manual lines or would not transact in the same way
over manual lines. To some such patrons, the presence of a private
exchange operator, even though employed and supervised by themselves,
seems to be a disadvantage. To meet such a feeling, it is not difficult
to arrange the circuits of a private exchange switchboard so that the
operator may listen in upon a cord circuit at any time and overhear what
is being said upon it _so long as two subscribers are not in
communication on that cord circuit_. That is, she may answer a call and
may speak to the calling person at any time she wishes until the called
person answers. When he does answer and conversation can take place,
some device operates to disconnect her listening circuit from the cord
circuit, not to be connected again until at least one of the subscribers
has hung up his receiver. With private exchange apparatus so arranged,
the secrecy of the system is complete.
=Battery Supply.= There are three available methods of supplying direct
current for talking and signaling purposes to private branch exchanges,
each of which represents good practice under certain conditions. First,
by means of pairs of wires extended from the central-office battery;
second, by means of a local storage battery at the private branch
exchange charged over wires from the central office; and third, by means
of a local storage battery at the private exchange charged from a local
source.
The choice of these three methods depends always on the local conditions
and it is a desirable feature, to be employed by large operating
companies, to have all private branch-exchange switchboards provided
with simple convertible features contained within the switchboard for
adapting it to any one of these methods of supplying current.
If a direct-current power circuit is available at the private branch
exchange, it may be used for charging the local storage battery by
inserting mere resistance devices in the charging leads. If the local
power circuit carries alternating current, a converting device of some
sort must be used and for this purpose, if the exchange is large enough
to warrant it, a mercury rectifier is an economical and simple device.
The supply of current to private branch exchanges over wires leading to
the central-office battery has the disadvantage of requiring one or
several pairs of wires in the cables carrying the trunk wires. No
special wires are run, regular pairs in the paper insulated line or
trunk cables being admirably suited for the purpose. Sufficient
conductivity may be provided by placing several such pairs in multiple.
If the amount of current required by the private exchange warrants it,
pairs of charging wires from the central office may be fewer if a
battery is charged over them than if they are used direct to the bus
bars of the private exchange switchboard. If they are used in the latter
way, and this is simpler for reasons of maintenance, some means must be
provided to prevent the considerable resistance of the supply wires from
introducing cross-talk into the circuit of the private exchange. This is
accomplished by bridging a considerable capacity across the supply pairs
at the private exchange--ten to twelve microfarads usually suffice. This
point has already been referred to and illustrated in connection with
Fig. 141.
The number of pairs of wires, or, in other words, the amount of copper
in the battery lead between the central office and the private
branch-exchange switchboard needs to be properly determined not only to
eliminate cross-talk when the proper condensers are used with them, but
to furnish the proper difference of potential at the private exchange
bus bars, so that the line and supervisory signals will receive the
proper current. It is a convenience in installing and maintaining
private exchange switchboards of this kind to prepare tables showing the
number of pairs of No. 19 gauge and No. 22 gauge wires required for a
private exchange at a given distance from its central office and of a
probable amount of traffic. The traffic may be expressed in the maximum
number of pairs of cords which will be in use at one time. With this
fact and the distance, the number of pairs of wires required may be
determined.
=Ringing Current.= The ringing current may be provided in two ways: over
pairs of wires from the city-office ringing machines or by means of a
local hand generator, or both. A key should enable either of these
sources of ringing current to be chosen at will.
=Marking of Apparatus.= All apparatus should be marked with permanent
and clear labels. That private exchange switchboard is best at which an
almost uninformed operator could sit and operate it at once. It is not
difficult to lay out a scheme of labels which will enable such a board
to be operated without any detailed instructions being given.
=Desirable Features.= The board should contain means of connecting
certain of the local private exchange lines to the central-office trunks
when the board is unattended. Also, it is desirable that it should
contain means whereby any local private exchange line may be connected
to the trunk so that its station will act as an ordinary subscriber's
station. Whether the trunks of the private exchange lead to a manual or
an automatic equipment, it often is desired to connect a local line
through in that way, either so that the calling person may make his
calls without the knowledge of the private exchange operator, because he
wishes to make a large number of calls in succession, or because for
some other reason he prefers to transact his business directly with or
through the exchange than to entrust it to his operator.
CHAPTER XXXV
INTERCOMMUNICATING SYSTEMS
=Definition.= The term "intercommunicating" has been given to a
specialized type of telephone system wherein the line belonging to each
station is extended to each of the other stations, resulting in all
lines extending to all stations. Each station is provided with apparatus
by means of which the telephone user there may connect his own telephone
with the line of the station with which he wishes to communicate,
enabling him to signal and talk with the person at that station.
=Limitations.= The idea is simple. Each person does his own switching
directly, and no operator is required. It is easy to see, however, that
the system has limitations. The amount of line wire necessary in order
to run each line to each station is relatively great, and becomes
prohibitive except in exchanges involving a very small number of
subscribers, none of which is remote from the others. Again, the amount
of switching apparatus required becomes prohibitive for any but a small
number of stations. As a result, twenty-five or thirty stations are
considered the usual practical limit for intercommunicating systems.
=Types.= An intercommunicating system may be either magneto or
common-battery, according to whether it uses magneto or common-battery
telephones. The former is the simpler; the latter is the more generally
used.
[Illustration: WESTERN ELECTRIC COMPANY BATTERY ROOM AT MONMOUTH,
ILLINOIS]
=Simple Magneto System.= The schematic circuit arrangement of an
excellent form of magneto intercommunicating system is given in Fig.
441. In this, five metallic circuit lines are led to as many stations,
an ordinary two-contact open jack being tapped off of each line at each
station. A magneto bell of the bridging type is permanently bridged
across each line at the station to which that line belongs. The
telephone at each station is an ordinary bridging magneto set except
that its bell is, in each case, connected to the line as just stated.
Each telephone is connected through a flexible cord to a two-contact
plug adapted to fit into any of the jacks at the same station.
The operation is almost obvious. If a person at Station _A_ desires to
call Station _E_, he inserts his plug into the jack of line _E_ at his
station and turns his generator crank. The bell of Station _E_ rings
regardless of where the plug of that station may be. The person at
Station _E_ responds by inserting his own plug in the jack of line _E_,
after which the two parties are enabled to converse over a metallic
circuit. It makes no difference whether the persons, after talking,
leave these plugs in the jacks or take them out, since the position of
the plug does not alter the relation of the bell with the line.
[Illustration: Fig. 441. Magneto Intercommunicating System]
This system has the advantage of great simplicity and of being about as
"fool proof" as possible. It is, however, not quite as convenient to use
as the later common-battery systems which require no turning of a
generator crank.
=Common-Battery Systems.= In the more popular common-battery systems two
general plans of operation are in vogue, one employing a plug and jacks
at each station for switching the "home" instrument into circuit with
any line, and the other employing merely push buttons for doing the
same thing. These may be referred to as the plug type and the
push-button type, respectively.
[Illustration: Fig. 442. Plug Type of Common-Battery Intercommunicating
System]
_Kellogg Plug Type._ The circuits of a plug type of intercommunicating
system, as manufactured by the Kellogg Company, are shown in Fig. 442.
While only three stations are shown, the method of connecting more will
be obvious.
This system requires as many pairs of wires running to all stations as
there are stations, and in addition, two common wires for ringing
purposes. The talking battery feed is through retardation coils to each
line. When all the hooks are down, each call bell is connected between
the lower common wire and the tip side of the talking circuit individual
to the corresponding station. The ringing buttons at each station are
connected between the tip of the plug at that station and the upper
common wire. As a result, when a person at one station desires to call
another, it is only necessary for him to insert his plug in the jack of
the desired station and press his ringing button; the circuit being
traced from one pole of the ringing battery through the upper common
ringing wire, ringing key of the station making the call, tip of plug,
tip conductor of called station's line, bell of called station, and back
to the ringing battery through the lower common ringing wire.
[Illustration: Fig. 443. Push-Button Wall Set]
_Kellogg Push-Button Type._ Fig. 443 shows a Kellogg wall-type
intercommunicating set employing the push-button method of selecting,
and Fig. 444 shows the internal arrangement of this set.
[Illustration: Fig. 444. Push-Button Wall Set]
_Western Electric System._ The method of operation of the push-button
key employed in the intercommunicating system of the Western Electric
Company is well shown in Fig. 445. When the button is depressed all the
way down, as shown in the center cut of Fig. 445, which represents the
ringing position of the key, contact is made with the line wires of the
station called, and ringing current is placed on the line. When the
pressure is released, the button assumes an intermediate position, as
shown in the right-hand cut, which represents the talking position of
the key and in which the ringing contacts _1_ and _2_ are open, but
contact with the line for talking purposes is maintained. The key is
automatically held in this intermediate position by locking plate _3_
until this plate is actuated by the operation of another button which
releases the key so that it assumes its normal position as shown in the
left-hand cut. When a button is depressed to call a station, it first
connects the called station's line to the calling station through the
two pairs of contacts _4_ and _5_ and then connects the ringing battery
to that line by causing the spring _1_ to engage the contact _2_. The
ringing current then passes through the bell at the called station,
through the back contacts of the switch hook at that station, over one
side of the line, and through the "way-down" contact _1_ of the button
at the calling station, thence over the other side of the battery line
back to the ringing battery, operating the bell at the called station.
[Illustration: Fig. 445. Push-Button Action, Western Electric System]
The circuits of the Western Electric system are similar to those of Fig.
442, but adapted, of course, to the push-button arrangement of switches.
Two batteries are employed, one for ringing and the other for talking,
talking current being fed to the lines through retardation coils to
prevent interference or cross-talk from other stations which might be
connected together at the same time.
_Monarch System._ As the making of connections in an intercommunicating
system is entirely in the hands of the user, it is desirable that the
operation be simple and that carelessness on the part of the user result
in as few evil effects as possible. For instance, the leaving of the
receiver off its hook will, in many systems, result in such a drain on
the battery as to greatly shorten its life.
The system of the Monarch Company has certain distinctive features in
this respect. It is of the push-button type and as in the system just
discussed, one pressure of the finger on one button clears the station
of previous connections, rings the station called, and establishes a
talking connection between the caller's telephone and the line desired.
In addition to this, the system is designed to eliminate battery waste
by so arranging the circuits that the battery current does not flow
through either called or calling instrument until a complete connection
is made--the calling button down at one station, the home button down at
the called station, and both receivers off the hook. It does not hurt
the batteries, therefore, if one neglects to hang up his receiver.
[Illustration: Fig. 446. Push-Button Wall Set]
[Illustration: Fig. 447. Push-Button Action, Monarch System]
Three views of the wall set of this system are shown in Fig. 446, which
illustrates how both the door and the containing box are separately
hinged for easy access to the apparatus and connecting rack. As in the
Western Electric and Kellogg push-button systems, each push-button key
has three positions, as shown in Fig. 447. The first button shows all
the springs open, the normal position of the key. The second button is
in the half-way or talking position with all the springs, except the
ringing spring, in contact. The third button shows the springs all in
contact, the condition which exists when ringing a station.
The mechanical construction of the key is shown in Fig. 448. Each button
has a separate frame upon which the springs are mounted. Any one of the
frames with its group of contact springs may be removed without
interfering with either the electrical or the mechanical operation of
the others. This is a convenient feature, making possible the
installation of as few stations as are needed at first, and the
subsequent addition of buttons as other stations are added.
[Illustration: Fig. 448. Push-Button Keys]
The restoring feature is a horizontal metal carriage, in construction
very much like a ladder--one round pressing against each key frame, due
to the tension on the carriage exerted by a single flat spring. The
plunger of each button is equipped with a shoulder, which normally is
above the round of the ladder. When the button is operated, this
shoulder presses against a round of the carriage forcing it over far
enough so that the shoulder can slip by. The upper surface of the
shoulder is flat, and on passing below the pin, allows the carriage to
slip back into its normal position and the pin rests on the top of the
shoulder holding the plunger down. This position places the talking
springs in contact. The ringing springs are open until the plunger is
pressed all the way down, then the ringing contact is made. When the
pressure is released, the plunger comes back to the half-way or talking
position, leaving the ringing contacts open again.
When another button is pressed, the same operation takes place and, by
virtue of the carriage being temporarily displaced, the original key is
left free to spring back to its normal position.
Each station is provided with a button for each other station and a
"home" button. The salient feature of the system is that before a
connection may be established, the button at the calling station
corresponding to the station called and also the home button of the
station called must be depressed, if it is not already down. The home
key at any station, when depressed, transposes the sides of the line
with respect to the talking apparatus. The home key also has a spring
which changes the normal connection of the line at that station from the
negative to the positive side of the talking battery. Unless, therefore,
a connection between two stations is made through the calling key at one
station and the home key at the other, no current can flow even though
both receivers are off their hooks, because in that case no connection
will exist with the positive side of the battery. This relation is shown
in Fig. 449, which gives a simplified circuit arrangement for two
connected stations.
[Illustration: Fig. 449. Monarch Intercommunicating System]
Referring to Fig. 449, when the station called depresses the home button
the talking circuit is then completed after the hook switch is raised.
This is because the talking battery is controlled by the home key.
Conductors from both the negative and the positive sides of the battery
enter this key. In the normal position of the springs, the negative side
of the battery is in contact with the master spring in the home key and
through these springs the negative battery is applied to all the calling
keys, and from there on to the hook switch. When, however, the home
button is operated, the spring which carries the negative battery to
the home key is opened, and the spring which carries the positive
battery is closed. This puts the positive battery on at the hook switch
instead of the negative battery, as in its normal condition.
In this system it is seen that a separate pair of line wires is used for
each station, and in addition to these, two common pairs are run to all
stations, one for ringing and one for talking battery connections.
=For Private Branch Exchanges.= So far the intercommunicating system has
been discussed only with respect to its use in small isolated plants. It
has a field of usefulness in connection with city exchange work, as it
may be made to serve admirably as a private branch exchange. Where this
is done, one or more trunk lines leading to an office of the city
exchange are run through the intercommunicating system exactly as a
local line in that system, being tapped to a jack or push button at
every station. A person at any one of the stations may originate a call
to the main office by inserting his plug in the trunk jack, or pushing
his trunk push button. Also any station, within hearing or sight of the
trunk-line signal from the main office, may answer a main-office call in
the same way. In order that the convenience of a private branch exchange
may be fully realized, however, it is customary to provide an
attendant's station at which is placed the drop or bell on which the
incoming trunk signal is received. The duty of this attendant during
business hours is to answer trunk calls from the main office and finding
out what party is desired, call up the proper station on the
intercommunicating system. The party at that station may then connect
himself with the trunk.
The practice of the Dean Company, for instance, is as follows in regard
to trunking between intercommunicating systems and main offices with
common-battery equipment. The attendant's station telephone cabinet
contains, besides the push-button keys for local and trunk connections,
a drop signal and release key, together with relays in each trunk
circuit. The latter are used to hold the trunks until the desired party
responds.
The main-exchange trunk lines, besides terminating at the attendant's
station, are wired through the complete intercommunicating system so
that any intercommunicating telephone can be connected direct to the
central office by depressing the trunk key, which is provided with a
button of distinctive color. The pressing of the trunk key allows the
telephone to take its current from the main-office storage battery and
to operate the main-office line and supervisory signals direct, without
making it necessary to call on the attendant to set up the connection.
[Illustration: Fig. 450. Junction Box]
[Illustration: Fig. 451. Typical Arrangement of Intercommunicating
System]
Incoming calls from the common-battery main office to the
intercommunicating system are all handled by the attendant. The
main-office operator signals the intercommunicating system by ringing,
the same as for a regular subscriber's line. This will operate a drop in
the attendant's station cabinet, and through an armature contact, give a
signal on a low-pitched buzzer. This alarm buzzer operates only when the
main exchange is ringing and, therefore, does not require that the drop
shutter be restored immediately. An extra key may be provided for an
extension night-alarm bell, for use where the attendant also does work
in a room separate from that containing the attendant's station
telephone equipment.
The attendant operator answers the main-line signal by pressing the
proper trunk button, as designated by the operated drop on the
attendant's cabinet. The answering of the trunk connects a locking relay
across the circuit so that the attendant may call the desired party on
the intercommunicating system without having to hold the trunk manually.
The party desired is then notified which trunk to use and the attendant
operator hangs up her receiver, no further attention being necessary on
her part.
The trunk-holding relay is automatically released when the desired party
(with the telephone receiver off the hook) depresses the proper trunk
button, thus clearing the trunk line of all bridged apparatus and making
the talking circuit the same as in the regular type of private
branch-exchange switchboard.
The most convenient way of installing the wires of an intercommunicating
system is to run a cable containing the proper number of pairs to
provide for the ultimate number of stations to all the stations, tapping
off from the conductors in the cable to the jacks or push buttons at
each station. These tap connections are best made by means of junction
boxes which contain terminals for all the conductors.
Such a junction box, with the through cable and the tap cable in place,
is illustrated in Fig. 450. A schematic lay-out of the various parts of
a Dean intercommunicating system, provided with an attendant's station
and with trunks to a city office, is given in Fig. 451.
CHAPTER XXXVI
LONG-DISTANCE SWITCHING
=Definitions.= Telephone messages between communities are called
long-distance messages. They are also called toll messages. Almost all
long-distance traffic is handled by message-rate (measured-service)
methods of charge. All measured-service messages are toll messages,
whether they are completed within a given community or between
communities. The term "long-distance," therefore, is more descriptive
than the term "toll." The subject of local and long-distance measured
service is treated exhaustively in a chapter of its own.
Some telephone-exchange operating companies call their own inter-city
business "toll," and use the term "long-distance" for business carried
between exchanges for them by another company. The distinction seems to
be unwarranted.
=Use of Repeating Coil.= Most long-distance lines are magneto circuits.
If they are switched to grounded circuits, repeating coils need to be
inserted. Toll switching equipments contain means of inserting repeating
coils in the connecting cords when required. Their use reduces the
volume of transmitted speech, but often is essential even in connecting
metallic circuit lines, as a quiet local metallic circuit may have a
ground upon it which will cause excessive noises when a quiet
long-distance line is connected to it.
=Switching through Local Board.= In the simplest form of long-distance
switching, the lines terminate in switchboards with local lines and may
be connected with each other and with the local lines through the
regular cord circuits, if the equipment be of the magneto type. The
waystations on such a line are equipped with magneto generators. These
waystations may signal each other by bell ringing; the central office
may call any waystation by ringing the proper signal and may supervise
in a way all traffic on such lines by noting the calls for other
stations than the supervising exchange.
=Operators' Orders.= _By Call Circuits._ Where the long-distance traffic
between two communities is large, economy requires that the sending of
signals by ringing over the line, waiting for an answer, and then
reciting the details of the call, be improved upon. If the traffic is
large and the distance between communities small, call circuits are
established in the same way as between the switchboards in several
manual central offices of an exchange. The long-distance operator
handling the originating call passes the necessary details to the
distant operator by telephone over the call circuit. Such circuits also
are known as order circuits. They are accessible to originating
operators at keys and are connected directly and permanently to the
telephone sets of receiving operators. One call circuit can handle the
orders for a large number of actual conversation circuits. The operator
at the receiving end designates the conversation circuit which shall be
used, the originating operator following that instruction.
_By Telegraph._ Where traffic and distance are large, conversation lines
cost more than in the case last assumed. It then is of greater
importance to use all the possible talking circuits for actual
conversations in order that the revenue may be as high as possible. A
phantom circuit good enough for call circuit purposes would be good
enough for actual commercial messages, therefore, it is customary to
furnish such originating and receiving operators with Morse telegraph
sets. The lines are obtained by applying composite apparatus to the
conversation circuits. Two Morse circuits can be had from each
long-distance line without impairing any quality of that line except the
ability to ring over it. As one Morse circuit can carry information
enough between two operators to enable them to keep many telephone
circuits busy, they do not need to ring upon the composited lines, so
that nothing is lost while revenue is gained.
=Two-Number Calls.= In cases where the traffic between communities is
large, where the rate is small, and where the conversations are short
and more on the general order of local calls, it is usual to handle the
switches exactly as local calls are trunked between central offices of
the same exchange. That is, the subscriber's operator who answers the
call trunks it, by the assistance of a call circuit and an incoming
trunk operator. The subscriber's operator records only the numbers of
the calling and called subscribers. No long-distance operators at all
assist in these connections. They are known as "two-number calls." The
calling subscriber remains at his telephone until the conversation is
finished.
=Particular-Party-Calls.= In cases where the traffic is smaller, and
where the rate is large, it is customary to handle the calls through
long-distance operators. The ticket records the particular party wished,
and the calls are named "particular party" calls. In such connections
the calling patron is allowed to hang up his receiver, after his call is
recorded, and is called again when his correspondent is found and is
ready to talk. This makes _all calls for conversations_ outgoing ones.
Only recording operators receive calls _from_ patrons. Line operators
make calls _to_ patrons.
=Trunking.= Long-distance lines entering a city usually terminate in one
office only, no matter how many offices the local exchange may have. It
is possible to terminate these long-distance lines on a position of the
multiple switchboard for local lines. For a variety of reasons this is
not practiced except in special cases. The usual method is to terminate
them in a special long-distance board and to provide trunk lines from
this board to the one or more local switchboards of the exchange. In
common-battery systems these toll trunks are so arranged that the called
local subscriber receives transmitter current from the office nearest to
him, yet is able to show the long-distance operator the position of his
switch hook and is able to be called by the long-distance operator
without the intervention of the switching operator in the local office,
even though two repeating coils may be in the trunk circuit.
_Through Ringing._ There is a distinct traffic advantage in having the
ringing of the subscriber under the control of the long-distance
operator. The latter may call for the subscriber by stating her wish
over the call circuit associated with the long-distance trunk. The
connection having been made by the switching operator, the long-distance
operator may withhold ringing the subscriber's bell until all is in
readiness for the conversation.
_High-Voltage Toll Trunks._ In some systems, the long-distance trunks
are further specialized by being enabled to furnish transmitter current
to subscribers at a higher voltage than is used in local conversations.
With a given construction of transmitters there is a critical maximum
current which can be carried by the granular carbon of the instrument
without excessive heating, consequent noises, and permanent damage. The
shortest lines and the longest lines of an exchange district being
served by a source of current common to all, the standard potential of
this source must be such as to give the longest lines current enough
without giving the shortest lines too much. The very longest local
lines, however, do not receive current enough from the standard
potential to give maximum efficiency when talking over long distances,
though they get enough for local conversations. By providing a battery
with a voltage twice that used for local conversations and connecting it
into the current supply element of the toll trunk through non-inductive
resistances, not too much current may be given to the shortest lines and
considerably more than normal current to the longest lines.
=Ticket Passing.= When only one operator is necessary in a town, her
duty being to switch both local and long-distance lines, she may write
her own tickets and execute them entire. In larger communities with
larger long-distance traffic, the duties need to be specialized. The
subscribers' wants as to long-distance connections are given by
themselves to recording long-distance operators, who write them on
tickets and pass these to operators who get the parties together. The
problem of ticket-passing becomes important and many mechanical carriers
have been tried, culminating in the system which utilizes vacuum tubes.
This is in some ways similar to vacuum or compressed-air tube systems
for carrying cash in retail stores. The ticket is carried, however,
without any enclosing case and the tubes are flat instead of round, _i.
e._, they are rectangular in section. By suitable means a vacuum is
maintained in a large common tube having a tap to a box-like valve at
each line operator's position. A ticket tube connects this valve with a
distributing table at or near which the tickets are written. The tickets
are of uniform size and are so made as to enable a flap to be bent up
easily along one edge. The distributing operator has merely to insert
the ticket, bent edge foremost, in the open end of the tube, whereupon
the air pressure behind it will drive it through to its destination,
near by or far away. The tickets travel thirty feet a second. The tube
may be bent into almost any required form. The ticket, on arriving at a
line operator's position, slides between two springs, breaking a shunt
around a relay and allowing the latter to light the lamp.
=Waystations.= Waystations on long-distance lines may be equipped in
several ways. Most of them have magneto sets and can ring each other.
Some are equipped with common-battery sets and get all current for
signaling and transmission from a terminal central office. In the latter
case, there is the advantage that the ringers are in series with
condensers, assisting greatly in tests for fault locations. Such tests
are hindered by the presence of ringer bridges across the line, as in
magneto practice. Condensers can be inserted in series with ringers of
magneto sets if the testing advantage is valued highly enough. A
disadvantage of the use of common-battery sets in waystations on
long-distance lines is the lessened transmission volume of the stations
farthest from the current source.
_Center Checking._ An operating advantage of common-battery sets on
long-distance lines is that all calls are forced to be answered by the
terminal station. Waystations can not call each other, as they have no
calling means. With magneto sets, waystation agents sometimes call each
other direct and neglect to record the call and to remit its price. When
they can not call each other direct, the revenues of the company
increase.
A traffic method which requires all calls from waystations to be made to
a central switching office is called a center-checking system. It is so
called because all checking for stations so switched is done at the
central point instead of each waystation keeping its own records of
calls sent and received. In such practice it is usual to bill each
station once a month for the messages it sent. Where center checking is
not practiced, the agent makes a report and sends a remittance. Center
checking comes about naturally for waystations having no ringing
equipment.
Center checking originated long before the invention of common-battery
systems. It requires merely that no waystation shall have a generator
which can ring a bell. The method most widely used is to equip the
waystations with magneto generators which produce direct currents only;
such a generator cannot operate a polarized ringer. It is not usual to
produce the direct current by actually rectifying the alternating
current, but merely by omitting half the impulses, sending to the line
only alternate half-cycles of the current generated. Any drop or relay
adapted to respond to regular ringing current will respond to this
modified form of generator.
CHAPTER XXXVII
TELEPHONE TRAFFIC
The term "traffic," with reference to telephone service, has come to
mean the gross transaction of communication between telephone users.
This traffic may be expressed in whatever terms are found convenient for
the particular phase considered.
=Unit of Traffic.= With reference to payment for local telephone
service, the conversation is the unit of traffic. In the daily
operations of telephone systems there are fewer conversations than there
are connections and fewer connections than there are calls, because
lines are found busy and all calls to subscribers are not answered.
For these reasons, in traffic inquiries which have to do with the amount
of business which subscribers attempt to transact, the total traffic in
a given time usually is considered as so many calls originated by the
subscribers in the community. From this condition arises the term
"originating calls."
For the reason that the purpose of the switching equipment in a central
office is to make connections, the abilities of operators and of
equipments frequently are measured in terms of connections per hour or
per other unit of time.
For the reason that in charging for service all unavailing calls are
omitted, the conversation is the unit of traffic.
=Traffic Variations.= Telephone-exchange traffic is subject to such
general variations as are noted in the way a compass needle points
north, the migrations of birds, the blowing of the trade winds, and
other natural phenomena. There are variations in traffic which occur
each day, others which change with the seasons, and still others which
are related to holidays and other special commercial and social events.
For instance, the day before Thanksgiving Day, in many regions, is the
busiest telephone traffic day in the year.
[Illustration: WESTERN ELECTRIC MOTOR-GENERATOR CHARGING SET]
The daily variations in telephone traffic are closely related to
commercial activities and certain general features of this daily
variation are common to all telephone systems everywhere. Fig. 452 is a
typical graphic record of the traffic of a telephone exchange and
represents what happens in almost every town or city. The total calls in
this figure are not given as absolute units but would vary to adapt the
figure to a particular case. The figure shows principally that the
traffic in the night is light; that it rises to its maximum height
somewhere between 10 o'clock A.M. and noon; that though it is never as
high again during that day, the afternoon peak is over 80 per cent as
great; and that two minor peaks appear about the dinner hour and after
evening entertainments.
[Illustration: Fig. 452. Load Curve]
_Busy-Hour Ratio._ If the story told by Fig. 452 were to be turned into
a table of calls per hour, the busiest hour of the day would be found to
correspond to the highest portion of the figure, and in that busiest
hour of the day, if a number of selected days were to be compared, would
be found a very constant traffic. The number of calls made, or the
number of connections completed, in that particular hour, day by day,
would be found to be much the same. The ratio of the number of units in
that hour to the number of units in that entire day would be found to
be practically the same ratio day by day. This ratio of busy hour to
total day would be found to be much more nearly constant than the gross
number of calls per hour or per day.
In a large, busy city, about one-eighth of the total daily calls are in
some one hour; in a smaller, less active city, probably one-tenth are so
congested. This is reasonable when one remembers that in the larger city
the active business of the day begins later and ends earlier.
=Importance of Traffic Study.= A knowledge of the amount of traffic in
an exchange, and its distribution as to time and as to the divisions of
the exchange, is important for a number of reasons. Traffic knowledge is
essential in order that the equipment may be designed and placed in the
proper way and the total load distributed properly on that apparatus and
its operators.
For example, in an office equipped with a manual multiple switchboard,
the length of the switchboard is governed entirely by the number of
operators who must work before it. It is mechanically possible to make a
switchboard for ten thousand lines only 15 feet long, seating seven
operators. The entire multiple of ten thousand lines could appear three
times in such a switchboard. The seven operators could not handle the
traffic we know would be originated by ten thousand lines, with any
present system of charging for service. Even a rough knowledge of the
probable traffic would enable us to approximate the number of operators
needed and to equip each position, not only with access to the ten
thousand lines to be called, but also with just enough keyboard
equipment, serving as tools, and just enough answering jacks, serving as
means of bringing the traffic to her. It is foreknowledge of traffic
which enables a switchboard to fit the task it is to perform.
=Rates of Calling.= The rates of calling of different kinds of lines
vary. The lines of business stations originate more calls than do the
lines of residences. Some kinds of business originate more calls than
others. Some kinds of business have a higher rate of calling in one
season than in others. Flat-rate lines originate more calls than do
message-rate lines. When a line changes from a flat rate to a message
rate, the number of originating calls per day decreases. An operator's
position, handling message-rate lines only, can serve more lines than if
all of them were at flat rates. The number of message-rate or
coin-prepayment lines which an operator's position can care for depends
not only on the traffic but on the method of charging for service,
whether by tickets or meters and upon the kind of meters; or it depends
on the method of collecting the coins. In some regions, the rate of
calling, on the introduction of a complete measured-service plan, has
been reduced to one-fourth of what it was on the flat-rate plan.
In manual switchboards of early types, wherein the position of the
subscriber's answering jack was fixed by his telephone number, the
inequality of traffic became a serious problem. Most of the subscribers
who first installed telephones when the exchange was small, retained
their telephones and numbers; as their use of the telephone grew with
their business, it was customary to find the positions answering the
lower numbers much more busy than the positions answering the higher
numbers, the latter belonging to later and usually less active business
places.
_Functions of Intermediate Distributing Frame._ The intermediate
distributing board was invented to meet these conditions of unequal
traffic upon lines and of variations in traffic with changes of seasons
and of charges. The intermediate distributing board enables a line to
retain its number and its position in the multiple, but to keep its
answering jack and lamp signal in any desired position. If a flat-rate
subscriber changes to a message rate, his line may be moved to a
message-rate position and be answered, in company with others like it,
by an operator serving many more lines than she could serve if all of
them were flat rate.
=Methods of Traffic Study.= The best way to learn traffic facts for the
purposes of designing and operating equipment is to conduct systematic
series of observations in all exchanges; to record them in company with
all related facts; and to compare them from time to time, recording the
results of the comparisons. Then when it is required to solve a new
problem, the traffic data will enable the probable future conditions to
be known with as great exactness as is possible in studies with relation
to transportation or any other human activity.
TABLE XIII
Calling Rates
+-------------------------+-------------------------------+
| | CALLS PER DAY WITH DIFFERENT |
| KIND OF SERVICE | METHODS OF CHARGE |
| +-------------+-----------------+
| | FLAT RATE | MESSAGE RATE |
+-------------------------+-------------+-----------------+
|Residence | 8 | 4 |
|Business | 12 to 20 | 8 to 14 |
|Private Exchange Trunk | 40 | 25 |
|Hotel Exchange Trunk | 50 | 30 |
|Apartment House Trunk | 30 | 18 |
+-------------------------+-------------+-----------------+
There are three general ways of observing traffic. A record of
originating calls is known as a "peg count," because the counting
formerly was done by moving a peg from place to place in a series of
holes. The simplest exact way is to provide each operator with a small
mechanical counter, the key of which she can depress once for each call
to be counted. A second way is to determine a ratio which exists, for
the particular time and place, between the number of calls in a given
period and the average number of cord circuits in use. Knowing this
ratio, the cord circuits can be counted, the ratio applied, and the
probable total known. The third method, which is applicable to offices
having service meters on all lines, is to associate one master meter per
position or group of lines with all the meters of that position or
group, so that each time any service meter of that position is operated,
the master meter will count one unit. This method applies to either
manual or automatic equipments.
=Representative Traffic Data.= For purposes of comparison, the following
are representative facts as to certain traffic conditions.
_Calling Rates._ The number of calls originated per day by different
kinds of lines with different methods of charge are shown in Table XIII.
_Operators' Loads._ The abilities of subscribers' operators to switch
these calls depend on the type of equipment used, on the kind of
management exercised, and on the individual skill of operators. With
manual multiple equipment of the common-battery type, and good
management, the numbers of originating calls per busy hour given in
Table XIV can be handled by an average operator. The number of calls per
operator per busy hour depends upon the amount of trunking to other
offices which that operator is required to do. In a small city, for
example, where all the lines are handled by one switchboard, there is no
local switching problem except to complete the connection in the
multiple before each position. In a large city, where wire economy and
mechanical considerations compel the lines to be handled by a number of
offices with manual equipment, some portion of the total originating
load of each office must be trunked to others. Table XIV shows that an
increase of 90 per cent in the amount of out-trunking has decreased the
operator's ability to less than 70 per cent of the possible maximum.
TABLE XIV
Effect of Out-Trunking on Operator's Capacity
+----------------------------+---------------------------------------+
|PER CENT ORIGINATING CALLS | CAPACITY OF SUBSCRIBERS' OPERATOR'S |
|TRUNKED TO OTHER OFFICES | POSITION IN CALLS PER BUSY HOUR |
+----------------------------+---------------------------------------+
| 0 | 240 |
| 10 | 230 |
| 30 | 200 |
| 50 | 185 |
| 75 | 170 |
| 90 | 165 |
+----------------------------+---------------------------------------+
_Trunking Factor._ In providing the system of trunks interconnecting the
offices, whether the equipment be manual or automatic, it is essential
to know not only how much traffic originates in each office, but how
much of it will be trunked to each other office and how many trunks will
be required. An interesting phase of telephone traffic studies is that
it is possible to determine in advance the amount of traffic which can
be completed directly in the multiple of that office and how much must
be trunked elsewhere. Theoretical considerations would indicate that if
the local multiple contains one-eighth of the total lines of the city,
one-eighth of the calls originating in that office could be completed
locally and seven-eighths would be trunked out. In almost all cases,
however, it is found that more than the theoretical percentage of
originating calls are for the neighborhood of that office and can be
completed in the multiple. This results in the determination of a factor
by which the theoretical out-trunking can be multiplied to determine the
probable real out-trunking. In most cases, the ratio of actual to
theoretical out-trunking is 75 per cent, or approximately that. In
special cases, it may be far from 75 per cent.
_Trunk Efficiency._ The capacities of trunks vary with their methods of
operation and with the number of trunks in a group. For example, in the
manual system where trunk operators in distant offices are instructed
over call circuits and make disconnections in response to lamp signals,
such an incoming trunk operator can complete from 250 to 500 connections
per busy hour. The actual ability depends upon the number of distant
offices served by that operator and upon the amount of work she has to
perform on each call.
The number of messages which can be handled by one trunk in the busy
hour will depend upon the number of trunks in the group and upon the
system employed. It appears that the ability of trunks in this regard is
higher in the automatic system than in the manual system. For the
latter, Table XV gives representative facts.
TABLE XV
Messages per Trunk in Manual System
+----------------------------+------------------------+
| NUMBER OF TRUNKS IN GROUP, | MESSAGES PER TRUNK PER |
| MANUAL SYSTEM | BUSY HOUR |
+----------------------------+------------------------+
| 5 | 7 |
| 10 | 9 |
| 20 | 12 |
| 40 | 15 |
| 60 | 18 |
+----------------------------+------------------------+
Some of the reasons for the higher efficiencies of trunks in the
automatic system are not well defined, but unquestionably exist. They
have to do partly with the prompter answering observable in automatic
systems. The operation of calling being simple, a called subscriber
seems to fear that unless he answers promptly the calling party will
disconnect and perhaps may call a competitor. The introduction of
machine-ringing on automatic lines, where existing in competition with
manual ringing on manual lines, seems to encourage subscribers to answer
even more promptly. The length of conversation in automatic systems
seems to be shorter than in manual systems. Still more important,
disconnection in automatic systems is instantaneous during all hours,
whereas in manual systems it is less prompt in the busiest and least
busy hours than in the hours of intermediate congestion. The practical
results of trunk efficiencies in automatic systems are given in Table
XVI.
TABLE XVI
Messages per Trunk in Automatic System
+----------------------------+------------------------+
| NUMBER OF TRUNKS IN GROUP, | MESSAGES PER TRUNK PER |
| AUTOMATIC SYSTEM | BUSY HOUR |
+----------------------------+------------------------+
| 5 | 15 |
| 10 | 22 |
| 20 | 28 |
| 40 | 32 |
| 60 | 34 |
+----------------------------+------------------------+
_Toll Traffic._ Toll or long-distance traffic follows the general laws
of local or exchange traffic. Conversations are of greater average
length in long-distance traffic. The long-distance line is held longer
for an average conversation than is a local-exchange line. The local
trunks which connect long-distance lines with exchange lines for
conversation are held longer than are the actual long-distance trunks
between cities. Knowing the probable traffic to be brought to the
long-distance switching center by the long-distance trunks from exchange
centers, the number of trunks required may be determined by knowing the
capacity of each trunk. These trunk capacities vary with the method of
handling the traffic and they vary as do local trunks with the number of
trunks in a group. Table XVII illustrates this variation of capacity
with sizes of groups.
TABLE XVII
Messages per Trunk in Long-Distance Groups
+--------------------------+-------------------------+
| NUMBER OF LONG-DISTANCE | MESSAGES PER TRUNK PER |
| TRUNKS IN GROUP | BUSY HOUR |
+--------------------------+-------------------------+
| 5 | 2 |
| 10 | 3 |
| 20 | 3.2 |
| 40 | 3.5 |
| 60 | 4 |
| 100 | 4.6 |
+--------------------------+-------------------------+
=Quality of Service.= The quality of telephone service rendered by a
particular equipment managed in a particular way depends on a great
variety of elements. The handling of the traffic presented by patrons is
a true manufacturing problem. The quality of the service rendered
requires continuous testing in order that the management may know
whether the service is reaching the standard; whether the standard is
high enough; whether the cost of producing it can be reduced without
lowering the quality; and whether the patrons are getting from it as
much value as they might.
In manual systems, the quality of telephone service depends upon a
number of elements. The following are some principal ones:
1. Prompt answering.
2. Prompt disconnection.
3. Freedom from errors in connecting with the called line.
4. Promptness in connecting with the called line.
5. Courtesy and the use of form.
6. Freedom from failure by busy lines and failure to answer.
7. Clear enunciation.
8. Team work.
_Answering Time._ There is an interrelation between these elements. Team
work assists both answering and prompt disconnection. The quality of
telephone service can not be measured alone in terms of prompt
answering. Formerly telephone service was boasted of as being
"three-second service" if most of the originating calls were answered in
three seconds. Often such prompt answering reacts to prevent prompt
disconnecting. Patient, systematic work is required to learn the real
quality of the service.
As to answering, the clearest, truest statement concerning manual
service is found by making test calls to each position, dividing them
into groups of various numbers of whole seconds each, and comparing the
percentage of these groups to the whole number of telephones to that
position. For example, assume each of the calls to a given position to
have been answered in ten seconds or less, in which
100 per cent are answered in ten seconds or less;
80 per cent in eight seconds or less;
60 per cent in six seconds or less.
It is probable that a reasonably uniform manual service will show only a
small percentage answered in three seconds or under. Such percentages
may be drawn in the form of curves, so that at a glance one may learn
efficiency in terms of prompt answering.
_Disconnecting Time._ Prompt disconnection was improved enormously by
the introduction of relay manual boards. Just before the installation of
relay boards in New York City, the average disconnecting time was over
seventeen seconds. On the completion of an entire relay equipment, the
average disconnecting time was found to be under three seconds. The
introduction of relay manual apparatus has led subscribers to a larger
traffic and to the making of calls which succeed each other very
closely. A most important rule is, _that disconnect signals shall be
given prompt attention either by the operator who made the connection,
by an operator adjacent, or by a monitor who may be assisting_; and
another, still more important one is, _that a flashing keyboard lamp
indicating a recall shall be given precedence over all originating and
all other disconnect signals_.
_Accuracy and Promptness._ Promptness and accuracy in connecting with
the called line are vital, and yet a large percentage of errors in these
elements might exist in an exchange having a very high average speed of
answering the originating call. Indeed, it seems quite the rule that
where the effort of the management is devoted toward securing and
maintaining extreme speed of original answering, all the other elements
suffer in due proportion.
_Courtesy and Form._ It goes without saying that operators should be
courteous; but it is necessary to say it, and keep saying it in the most
effective form, in order to prevent human nature under the most
exasperating circumstances from lapsing a little from the standard,
however high. The use of form assists both the operators and the
subscribers, because in all matters of strict routine it is much easier
to secure high speed and great accuracy by making as many as possible of
the operations automatic. The use of the word "number" and other
well-accepted formalities has assisted greatly in securing speed, clear
understanding, and accurate performance. The simple expedient of
spelling numbers by repeating the figures in a detached form--as "1-2-5"
for 125--has taught subscribers the same expedient, and the percentage
of possible error is materially reduced by going one step further and
having the operator, in repeating, use always the opposite form from
that spoken by the calling subscriber.
_Busy and Don't Answer Calls._ Notwithstanding the old impression of the
public to the contrary, the operator has no control over the "busy line"
and "don't answer" situation. It is, however, of high importance that
the management should know, by the analysis of repeated and exhaustive
tests of the service, to what extent these troubles are degrading it. In
addition to improving the service by the elimination of busy reports,
there is no means of increasing revenue which is so easy and so certain
as that which comes from following up the tabulated results of busy
calls.
_Enunciation._ It must be remembered that clear enunciation for
telephone purposes is a matter wholly relative, and the ability of an
operator in this regard can be determined only by a close analysis of
many observations from the standpoint of a subscriber. A trick of speech
rather than a pleasant voice and an easy address has made the answering
ability of many an operator captivating to a group of satisfied
subscribers.
_Team Work._ By team work is meant the ability of a group of operators,
seated side by side, to work together as a unit in caring for the
service brought to them by the answering jacks within their reach. In
switchboards of the construction usual today, a call before any operator
may be answered by her, or by the operator at either the right or the
left of her position. In many exchanges this advantage is wholly
overlooked. In the period of general re-design of central-office
equipments about fourteen years ago, a switchboard was installed with
mechanical visual signals and answering-jacks on a flat-top board, and
an arrangement of operators such that the signal of any call was
extremely prominent and in easy reach of each one of four or possibly
five operators. Associated with the line signals within the reach of
such a group was an auxiliary lamp signal which would light when a call
was made by any of the lines so terminating. It was found that with this
arrangement the calls were answered in a strictly even manner, special
rushes being cared for by the joint efforts of the group rather than
serving to swamp the operator who happened to be in charge of the
particular section affected by the rush.
This principle has been tried out in so many ways that it is astonishing
that it is not recognized as being a vital one. The whole matter is
accomplished by impressing upon each operator that her duty is, _not_ to
answer the calls of a specific number of lines before her, but to
answer, with such promptness as is possible, _any call which is within
the reach of her answering equipment_.
=Observation of Service.= All that is required to be known concerning
the form of address and courtesy may be learned by a close observation
of the operators' work by the chief operators and monitors, and by the
use of listening circuits permanently connected to the operators' sets.
It is naturally necessary that the use of these listening circuits by
the chief operator or her assistants must not be known to the operators
at the times of use, even though they may know of the existence of such
facilities.
With a well-designed and properly maintained automatic equipment, the
eight elements of good manual service reduce themselves to only one or
two. Freedom from failure by busy lines and failure to answer are
service-qualities independent of the kind of switching apparatus. Too
great a percentage of busy calls for a given line indicates that the
telephone facilities for calls incoming to that subscriber are
inadequate. The best condition would be for each subscriber to have
lines enough so that none of them ever would be found busy. This is the
condition the telephone company tries to establish between its various
offices.
In manual practice it is possible to keep such records as will enable
the traffic department to know when the lines to a subscriber are
insufficient for the traffic trying to reach him. As soon as such facts
are known, they can be laid before the subscriber so that he may arrange
for additional incoming lines. In automatic practice this is not so
simple, as the source and destination of traffic in general is not so
clearly known to the traffic department. Automatic recorders of busy
calls are necessary to enable the facts to be tabulated.
CHAPTER XXXVIII
MEASURED SERVICE
In the commercial relation between the public and a telephone system,
the commodity which is produced by the latter and consumed by the former
is telephone service. Users often consider that payment is made for
rental of telephone apparatus and to some persons the payment per month
seems large for the rental of a mere telephone which could be bought
outright for a few dollars.
The telephone instrument is but a small part of the physical property
used by a patron of a telephone system. Even the _entire_ group of
property elements used by a patron in receiving telephone service
represents much less than what really is his proportion of the
service-rendering effort. What the patron receives is service and its
value during a time depends largely on how much of it he uses in that
time, and less on the number of telephones he can call.
_The cost of telephone service varies as the amount of use._ It is just,
therefore, that the selling price should vary as the amount of use.
=Rates.= There are two general methods of charging for telephone service
and of naming rates for this charge. These are called flat rates and
measured-service rates. The latter are also known as message rates,
because the message or conversation is the unit. Flat rates are those
which are also known as rentals. The service furnished under flat rates
is also known as unlimited service, for the reason that under it a
patron pays the same amount each month and is entitled to hold as many
conversations--send as many messages and make as many calls--as he
wishes, without any additional payment. In the measured-service plan,
the amount of payment in a month varies in some way with the amount of
use, depending on the plan adopted. The patron may pay a fixed base
amount per month, entitling him to have equipment for telephone service
and to receive messages, but being required to pay, in addition to this
base amount, a sum which is determined by the number of messages which
he sends. Or he may pay a base amount per month and be entitled to have
the equipment, to receive calls, and to send a certain number of
messages, paying specifically in addition only for messages exceeding
that certain number.
Whether flat rates or measured-service rates are practiced, the general
tendency is to establish lower rates for service in homes than in
business places. This is another recognition of the justice of
graduating the rates in accordance with the amount of use.
=Units of Charging.= While both the flat-rate and the measured-rate
methods of charging for unlimited and measured service are practiced in
local exchanges, long-distance service universally is sold at message
rates. The unit of message rates in long-distance service is time. The
charge for a message between two points joined by long-distance lines
usually is a certain sum for a conversation three minutes long plus a
certain sum for each additional minute or fraction of a minute. In local
service, the message-rate time charge per message takes less account of
the time unit. The conversation is almost universally the unit in
exchanges. Some managements restrict messages of multi-party lines to
five minutes per conversation, because of the desire to avoid
withholding the line from other parties upon it for too long periods.
Service sold at public stations similarly is restricted as to time, even
though the message be local to the exchange. Three to five minutes local
conversation is sold generally for five cents in the United States. The
time of the average local message, counting actual conversation time
only, is one hundred seconds.
=Toll Service.= _Long Haul._ In long-distance service, there are two
general methods of handling traffic, as to the relations between the
calling and the called stations. For the greater distances, as between
cities not closely related because not belonging to one general
community, the calling patron calls a particular person and pays nothing
unless he holds conversation with that person. In this method, the
operator records the name of the person called for; the name, telephone
number, or both, of the person calling; the names of the towns where the
message originated and ended; the date, the time conversation began, and
the length of time it lasted.
_Short Haul._ Where towns are closely related in commercial and social
ways and where the traffic is large and approaches local service in
character, and yet where conversations between them are charged at
different rates than are local calls within them, a more rapid system of
toll charging than that just described is of advantage. In these
conditions, patrons are not sold a service which allows a particular
party to be named and found, nor is the identity of the calling person
required. The operator needs to know merely of these calls that they
originate at a certain telephone and are for a certain other. The facts
she must record are fewer and her work is simpler. Therefore, the cost
of such switching is less than for true long-distance calls and it can
be learned by careful auditing just when traffic between points becomes
great enough to warrant switching them in this way. Such switching, for
example, exists between New York and Brooklyn, between Chicago and
suburbs around it which have names of their own but really are part of
the community of Chicago, and between San Francisco and other cities
which cluster around San Francisco Bay.
Calls of the "long-haul" class are known as "particular person" or
"particular party" calls, while "short-haul" calls are known as
"two-number" long-distance calls. It is customary to handle particular
party calls on long-distance switchboards and to handle two-number calls
in manual systems on subscribers' switchboards exactly like local calls,
except that the two-number calls are ticketed. It is customary in
automatic systems to handle two-number calls by means of the regular
automatic equipment plus ticketing by a suburban or two-number operator.
_Timing Toll Connections._ It formerly was customary to measure the time
of long-distance conversations by noting on the ticket the time of its
beginning and the time of its ending, the operator reading the time from
a clock. For human and physical reasons, such timing seems not to be
considered infallible by the patron who pays the charge, and in cases of
dispute concerning overtime charges so timed, telephone companies find
it wisest to make concessions. The physical cause of error in reading
time from a clock is that of parallax; that is, the error which arises
from the fact that the minute hand of a clock is some distance from the
surface of the dial so that one can "look under it." On an ordinary
clock having a large face and its minute hand pointing upward or
downward, five people standing in a row could read five different times
from it at the same instant. The middle person might see the minute
hand pointing at 6, indicating the time to be half-past something;
whereas, person No. 1 and person No. 5 in the row might read the time
respectively 29 and 31 minutes past something. Operators far to the
right or to the left of a clock will get different readings, and an
operator below a clock will get different kinds of readings at different
times and correct readings at few times.
Timing Machines:--Machines which record time directly on long-distance
tickets are of value and machines which automatically compute the time
elapsing during a conversation are of much greater value. The
calculagraph is a machine of the latter class. The use of some such
machine uniformly reduces controversy as to time which really elapsed.
Parallax errors are avoided. The record possesses a dignity which
carries conviction.
[Illustration: Fig. 453. Calculagraph Records]
Calculagraph records are shown in Fig. 453. In the one shown in the
upper portion of this figure, the conversation began at 10.44 P.M. This
is shown by the right-hand dial of the three which constitute the
record. The minutes past 10 o'clock are shown by the hand within the
dial and the hour 10 is shown by the triangular mark just outside the
dial between X and XI.
The duration of the conversation is shown by the middle and the
left-hand dials. The figures on both these dials indicate minutes. The
middle dial indicates roughly that the conversation lasted for a time
between 0 and 5 minutes. The left-hand dial indicates with greater
exactness that the conversation lasted one and one-quarter minutes.
The hand of the left-hand dial makes one revolution in five minutes; of
the middle dial, one revolution in an hour. The middle dial tells how
many full periods of five minutes have elapsed and the left-hand dial
shows the excess over the five-minute interval.
The lower portion of Fig. 453 is a similar record beginning at the same
time of day, but lasting about five and one-half minutes. As before, the
readings of the two dials are added to get the elapsed time.
[Illustration: Fig. 454. Relative Position of Hands and Dials]
The right-hand dial, showing merely time of day, stands still while its
hands revolve. The dies which print the dials and hands of the middle
and the left-hand records rotate together. Examining the machine, one
finds that the hands of these dials always point to zero. The middle
dial and hand make one complete revolution in an hour; the left-hand
dial and hand, one in five minutes. In making the records, the dials are
printed at the beginning and the hands at the end of the conversation.
Therefore, the hands will have moved forward during the
conversation--still pointing to zero in both cases--but when printed the
hands will point to some other place than they were pointing when the
dials were printed. In this way, their angular distances truly indicate
the lapse of time. Fig. 454 shows the relative position of the hands and
dials within the machine at all times. It will be noted that the arrow
of the left-hand dial does not point exactly to zero. This is due to the
fact that the dials and hands are printed by separate operations and
cannot be printed simultaneously.
[Illustration: WESTERN ELECTRIC RINGING MACHINE]
Another method of timing toll connections has been developed by the
Monarch Telephone Manufacturing Company. This employs a master clock of
great accuracy, which may be mounted on the wall anywhere in the
building or another building if desired. A circuit leads from this clock
to a time-stamp device on the operator's key shelf, and the clock closes
this circuit every quarter minute. The impulses thus sent over the
circuit energize the magnet of the time stamp, which steps a train of
printing wheels around so as always to keep them set in such position as
to properly print the correct time on a ticket whenever the head of the
stamp is moved by the operator into contact with the ticket. A large
number of such stamps may be operated from the same master clock. By
printing the starting time of a connection below the finishing time the
computation of lapsed time becomes a matter of subtraction. A typical
toll ticket with the beginning and ending time printed by the time stamp
in the upper left-hand corner and the elapsed time recorded by hand in
the upper right-hand corner is shown in Fig. 455. It is seen that this
stamp records in the order mentioned the month, the day, the hour, the
minute and quarter minute, the A.M. and P.M. division of the day, and
the year.
[Illustration: Fig. 455. Toll Ticket Used with Monarch System]
An interesting feature of this system is that the same master clock may
be made in a similar manner to actuate secondary clocks placed at
subscribers' stations, the impulses being sent over wires in the same
cables as those containing the subscribers' lines. This system,
therefore, serves not only as a means for timing the toll tickets and
operating time stamps wherever they are required in the business of the
telephone company, but also to supply a general clock and time-stamp
service to the patrons of the telephone company as a "by-product" of the
general telephone business.
Exchange service is measured in terms of conversations without much
regard to their length. The payment for the service may be made at the
time it is received, as in public stations and at telephones equipped
with coin prepayment devices; or the calls from a telephone may be
recorded and collection for them made at agreed intervals. In the
prepayment method the price per call is uniform. In the deferred payment
method the calls are recorded as they are made, their number summed up
at intervals, and the amount due determined by the price per call. The
price per call may vary with the number of calls sold. A large user may
have a lower rate per call than a small user.
=Local Service.= _Ticket Method._ Measured local service sometimes is
recorded by means of tickets, similarly to the described method of
charging long-distance calls, except that the time of day and the
duration of conversation are not so important. Where local ticketing is
practiced, it is usual to write on the ticket only the number of the
calling telephone and the date, and to pass into the records only those
tickets which represent actual conversations, keeping out tickets
representing calls for busy lines and calls which were not answered.
_Meter Method._ The requirements of speed in good local service are
opposed to the ticketing method. Where measured service is supplied to a
substantial proportion of the lines of a large exchange,
electro-mechanical service meters are attached to the lines. These
service meters register as a consequence of some act on the part of the
switchboard operator, or may be caused to register by the answering of
the called subscriber.
[Illustration: Fig. 456. Connection Meter]
In manual practice, meters of the type shown in Fig. 456 are associated
with the lines as in Fig. 457. The meters are mounted separately from
the switchboard, needing only to be connected to the test-strand of the
line by cabled wires. If desired, the meter may be mounted on racks in
quarters especially devoted to them, and the cases in which the racks
are mounted may be kept locked. In such an arrangement the meters are
read from time to time through the glass doors of the cases.
The meters are caused to operate by pressure on the meter key _MK_,
associated with the answering cord as in Fig. 458. This increases the
normal potential to 30 volts. When the armature of the meter has made a
part of its stroke, it closes a contact which places its 40-ohm winding
in shunt with its 500-ohm winding, thus furnishing ample power for
turning the meter wheels.
[Illustration: Fig. 457. Western Electric Line Circuit and Service
Meter]
Such meters are in common use in large exchanges, notable examples being
the cities of New York and London. In London, there is a zone within
which the price per call is one penny and between which and other zones
the price is twopence. Calls within the zone either are completed by the
answering operator directly in the multiple before her or are trunked to
other offices in that zone. Calls for points outside of that zone are
trunked to other offices and in giving the order the operator finds that
the call circuit key lights a special signal lamp before her. This
reminds her that the call is at a twopence price, so in recording it she
presses the meter key twice. This counts two units on the meter and the
units are billed at a penny each.
In automatic systems it is not possible to operate a meter system in
which the operator will press a key for each call to be charged, because
there is no operator. In such systems--a notable example being the
measured-service automatic system in San Francisco--the meter registers
only upon the answering of the called subscriber. Calls for lines found
busy and calls which are not answered do not register. Calls for
long-distance recording operators, two-number ticket operators,
information, complaint, and other company departments are not
registered. In the Chinatown quarter of San Francisco, where most calls
begin and end in the neighborhood, service is sold at an unlimited flat
rate for neighborhood calls and at a message rate for other calls. The
meter system recognizes this condition and does not register calls
_from_ Chinese subscribers _for_ Chinese subscribers, though it does
register calls from Chinese subscribers to Caucasian subscribers. The
nature of the system is such as to enable it to discriminate as to
races, localities, or other peculiarities as may be desired.
[Illustration: Fig. 458. Western Electric Cord Circuit and Service Meter
Key]
In the manual meter circuits of Figs. 457 and 458, the meter windings
have no relation to the line conductors. In the automatic arrangement
just described, there are meter windings in the line during times of
calling, but none in the line during times of conversation. The balance
of the line, therefore, is undisturbed at all times wherein balance is
of any importance.
In both systems just described, the meters of all lines are in their
respective central offices. Meters for use at subscribers' stations have
been devised and there is no fundamental reason why the record might not
be made at the subscriber's station instead of, or in addition to, a
central-office record. Experience has shown that confidence in a meter
system can be secured if the meters be positive, accurate, and reliable.
The labor of reading the meters is much less when they are kept in
central offices. Subscribers may have access to them if they wish.
_Prepayment Method._ Prepayment measured-service mechanisms permit a
coin or token to be dropped into a machine at the subscriber's telephone
at the time the conversation is held. A variety of forms of telephone
coin collectors are in use, their operations being fundamentally either
electrical or mechanical.
Electrically operated coin collectors require either that the coin be
dropped into the machine in order to enable the central office to be
signaled in manual systems, or the switches to be operated in automatic
systems, or they require that the coin be dropped into the machine after
calling, but before the conversation is permitted.
Western Electric Company coin collectors, shown in Fig. 459, may be
operated in either way in connection with manual systems. The usual way
is to require the coin to be dropped before the central-office line lamp
can glow. The operator then rings the called subscriber and upon his
answering places a sufficient potential upon the calling line to operate
the polarized relay and to drop the coin into the cash box. If the
called subscriber does not answer or his line is busy, potential is
placed on the calling line, moving the polarized relay in the other
direction and dropping the coin into a return chute so that the
subscriber may take it. If it is preferred that the coin be paid only on
the request of the operator, the return feature need not be provided.
In both forms of operation, the Western Electric coin collector is
adapted to bridge its polarized relay between one limb of the line and
ground during the time a coin rests on the pins, as shown in Fig. 459.
When no coin is on the pins--_i. e._, before calling and after the
called station responds--the relay is not so bridged.
[Illustration: Fig. 459. Principle of Western Electric Coin Collector]
The armature of the relay responds only to a high potential and this is
applied by the operator. If the coin is to be taken by the company, one
polarity is sent; if it is to be returned to the patron, the other
polarity is sent. These polarities are applied to a limb of the line
proper. It will be recalled that pressures to actuate service meters are
applied to the test-strand. If wished, keys may be arranged so as to
apply 30 volts to the test-strand and the collecting potential to the
line at the same operation. This enables the service meter to count the
tokens placed in the cash box of the coin collector, and serves as a
valuable check.
In automatic systems, in one arrangement, coin collectors are arranged
so that no impulses can be sent unless a coin has been deposited, the
coin automatically passing to the cash box when the called subscriber
answers, or to the patron if it is not answered. In another arrangement,
calls are made exactly as in unlimited service, but a coin must be
deposited before a conversation can be held. The calling person can hear
the called party speak and may speak himself but can not be heard until
the coin is deposited. No coin-return mechanism is required in this
method.
Coin collectors of these types usually are adapted to receive only one
kind of coin, these, in the United States, being either nickels or
dimes. For long-distance service, where the charges vary, it is
necessary to signal to an operator just what coins are paid. It is
uniformly customary to send these signals by sound, the collector being
so arranged that the coins strike gongs. In coin collectors of the Gray
Telephone Paystation Company, the coins strike these gongs by their own
weight in falling through chutes. In coin collectors of the Baird
Electric Company, the power for the signals is provided by hand power, a
lever being pulled for each coin deposited. Both methods are in wide
use.
CHAPTER XXXIX
PHANTOM, SIMPLEX, AND COMPOSITE CIRCUITS
=Definitions.= Phantom circuits are arrangements of telephone wires
whereby more working, non-interfering telephone lines exist than there
are sets of actual wires. When four wires are arranged to provide three
metallic circuits for telephone purposes, two of the lines are physical
circuits and one is a phantom circuit.
Simplex and composite circuits are arrangements of wires whereby
telephony and telegraphy can take place at the same time over the same
wires without interference.
[Illustration: Fig. 460. Phantom Circuit]
=Phantom.= In Fig. 460 four wires join two offices. _RR_ are repeating
coils, designed for efficient transforming of both talking and ringing
currents. The devices marked _A_ in this and the following figures are
air-gap arresters. Currents from the telephones connected to either
physical pair of wires pass, at any instant, in opposite directions in
the two wires of the pair. The phantom circuit uses one of the physical
pairs as a _wire_ of its line. It does this by tapping the middle point
of the line side of each of the repeating coils. The impedance of the
repeating-coil winding is lowered because, all the windings being on
the same core, the phantom line currents pass from the middle to the
outer connections so as to neutralize each other's influence. The
currents of the phantom circuit, unlike those of the physical circuits,
are _in the same direction_ in both wires of a pair at any instant.
Their potentials, therefore, are equal and simultaneous.
A phantom circuit is formed most simply when both physical lines end in
the same two offices. If one physical line is longer than the other, a
phantom circuit may be formed as in Fig. 461, wherein the repeating coil
is inserted in the longer line where it passes through a terminal
station of the shorter.
[Illustration: Fig. 461. Phantom from Two Physical Circuits of Unequal
Length]
[Illustration: Fig. 463. Two Phantoms Joined by Physical Circuit]
A circuit may be built up by adding a physical circuit to a phantom. A
circuit may be made up of two or more phantom circuits, joined by
physical ones. In Fig. 462 a phantom circuit is extended by the use of a
physical circuit, while in Fig. 463, two phantom circuits are joined by
placing between them a physical circuit.
[Illustration: Fig. 462. Phantom Extended by Physical Circuit]
_Transpositions._ In phantom circuits formed merely by inserting
repeating coils in physical circuits and doing nothing else, an exact
balance of the sides of the phantom circuit is lacking. The resistances,
insulations, and capacities to earth of the sides may be equal, but the
exposures to adjacent telephone and telegraph circuits and to power
circuits will not be equal unless the phantom circuits are transposed.
To transpose a set of lines of two physical wires each, is not
complicated, though it must be done with care and in accordance with a
definite, foreknown plan. Transposing phantom circuits is less simple,
however, as four wires per circuit have to be transposed, instead of
two.
[Illustration: Fig. 464. Transposition of Phantom Circuits]
In Fig. 464, the general spacing of transposition sections is the usual
one, 1,300 feet, of the _ABCB_ system widely in use. The pole circuit,
on pins _5_ and _6_ of the upper arm, is transposed once each two miles.
The pole circuit of the second arm transposes either once or twice a
mile. But neither pole circuit differs in transposition from any other
regular scheme except in the frequency of transposition. All the other
wires of each arm, however, are so arranged that each wire on either
side of the pole circuit moves from pin to pin at section-ends, till it
has completed a cycle of changes over all four of the pins on its side.
In doing so, each phantom circuit is transposed with proper regard to
each of the other three on that twenty-wire line.
The "new transposition" lettering in Fig. 464 is for the purpose of
identifying the exact scheme of wiring each transposition pole. The
complication of wiring at each transposition pole is increased by the
adoption of phantom circuits. Maintenance of all the circuits is made
more costly and less easy unless the work at points of transposition is
done with care and skill. Phantom circuits, to be always successful,
require that the physical circuits be balanced and kept so.
_Transmission over Phantom Circuits._ Under proper conditions phantom
circuits are better than physical circuits, and in this respect it may
be noted that some long-distance operating companies instruct their
operators always to give preference to phantom circuits, because of the
better transmission over them. The use of phantom circuits is confined
almost wholly to open-wire circuits; and while the capacity of the
phantom circuit is somewhat greater than that of the physical circuit,
its resistance is considerably smaller. In the actual wire the phantom
loop is only half the resistance of either of the physical lines from
which it is made, for it contains twice as much copper. The resistance
of the repeating coils, however, is to be added.
=Simplex.= Simplex telegraph circuits are made from metallic circuit
telephone lines, as shown in Fig. 465. The principle is identical with
that of phantom telephone circuits. The potentials placed on the
telephone line by the telegraph operations are equal and simultaneous.
They cause no current to flow _around_ the telephone loop, only _along_
it. If all qualities of the loop are balanced, the telephones will not
overhear the telegraph impulses. In the figure, _AA_ are arresters, as
before, _GG_ are Morse relays; a 2-microfarad condenser is shunted
around the contact of each Morse key _F_ to quench the noises due to the
sudden changes on opening the keys between dots and dashes.
[Illustration: Fig. 465. Simplex Telegraph Circuit]
A simplex arrangement even more simple substitutes impedance coils for
the repeating coils of Fig. 465. The operation of the Morse circuit is
the same. An advantage of such a circuit, as shown in Fig. 466, is that
the telephone circuit does not suffer from the two repeating-coil losses
in series. A disadvantage is, that in ringing on such a line with a
grounded generator, the Morse relays are caused to chatter.
[Illustration: Fig. 466. Simplex Telegraph Circuit]
The circuit of Fig. 465 may be made to fit the condition of a through
telephone line and a way telegraph station. The midway Morse apparatus
of Fig. 467 is looped in by a combination of impedance coils and
condensers. The plans of Figs. 465 and 466 here are combined, with the
further idea of stopping direct and passing alternating currents, as is
so well accomplished by the use of condensers.
[Illustration: Fig. 467. Simplex Circuit with Waystation]
[Illustration: Fig. 468. Composite Circuit]
=Composite.= Composite circuits depend on another principle than that of
producing equal and simultaneous potentials on the two wires of the
telephone loop. The opposition of impedance coils to alternating
currents and of condensers to direct currents are the fundamentals. The
early work in this art was done by Van Rysselberghe, of Belgium. In Fig.
468, one telephone circuit forms two Morse circuits, two wires carrying
three services. Each Morse circuit will be seen to include, serially,
two 50-ohm impedance coils, and to have shunts through condensers to
ground. The 50-ohm coils are connected differentially, offering low
consequent impedance to Morse impulses, whose frequency of interruption
is not great. As the impedance coils are large, have cores of
considerable length, and are wound with two separate though serially
connected windings each, their impedance to voice currents is great.
They act as though they were not connected differentially, so far as
voice currents are concerned.
Because of the condensers serially in the telephone line, voice currents
can pass through it, but direct currents can not. Impulses due to
discharges of cores, coils, and capacities in the Morse circuit _could_
make sounds in the telephones, but these are choked out, or led to earth
by the 30-ohm impedance coils and the heavy Morse condensers.
=Ringing.= Ringing over simplex circuits is done in the way usual where
no telegraph service is added. Both telegraphy and telephony over
simplex circuits follow their usual practice in the way of calling and
conversing. In composite working, however, ringing by usual methods
either is impossible because of heavy grounds and shunts, or if it is
possible to get ringing signals through at all, the relays of the Morse
apparatus will chatter, interfering with the proper use of the telegraph
portion of the service.
It is customary, therefore, either to equip composite circuits with
special signaling devices by which high-frequency currents pass over the
telephone circuits, operating relays which in turn operate local ringing
signals; or to refrain from ringing on composite circuits and to
transmit orders for connections by telegraph. The latter is wholly
satisfactory over composite lines between points having heavy telegraph
traffic, and it is between such points as these that composite practice
is most general.
=Phantoms from Simplex and Composite Circuits.= Phantom and simplex
principles are identical, and by adding the composite principle, two
simplex circuits may have a phantom superadded, as in Fig. 469.
Similarly, as in Fig. 470, two composite circuits can be phantomed. This
case gives seven distinct services over four wires: three telephone
loops--two physical and one phantom--and four Morse lines.
[Illustration: Fig. 469. Phantom of Two Simplex Circuits]
[Illustration: Fig. 470. Phantom of Two Composite Circuits]
=Railway Composite.= The foregoing are problems of making telegraphy a
by-product of telephony. With so many telegraph wires on poles over the
country, it has seemed a pity not to turn the thing around and provide
for telephony as a by-product of telegraphy. This has been accomplished,
and the result is called a railway composite system. For the reason that
the telegraph circuits are not in pairs, accurately matched one wire
against another, and are not always uniform as to material, it has not
been possible to secure as good telephone circuits from telegraph wires
as telegraph circuits from telephone wires.
Practical results are secured by adaptation of the original principle of
different frequencies. A study of Fig. 468 shows that over such a
composite circuit the usual method of ringing from station to station
over the telephone circuit by an alternating current of a frequency of
about sixteen per second is practically impossible. This is because of
the heavy short-circuit provided by the two 30-ohm choke coils at each
of the stations, the heavy shunt of the large condensers, and the
grounding through the 50-ohm choke coils. If high-frequency speech
currents can pass over these circuits with a very small loss, other
high-frequency circuits should find a good path. There are many easy
ways of making such currents, but formerly none very simple for
receiving them. Fig. 471 shows one simple observer of such
high-frequency currents, it being merely an adaptation of the familiar
polarized ringer used in every subscriber's telephone. In either
position of the armature it makes contact with one or the other of two
studs connected to the battery, so that in all times of rest the relay
_A_ is energized. When a high-frequency current passes through this
polarized relay, however, there is enough time in which the armature is
out of contact with either stud to reduce the total energy through the
relay _A_ and allow its armature to fall away, ringing a vibrating bell
or giving some other signal.
[Illustration: Fig. 471. Ringing Device for Composite Circuits]
Fig. 472 shows a form of apparatus for producing the high-frequency
current necessary for signaling. It is evident that if a magneto
generator, such as is used in ordinary magneto telephones, could be made
to drive its armature fast enough, it also might furnish the
high-frequency current necessary for signaling through condensers and
past heavy impedances.
[Illustration: Fig. 472. Ringing Current Device]
Applying these principles of high-frequency signals sent and received to
a single-wire telegraph circuit, the arrangement shown in Fig. 473
results, this being a type of railway composite circuit. The principal
points of interest herein are the insertion of impedances in series with
the telegraph lines, the shunting of the telegraph relays by small
condensers, the further shunting of the whole telegraph mechanism of a
station by another condenser, and thus keeping out of the line circuit
changes in current values which would be heard in the telephones if
violent, and might be inaudible if otherwise.
[Illustration: Fig. 473. Railway Composite Circuit]
[Illustration: FRONT OF LONG-DISTANCE POWER BOARD U.S. Telephone
Company, Cleveland, Ohio. _The Dean Electric Co._]
A further interesting element is the very heavy shunting of the
telephone receiver by means of an inductive coil. This shunt is applied
for by-path purposes so that heavy disturbing currents may be kept out
of the receiver while a sufficient amount of voice current is diverted
through the receiver. It is well to have the inductance of this shunt
made adjustable by providing a movable iron core for the shunt winding.
When the core is drawn out of the coil, its impedance is diminished
because the inductance is diminished. This reduces the amount of
disturbing noise in the receiver. The core should be withdrawn as little
as the amount of disturbance permits, as this also diminishes the
loudness of the received speech.
Because the signaling over lines equipped with this form of composite
working results in the ringing of a bell by means of local current, it
is of particular advantage in cases where the bell needs to ring loudly.
Switch stations, crossings, and similar places where the attendant is
not constantly near the telephone can be equipped with this type of
composite apparatus and it so offers a valuable substitute for regular
railway telegraph equipment, with which the attendant may not be
familiar. The success of the local bell-ringing arrangement, however,
depends on accurate relay adjustment and on the maintenance of a primary
battery. The drain on the ringing battery is greater than on the talking
battery.
A good substitute for the bell signal on railway composite circuits is a
telephone receiver responding directly to high-frequency currents over
the line. The receiver is designed specially for the purpose and is
known as a "howler." Its signal can be easily heard through a large
room. The condenser in series with it is of small capacity, limiting the
drain upon the line. Usually the howler is detached by the switch hook
during conversation from a station.
_Railway Composite Set._ The circuit of a set utilizing such an
arrangement together with other details of a complete railway composite
set is shown in Fig. 474. The drawing is arranged thus, in the hope of
simplifying the understanding of its principles. It will be seen that
the induction coil serves as an interrupter as well as for transmission.
All of the contacts are shown in the position they have during
conversation. The letters _Hc1_, _Hc2_, etc., and _Kc1_, _Kc2_, etc.,
refer to hook contacts and key contacts, respectively, of the numbers
given. The arrangements of the hook and key springs are shown at the
right of the figure. _RR_ represent impedance coils connected serially
in the line and placed at terminal stations. The composite telephone
sets are bridged from the line to ground at any points between the
terminal impedance coils.
The direct currents of telegraphy are prevented from passing to ground
through the telephone set during conversation by the 2-microfarad
condenser which is in series with the receiver. They are prevented from
passing to ground through the telephone set when the receiver is on the
hook by a .05 microfarad condenser in series with the howler. The
alternating currents of speech and interrupter signaling are kept from
passing to ground at terminals by the impedance coils.
Signals are sent from the set by pressing the key _K_. This operates the
vibrator by closing contacts _Kc6_ and _Kc7_. The howler is cut off and
the receiver is short-circuited by the same operation of the key. The
impedance of the coil _I_ is changed by moving its adjustable core.
[Illustration: Fig. 474. Railway Composite Set]
=Applications.= A chief use of composite and simplex circuits is for
ticket wire purposes. These are circuits over which long-distance
operators instruct each other as to connecting and disconnecting lines,
the routing of calls, and the making of appointments. One such wire will
care for all the business of many long-distance trunks. The public also
absorbs the telegraph product of telephone lines. Such telegraph service
is leased to brokers, manufacturers, merchants, and newspapers. Railway
companies use portable telephone adjuncts to telegraph circuits on
trains for service from stations not able to support telegraph
attendants, and in a limited degree for the dispatching of trains.
Telephone train dispatching, however, merits better equipment than a
railway composite system affords.
CHAPTER XL
TELEPHONE TRAIN DISPATCHING[A]
It has been only within the past three few that the telephone has begun
to replace the telegraph for handling train movements. The telegraph and
the railroads have grown up together in this country since 1850, and in
view of the excellent results that the telegraph has given in train
dispatching and of the close alliance that has always naturally existed
between the railway and the telegraph, it has been difficult for the
telephone, which came much later, to enter the field.
=Rapid Growth.= The telephone has been in general use among the
railroads for many years, but only on a few short lines has it been used
for dispatching trains. In these cases the ordinary magneto circuit and
instruments have been employed, differing in no respect from those used
in commercial service at the present time. Code ringing was used and the
number of stations on a circuit was limited by the same causes that
limit the telephones on commercial party lines at present.
The present type of telephone dispatching systems, however, differs
essentially from the systems used in commercial work, and is, in fact, a
highly specialized party-line system, arranged for selective ringing and
_many stations_. The first of the present type was installed by the New
York Central and Hudson River Railroad in October, 1907, between Albany
and Fonda, New York, a distance of 40 miles. This section of the road is
on the main line and has four tracks controlled by block signals.
The Chicago, Burlington, and Quincy Railroad was the second to install
train-dispatching circuits. In December, 1907, a portion of the main
line from Aurora to Mendota, Illinois, a distance of 46 miles, was
equipped. This was followed in quick succession by various other
circuits ranging, in general, in lengths over 100 miles. At the present
time there are over 20 train-dispatching circuits on the Chicago,
Burlington, and Quincy Railroad covering 125 miles of double track, 28
miles of multi-track, and 1,381 miles of single track, and connecting
with 286 stations.
Other railroads entered this field in quick order after the initial
installations, and at the present time nearly every large railroad
system in the United States is equipped with several telephone
train-dispatching circuits and all of these seem to be extending their
systems.
In 1910, several railroads, including the Delaware, Lackawanna, and
Western, had their total mileage equipped with telephone dispatching
circuits. The Atchison, Topeka, and Santa Fe Railroad is equipping its
whole system as rapidly as possible and already is the largest user of
this equipment in this country. From latest information, over 55
railroads have entered this field, with the result that the telephone is
now in use in railroad service on over 29,000 miles of line.
=Causes of Its Introduction.= The reasons leading to the introduction of
the telephone into the dispatching field were of this nature: First, and
most important, was the enactment of State and Federal Laws limiting to
nine hours the working day of railroad employes transmitting or
receiving orders pertaining to the movement of trains. The second, which
is directly dependent upon the first, was the inability of the railroads
to obtain the additional number of telegraph operators which were
required under the provisions of the new laws. It was estimated that
15,000 additional operators would be required to maintain service in the
same fashion after the new laws went into effect in 1907. The increased
annual expense occasioned by the employment of these additional
operators was roughly estimated at $10,000,000. A third reason is found
in the decreased efficiency of the average railway and commercial
telegraph operator. There is a very general complaint among the
railroads today regarding this particular point, and many of them
welcome the telephone, because, if for no other reason, it renders them
independent of the telegrapher. What has occasioned this decrease in
efficiency it is not easy to say, but there is a strong tendency to lay
it, in part, to the attitude of the telegraphers' organization toward
the student operator. It is a fact, too, that the limits which these
organizations have placed on student operators were directly
responsible for the lack of available men when they were needed.
=Advantages.= In making this radical change, railroad officials were
most cautious, and yet we know of no case where the introduction of the
telephone has been followed by its abandonment, the tendency having been
in all cases toward further installations and more equipment of the
modern type. The reasons for this are clear, for where the telephone is
used it does not require a highly specialized man as station operator
and consequently a much broader field is open to the railroads from
which to draw operators. This, we think, is the most far-reaching
advantage.
The telephone method also is faster. On an ordinary train-dispatching
circuit it now requires from 0.1 of a second to 5 seconds to call any
station. In case a plurality of calls is desired, the dispatcher calls
one station after another, getting the answer from one while the next is
being called, and so on. By speaking into a telephone many more words
may be transmitted in a given time than by Morse telegraphy. It is
possible to send fifty words a minute by Morse, but such speed is
exceptional. Less than half that is the rule. The gain in high speed,
therefore, which is obtained is obvious and it has been found that this
is a most important feature on busy divisions. It is true that in the
issuance of "orders," the speed, in telephonic train dispatching, is
limited to that required to write the words in longhand. But all
directions of a collateral character, the receipt of important
information, and the instantaneous descriptions of emergency situations
can be given and received at a speed limited only by that of human
speech.
The dispatcher is also brought into a closer personal relation with the
station men and trainmen, and this feature of direct personal
communication has been found to be of importance in bringing about a
higher degree of co-operation and better discipline in the service.
Telephone dispatching has features peculiar to itself which are
important in improving the class of service. One of these is the
"answer-back" automatically given to the dispatcher by the waystation
bell. This informs the dispatcher whether or not the bell at the station
rang, and excuses by the operators that it did not, are eliminated.
Anyone can answer a telephone call in an emergency. The station
operator is frequently agent also, and his duties often take him out of
hearing of the telegraph sounder. The selector bell used with the
telephone can be heard for a distance of several hundred feet. In
addition, it is quite likely that anyone in the neighborhood would
recognize that the station was wanted and either notify the operator or
answer the call.
In cases of emergency the train crews can get into direct communication
with the dispatcher immediately, by means of portable telephone sets
which are carried on the trains. It is a well-known fact that every
minute a main line is blocked by a wreck can be reckoned as great loss
to the railroad.
It is also possible to install siding telephone sets located either in
booths or on poles along the right-of-way. These are in general service
today at sidings, crossings, drawbridges, water tanks, and such places,
where it may be essential for a train crew to reach the nearest
waystation to give or receive information.
The advantage of these siding sets is coming more and more to be
realized. With the telegraph method of dispatching, a train is ordered
to pass another train at a certain siding, let us say. It reaches this
point, and to use a railroad expression, "goes into the hole." Now, if
anything happens to the second train whereby it is delayed, the first
train remains tied up at that siding without the possibility of either
reaching the dispatcher or being reached by him. With the telephone
station at the siding, which requires no operator, this is avoided. If a
train finds itself waiting too long, the conductor goes to the siding
telephone and talks to the dispatcher, possibly getting orders which
will advance him many miles that would otherwise have been lost.
It is no longer necessary for a waystation operator to call the
dispatcher. When one of these operators wishes to talk to the
dispatcher, he merely takes his telephone receiver off the hook, presses
a button, and speaks to the dispatcher.
With the telephone it is a simple matter to arrange for provision so
that the chief dispatcher, the superintendent, or any other official may
listen in at will upon a train circuit to observe the character of the
service. The fact that this can be done and that the operators know it
can be done has a very strong tendency to improve the discipline.
The dispatchers are so relieved, by the elimination of the strain of
continuous telegraphing, and can handle their work so much more quickly
with the telephone, that in many cases it has been found possible to
increase the length of their divisions from 30 to 50 per cent.
=Railroad Conditions.= One of the main reasons that delayed the
telephone for so many years in its entrance to the dispatching field is
that the conditions in this field are like nothing which has yet been
met with in commercial telephony. There was no system developed for
meeting them, although the elements were at hand. A railroad is divided
up into a number of divisions or dispatchers' districts of varying
lengths. These lengths are dependent on the density of the traffic over
the division. In some cases a dispatcher will handle not more than 25
miles of line. In other cases this district may be 300 miles long. Over
the length of one of these divisions the telephone circuit extends, and
this circuit may have upon it 5 or 50 stations, _all of which may be
required to listen upon the line at the same time_.
It will be seen from this that the telephone dispatching circuit
partakes somewhat of the nature of a long-distance commercial circuit in
its length, and it also resembles a rural line in that it has a large
number of telephones upon it. Regarding three other characteristics,
namely, that many of these stations may be required to be in on the
circuit simultaneously, that they must all be signaled selectively, and
that it must also be possible to talk and signal on the circuit
simultaneously, a telephone train-dispatching circuit resembles nothing
in the commercial field. These requirements are the ones which have
necessitated the development of special equipment.
=Transmitting Orders.= The method of giving orders is the same as that
followed with the telegraph, with one important exception. When the
dispatcher transmits a train order by telephone, he writes out the order
as he speaks it into his transmitter. In this way the speed at which the
order is given is regulated so that everyone receiving it can easily get
it all down, and a copy of the transmitted order is retained by the
dispatcher. All figures and proper names are spelled out. Then after an
order has been given, it is repeated to the dispatcher by each man
receiving it, and he underlines each word as it comes in. This is now
done so rapidly that a man can repeat an order more quickly than the
dispatcher can underline. The doubt as to the accuracy with which it is
possible to transmit information by telephone has been dispelled by this
method of procedure, and the safety of telephone dispatching has been
fully established.
=Apparatus.= The apparatus which is employed at waystations may be
divided into two groups--the selector equipment and the telephone
equipment. The selector is an electro-mechanical device for ringing a
bell at a waystation when the dispatcher operates a key corresponding to
that station. At first, as in telegraphy, the selector magnets were
connected in series in the line, but today all systems bridge the
selectors across the telephone circuit in the same way and for the same
reasons that it is done in bridging party-line work. There are at the
present time three types of selectors in general use, and the mileage
operated by means of these is probably considerably over 95 per cent of
the total mileage so operated in the country.
[Illustration: Fig. 475. Western Electric Selector]
[Illustration: Fig. 476. Western Electric Selector]
_The Western Electric Selector._ This selector is the latest and perhaps
the simplest. Fig. 475 shows it with its glass dust-proof cover on, and
Fig. 476 shows it with the cover removed. This selector is adapted for
operating at high speed, stations being called at the rate of ten per
second.
The operating mechanism, which is mounted on the front of the selector
so as to be readily accessible, works on the central-energy
principle--the battery for its operation, as well as for the operation
of the bell used in connection with it, both being located at the
dispatcher's office. The bell battery may, however, be placed at the
waystation if this is desired.
The selector consists of two electromagnets which are bridged in series
across the telephone circuit and are of very high impedance. It is
possible to place as many of these selectors as may be desired across a
circuit without seriously affecting the telephonic transmission.
Direct-current impulses sent out by the dispatcher operate these
magnets, one of which is slow and the other quick-acting. The first
impulse sent out is a long impulse and pulls up both armatures, thereby
causing the pawls above and below the small ratchet wheel, shown in Fig.
476, to engage with this wheel. The remaining impulses operate the
quick-acting magnet and step the wheel around the proper number of
teeth, but do not affect the slow-acting magnet which remains held up by
them. The pawl connected to the slow-acting magnet merely serves to
prevent the ratchet wheel from turning back. Attached to the ratchet
wheel is a contact whose position can be varied in relation to the
stationary contact on the left of the selector with which this engages.
This contact is set so that when the wheel has been rotated the desired
number of teeth, the two contacts will make and the bell be rung. Any
selector may thus be adjusted for any station, and the selectors are
thus interchangeable. When the current is removed from the line at the
dispatcher's office, the armatures fall back and everything is restored
to normal. An "answer-back" signal is provided with this selector
dependent upon the operation of the bell. When the selector at a station
operates, the bell normally rings for a few seconds. The dispatcher,
however, can hold this ring for any length of time desired.
The keys employed at the dispatcher's office for operating selectors are
shown in Fig. 477. There is one key for each waystation on the line and
the dispatcher calls any station by merely giving the corresponding key
a quarter turn to the right. Fig. 478 shows the mechanism of one of
these keys and the means employed for sending out current impulses over
the circuit. The key is adjustable and may be arranged for any station
desired by means of the movable cams shown on the rear in Fig. 478,
these cams, when occupying different positions, serving to cover
different numbers of the teeth of the impulse wheel which operate the
impulse contacts.
[Illustration: Fig. 477. Dispatcher's Keys]
[Illustration: Fig. 478. Dispatcher's Key Mechanism]
_The Gill Selector._ The second type of selector in extensive use
throughout the country today is known as the Gill, after its inventor.
It is manufactured for both local-battery and central-energy types, the
latter being the latest development of this selector. With the
local-battery type, the waystation bell rings until stopped by the
dispatcher. With the central-energy type it rings a definite length of
time and can be held for a longer period as is the case with the Western
Electric selector. The selector is operated by combinations of
direct-current impulses which are sent out over the line by keys in the
dispatcher's office.
[Illustration: Fig. 479. Gill Selector]
The dispatcher has a key cabinet, and calls in the same way as already
described, but these keys instead of sending a series of quick impulses,
send a succession of impulses with intervals between corresponding to
the particular arrangement of teeth in the corresponding waystation
selector wheel. Each key, therefore, belongs definitely with a certain
selector and can be used in connection with no other.
A concrete example may make this clearer. The dispatcher may operate key
No. 1421. This key starts a clockwork mechanism which impresses at
regular intervals, on the telephone line, direct-current impulses, with
intervals between as follows: 1-4-2-1. There is on the line one selector
corresponding to this combination and it alone, of all the selectors on
the circuit, will step its wheel clear around so that contact is made
and the bell is rung. In all the others, the pawls will have slipped out
at some point of the revolution and the wheels will have returned to
their normal positions.
The Gill selector is shown in Fig. 479. It contains a double-wound relay
which is bridged across the telephone circuit and operates the selector.
This relay has a resistance of 4,500 ohms and a high impedance, and
operates the selector mechanism which is a special modification of the
ratchet and pawl principle. The essential features of this selector are
the "step-up" selector wheel and a time wheel, normally held at the
bottom of an inclined track.
The operation of the selector magnet pushes the time wheel up the track
and allows it to roll down. If the magnet is operated rapidly, the wheel
does not get clear down before being pushed back again. A small pin on
the side of the pawl, engaging the selector wheel normally, opposes the
selector wheel teeth near their outer points. When the time wheel rolls
to the bottom of the track, however, the pawl is allowed to drop to the
bottom of the tooth. Some of the teeth on the selector wheel are formed
so that they will effectually engage with the pawl only when the latter
is in normal position, while others will engage only while the pawl is
at the bottom position; thus innumerable combinations can be made which
will respond to certain combinations of rapid impulses with intervals
between. The correct combination of impulses and intervals steps the
selector wheel clear around so that a contact is made. The selector
wheels at all other stations fail to reach their contact position
because at some point or points in their revolution the pawls have
slipped out, allowing the selector wheels to return "home."
The "answer-back" is provided in this selector by means of a few
inductive turns of the bell circuit which are wound on the selector
relay. The operation of the bell through these turns induces an
alternating current in the selector winding which flows out on the line
and is heard as a distinctive buzzing noise by the dispatcher.
[Illustration: Fig. 480. Cummings-Wray Dispatcher's Sender]
_The Cummings-Wray Selector._ Both of the selectors already described
are of a type known as the _individual-call_ selectors, meaning that
only one station at a time can be called. If a plurality of calls is
desired, the dispatcher calls one station after another. The third type
of selector in use today is of a type known as the _multiple-call_, in
which the dispatcher can call simultaneously as many stations as he
desires.
The Cummings-Wray selector and that of the Kellogg Switchboard and
Supply Company are of this type and operate on the principle of
synchronous clocks. When the dispatcher wishes to put through a call, he
throws the keys of all the stations that he desires and then operates a
starting key. The bells at all these stations are rung by one operation.
The dispatcher's sending equipment of the Cummings-Wray system is shown
in Fig. 480, and the waystation selector in Fig. 481. It is necessary
with this system for the clocks at all stations to be wound every eight
days.
[Illustration: Fig. 481. Cummings-Wray Selector]
In the dispatcher's master sender the clock-work mechanism operates a
contact arm which shows on the face of the sender in Fig. 480. There is
one contact for every station on the line. The clock at this office and
the clocks at all the waystation offices start together, and it is by
this means that the stations are signaled, as will be described later,
when the detailed operation of the circuits is taken up.
=Telephone Equipment.= Of no less importance than the selective devices
is the telephone apparatus. That which is here illustrated is the
product of the Western Electric Company, to whom we are indebted for all
the illustrations in this chapter.
_Dispatcher's Transmitter._ The dispatcher, in most cases, uses the
chest transmitter similar to that employed by switchboard operators in
every-day service. He is connected at all times to the telephone
circuit, and for this reason equipment easy for him to wear is
essential. In very noisy locations he is equipped with a double head
receiver. On account of the dispatcher being connected across the line
permanently and of his being required to talk a large part of the time,
there is a severe drain on the transmitter battery. For this reason
storage batteries are generally used.
[Illustration: Fig. 482. Waystation Desk Telephone]
_Waystation Telephones._ At the waystations various types of telephone
equipment may be used. Perhaps the most common is the familiar desk
stand shown in Fig. 482, which, for railroad service, is arranged with a
special hook-switch lever for use with a head receiver.
Often some of the familiar swinging-arm telephone supports are used, in
connection with head receivers, but certain special types developed
particularly for railway use are advantageous, because in many cases the
operator who handles train orders is located in a tower where he must
also attend to the interlocking signals, and for such service it is
necessary for him to be able to get away from the telephone and back to
it quickly. The Western Electric telephone arm developed for this use is
shown in Fig. 483. In this the transmitter and the receiver are so
disposed as to conform approximately to the shape of the operator's
head. When the arm is thrown back out of the way it opens the
transmitter circuit by means of a commutator in its base.
[Illustration: Fig. 483. Telephone Arm]
_Siding Telephones._ Two types of sets are employed for siding
purposes. The first is an ordinary magneto wall instrument, which
embodies the special apparatus and circuit features employed in the
standard waystation sets. These are used only where it is possible to
locate them indoors or in booths along the line. These sets are
permanently connected to the train wire, and since the chances are small
that more than one of them will be in use at a time, they are rung by
the dispatcher, by means of a regular hand generator, when it is
necessary for him to signal a switching.
[Illustration: Fig. 484. Weather-Proof Telephone Set]
In certain cases it is not feasible to locate these siding telephone
sets indoors, and to meet these conditions an iron weather-proof set is
employed, as shown in Figs. 484 and 485. The apparatus in this set is
treated with a moisture-proofing compound, and the casing itself is
impervious to weather conditions.
[Illustration: Fig. 485. Weather-Proof Telephone Set]
_Portable Train Sets._ Portable telephone sets are being carried
regularly on wrecking trains and their use is coming into more and more
general acceptance on freight and passenger trains. Fig. 486 shows one
of these sets equipped with a five-bar generator for calling the
dispatcher. Fig. 487 shows a small set without generator for conductors'
and inspectors' use on lines where the dispatcher is at all times
connected in the circuit.
[Illustration: Fig. 486. Portable Telephone Set]
[Illustration: Fig. 487. Portable Telephone Set]
These sets are connected to the telephone circuit at any point on the
line by means of a light portable pole arranged with terminals at its
outer extremity for hooking over the line wires, and with flexible
conducting cords leading to the portable set. The use of these sets
among officials on their private cars, among construction and bridge
gangs working on the line, and among telephone inspectors and repairmen
for reporting trouble, is becoming more and more general.
=Western Electric Circuits.= As already stated, a telephone
train-dispatching circuit may be from 25 to 300 miles in length, and
upon this may be as many stations as can be handled by one dispatcher.
The largest known number of stations upon an existing circuit of this
character is 65.
[Illustration: Fig. 488. Dispatcher's Station--Western Electric System]
_Dispatcher's Circuit Arrangement._ The circuits of the dispatcher's
station in the Western Electric system are shown in Fig. 488, the
operation of which is briefly as follows: When the dispatcher wishes to
call any particular station, he gives the key corresponding to that
station a quarter turn. This sends out a series of rapid direct-current
impulses on the telephone line through the contact of a special
telegraph relay which is operated by the key in a local circuit. The
telegraph relay is equipped with spark-eliminating condensers around its
contacts and is of heavy construction throughout in order to carry
properly the sending current.
_Voltage._ The voltage of the sending battery is dependent on the length
of the line and the number of stations upon it. It ranges from 100 to
300 volts in most cases. When higher voltages are required in order
successfully to operate the circuit, it is generally customary to
install a telegraph repeater circuit at the center of the line, in order
to keep the voltage within safe limits. One reason for limiting the
voltage employed is that the condensers used in the circuit will not
stand much higher potentials without danger of burning out. It is also
possible to halve the voltage by placing the dispatcher in the center of
the line, from which position he may signal in two directions instead of
from one end.
_Simultaneous Talking and Signaling._ Retardation coils and condensers
will be noticed in series with the circuit through which the signaling
current must pass before going out on the line. These are for the
purpose of absorbing the noise which is caused by high-voltage battery,
thus enabling the dispatcher to talk and signal simultaneously. The
250-ohm resistance connected across the circuit through one back contact
of the telegraph relay absorbs the discharge of the 6-microfarad
condenser.
[Illustration: Fig. 489. Selector Set--Western Electric System]
=Waystation Circuit.= The complete selector set for the waystations is
shown in Fig. 489, and the wiring diagram of its apparatus in Fig. 490.
The first impulse sent out by the key in the dispatcher's office is a
long direct-current impulse, the first tooth being three or four times
as wide as the other teeth. This impulse operates both magnets of the
selector and attracts their armatures, which, in turn, cause two pawls
to engage with the ratchet wheel, while the remaining quick impulses
operate the "stepping-up" pawl and rotate the wheel the requisite number
of teeth. Retardation coils are placed in series with the selector in
order to choke back any lightning discharges which might come in over
the line. The selector contact, when operated, closes a bell circuit,
and it will be noted that both the selector and the bell are operated
from battery current coming over the main line through variable
resistances. There are, of course, a number of selectors bridged across
the circuit, and the variable resistance at each station is so adjusted
as to give each approximately 10 milliamperes, which allows a large
factor of safety for line leakage in wet weather. The drop across the
coils at 10 milliamperes is 38 volts. If these coils were not employed,
it is clear that the selectors nearer the dispatcher would get most of
the current and those further away very little.
[Illustration: Fig. 490. Selector Set--Western Electric System]
A time-signal contact is also indicated on the selector-circuit diagram
of Fig. 490. This is common to all offices and may be operated by a
special key in the dispatcher's office, thereby enabling him to send out
time signals over the telephone circuit.
[Illustration: Fig. 491. Gill Dispatcher's Station]
=Gill Circuits.= The circuit arrangement for the dispatcher's outfit of
the Gill system is shown in Fig. 491. This is similar to that of the
Western Electric system just described. The method of operation also is
similar, the mechanical means of accomplishing the selection being the
main point of difference. In Fig. 492 the wiring of the Gill selector at
a waystation for local-battery service is shown. The selector contact
closes the bell circuit in the station and a few windings of this
circuit are located on the selector magnets, as shown. These provide the
"answer-back" by inductive means.
[Illustration: Fig. 492. Gill Selector--Local Battery]
Fig. 493 shows the wiring of the waystation, central-energy Gill
selector. In this case, the local battery for the operation of the bell
is omitted and the bell is rung, as is the case of the Western Electric
selector, by the main sending battery in the dispatcher's office.
[Illustration: Fig. 493. Gill Selector--Central Energy]
The sending keys of these two types of circuits differ, in that with the
local-battery selector the key contact is open after the selector has
operated, and the ringing of the bell must be stopped by the dispatcher
pressing a button or calling another station. Either of these operations
sends out a new current impulse which releases the selector and opens
its circuit.
With the central-energy selector, however, the contacts of the sending
key at the dispatcher's office remain closed after operation for a
definite length of time. This is obviously necessary in order that
battery may be kept on the line for the operation of the bell. In this
case the contacts remain closed during a certain portion of the
revolution of the key, and the bell stops ringing when that portion of
the revolution is completed. If, however, the dispatcher desires to give
any station a longer ring, he may do so by keeping the key contacts
closed through an auxiliary strap key as soon as he hears the
"answer-back" signal from the called station.
=Cummings-Wray Circuits.= The Cummings-Wray system, as previously
stated, is of the multiple-call type, operating with synchronous clocks.
Instead of operating one key after another in order to call a number of
stations, all the keys are operated at once and a starting key sets the
mechanism in motion which calls all these stations with one operation.
Fig. 494 shows the circuit arrangement of this system.
[Illustration: Fig. 494. Cummings-Wray System]
In order to ring one or more stations, the dispatcher presses the
corresponding key or keys and then operates the starting key. This
starting key maintains its contact for an appreciable length of time to
allow the clock mechanism to get under way and get clear of the
releasing magnet clutch. Closing the starting key operates the
clock-releasing magnet and also operates the two telegraph-line relays.
These send out an impulse of battery on the line operating the bridged
2,500-ohm line relays and, in turn, the selector releasing magnets;
thus, all the waystation clocks start in unison with the master clock.
The second hand arbor of each clock carries an arm, which at each
waystation is set at a different angle with the normal position than
that at any other station. Each of these arms makes contact precisely at
the moment the master-clock arm is passing over the contact
corresponding to that station.
If, now, a given station key is pressed in the master sender, the
telegraph-line relays will again operate when the master-clock arm
reaches that point, sending out another impulse of battery over the
line. The selector contact at the waystation is closed at this moment;
therefore, the closing of the relay contact operates the ringing relay
through a local circuit, as shown. The ringing relay is immediately
locked through its own contact, thus maintaining the bell circuit closed
until it is opened by the key and the ringing is stopped.
As the master-clock arm passes the last point on the contact dial, the
current flows through the restoring relay operating the restoring magnet
which releases all the keys. A push button is provided by means of which
the keys may be manually released, if desired. This is used in case the
dispatcher presses a key by mistake. Retardation coils and variable
resistances are provided at the waystation just as with the other
selector systems which have been described and for the same reasons.
The circuits of the operator's telephone equipment shown in Fig. 495,
are also bridged across the line. This apparatus is of high impedance
and of a special design adapted to railroad service. There may be any
number of telephones listening in upon a railroad train wire at the same
time, and often a dispatcher calls in five or six at once to give
orders. These conditions have necessitated the special circuit
arrangement shown in Fig. 495.
[Illustration: Fig. 495. Telephone Circuits]
The receivers used at the waystations are of high impedance and are
normally connected, through the hook switch, directly across the line in
series with a condenser. When the operator, at a waystation wishes to
talk, however, he presses the key shown. This puts the receiver across
the line in series with the retardation coil and in parallel with the
secondary of the induction coil. It closes the transmitter battery
circuit at the same time through the primary of the induction coil.
The retardation coil is for the purpose of preventing excessive side
tone, and it also increases the impedance of the receiver circuit, which
is a shunt on the induction coil. This latter coil, however, is of a
special design which permits just enough current to flow through the
receiver to allow the dispatcher to interrupt a waystation operator when
he is talking.
The key used to close the transmitter battery is operated by hand and is
of a non-locking type. In some cases, where the operators are very busy,
a foot switch is used in place of this key. The use of such a key or
switch in practical operation has been found perfectly satisfactory, and
it takes the operators but a short time to become used to it.
The circuits of the dispatcher's office are similarly arranged, Fig.
495, being designed especially to facilitate their operation. In other
words, as the dispatcher is doing most of the work on the circuit, his
receiver is of a low-impedance type, which gives him slightly better
transmission than the waystations obtain. The key in his transmitter
circuit is of the locking type, so that he does not have to hold it in
while talking. This is for the reason that the dispatcher does most of
the talking on this circuit. Foot switches are also employed in some
cases by the dispatchers.
=Test Boards.= It is becoming quite a general practice among the
railroads to install more than one telephone circuit along their
rights-of-way. In many cases in addition to the train wire, a message
circuit is also equipped, and quite frequently a block wire also
operated by telephone, parallels these two. It is desirable on these
circuits to be able to make simple tests and also to be able to patch
one circuit with another in cases of emergency.
[Illustration: Fig. 496. Test Board]
Test boards have been designed for facilitating this work. These consist
of simple plug and jack boxes, the general appearance of which is shown
in Fig. 496. The circuit arrangement of one of these is shown in Fig.
497. Each wire comes into an individual jack as will be noted on one
side of the board, and passes through the inside contact of this jack,
out through a similar jack on the opposite side. The selector and
telephone set at an office are taken off these inside contacts through a
key, as shown. The outside contacts of this key are wired across two
pairs of cords. Now, assume the train wire comes in on jacks _1_ and
_3_, and the message wire on jacks _9_ and _11_. In case of an accident
to the train wire between two stations, it is desirable to patch this
connection with a message wire in order to keep the all-important train
wire working. The dispatcher instructs the operator at the last station
which he can obtain, to insert plugs _1_ and _2_ in jacks _1_ and _10_,
and plugs _3_ and _4_ in jacks _3_ and _12_, at the same time throwing
the left-hand key. Then, obtaining an operator beyond the break by any
available means, he instructs him likewise to insert plugs _1_ and _2_
in jacks _9_ and _2_, and plugs _3_ and _4_ in jacks _11_ and _4_,
similarly throwing the left-hand key. By tracing this out, it will be
observed that the train wire is patched over the disabled section by
means of the message circuit, and that the selector and the telephone
equipment are cut over on to the patched connections; in other words,
bridged across the patching cords.
[Illustration: Fig. 497. Circuits of Test Board]
It will also be seen that with this board it is possible to open any
circuit merely by plugging into a jack. Two wires can be short-circuited
or a loop made by plugging two cords of corresponding colors into the
two jacks. A ground jack is provided for grounding any wire. In this
way, a very flexible arrangement of circuits is obtained, and it is
possible to make any of the simple tests which are all that are usually
required on this type of circuit.
=Blocking Sets.= As was just mentioned, quite frequently in addition to
train wires and message circuits, block wires are also operated by
telephone. In some cases separate telephone instruments are used for the
blocking service, but in others the same man handles all three circuits
over the same telephone. The block wire is generally a converted
telegraph wire between stations, usually of iron and usually grounded.
It seldom ranges in length over six miles.
[Illustration: Fig. 498. Blocking Set]
Where the block wires are operated as individual units with their own
instruments, it is unnecessary to have any auxiliary apparatus to be
used in connection with them. Where, however, they are operated as part
of a system and the same telephone is used on these that is used on the
train wire and message wire, additional apparatus, called a blocking
set, is required. This blocking set, shown in Figs. 498 and 499, was
developed especially for this service by the Western Electric Company.
As will be noted, a repeating coil at the top and a key on the front of
the set are wired in connection with a pair of train wire cords. This
repeating coil is for use in connecting a grounded circuit to a metallic
circuit, as, for instance, connecting a block wire to the train wire,
and is, of course, for the purpose of eliminating noise. Below the key
are three combined jacks and signals. One block wire comes into each of
these and a private line may be brought into the middle one. When the
next block rings up, a visual signal is displayed which operates a bell
in the office by means of a local circuit. The operator answers by
plugging the telephone cord extending from the bottom of the set into
the proper jack. This automatically restores the signal and stops the
bell.
[Illustration: Fig. 499. Blocking Set]
Below these signals appear four jacks. One is wired across the train
wire; one across the message wire; and the other two are bridged across
the two pairs of patching cords on each side of the set. The operator
answers a call on any circuit by plugging his telephone cord into the
proper jack.
If a waystation is not kept open in the evening, or the operator leaves
it for any reason and locks up, he can connect two blocks together by
means of the block-wire cords. These are arranged simply for connecting
two grounded circuits together and serve to join two adjacent blocks,
thereby eliminating one station. A jack is wired across these cords, so
that the waystation operator can listen in on the connection if he so
desires.
In some cases not only are the telephone circuits brought into the test
board, but also two telegraph wires are looped through this board before
going to the peg switchboard. This is becoming quite a frequent practice
and, in times of great emergency, enables patches to be made to the
telegraph wires as well as to the telephone wires.
=Dispatching on Electric Railways.= As interurban electric railways are
becoming more extended, and as their traffic is becoming heavier, they
approximate more closely to steam methods of operation. It is not
unusual for an electric railway to dispatch its cars exactly as in the
case of a steam road. There is a tendency, however, in this class of
work, toward slightly different methods, and these will be briefly
outlined.
On those electric railways where the traffic is not especially heavy, an
ordinary magneto telephone line is frequently employed with standard
magneto instruments. In some cases the telephone sets are placed in
waiting rooms or booths along the line of the road. In other cases it is
not feasible to locate the telephone indoors and then iron weather-proof
sets, such as are shown in Figs. 484 and 485, are mounted directly on
the poles along the line of railway. With a line of this character there
is usually some central point from which orders are issued and the
trainmen call this number when arriving at sidings or wherever they may
need to do so.
Another method of installing a telephone system upon electric railways
is as follows: Instead of instruments being mounted in booths or on
poles along the line, portable telephone sets are carried on the cars
and jacks are located at regular intervals along the right-of-way on the
poles. The crew of the car wishing to get in touch with the central
office or the dispatcher, plugs into one of these jacks and uses the
portable telephone set. At indoor stations, in offices or buildings
belonging to the railroad, the regular magneto sets may be employed, as
in the first case outlined.
On electric railway systems where the traffic is heavy, the train or car
movements may be handled by a dispatcher just as on the steam railroad.
There is usually one difference, however. On a steam road, the operators
who give the train crews their orders and manipulate the semaphore
signals are located at regular intervals in the different waystations.
No such operators are usually found on electric railways, except,
perhaps, at very important points, and, therefore, it is necessary for
the dispatcher to be able to signal cars at any point and to get into
communication with the crews of these cars. He does this by means of
semaphores operated by telephone selectors over the telephone line. The
telephone circuit may be equipped with any number of selectors desired,
and the dispatcher can operate any particular one without operating any
other one on the circuit. Each selector, when operated, closes a pair of
contacts. This completes a local circuit which throws the semaphore arm
to the "danger" position, at the same time giving the dispatcher a
distinctive buzz in his ear, which informs him that the arm has actually
moved to this position. He can get this signal only by the operation of
the arm.
Each semaphore is located adjacent to a telephone booth in which is also
placed the restoring lever, by means of which the semaphore is set in
the "clear" position by the crew of the car which has been signaled. The
wall-type telephone set is usually employed for this class of service,
but if desired, desk stands or any of the various transmitter arms may
be used.
It is necessary for the crew of the car which first approaches a
semaphore set at "danger," to get out, communicate with the dispatcher,
and restore the signal to the "clear" position. The dispatcher can not
restore the signal. The signal is set only in order that the train crew
may get into telephonic communication with the dispatcher, and in order
to do this, it is necessary for them to go into the booth in any case.
[Footnote A: We wish particularly to acknowledge the courtesy of the
Western Electric Company in their generous assistance in the preparation
of this chapter.]
REVIEW QUESTIONS
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 11--68
* * * * *
1. What are the advantages of a common-battery system?
2. When is the local battery to be preferred to the common-battery?
3. Enumerate the different kinds of line signals.
4. Make a diagram of the arrangement of a direct line lamp signal.
5. What is a direct line lamp with ballast? Give sketch.
6. Describe a line lamp with relay.
7. What is a pilot lamp and what are its functions?
8. Sketch three different kinds of batteries applied to cord circuits.
9. What is a supervisory signal?
10. Make diagram of a complete simple common-battery switchboard
circuit.
11. When will the supervisory signal become operative?
12. What is the candle-power of incandescent lamps used for line and
supervisory signals?
13. At what voltages do they operate?
14. What are visual signals?
15. Describe the mechanical signal of the Western Electric Company.
16. Give a short description of the general assembly of the parts of a
simple common-battery switchboard.
17. What is a transfer switchboard?
18. Outline the limitations of a simple switchboard.
19. Describe and sketch a plug-ended transfer line.
20. Why is the plug-seat switch not more widely adopted for use?
21. Make diagram of an order-wire arrangement.
22. What are the limitations of the transfer system?
23. What are the fundamental features of the multiple switchboard?
24. What is a multiple jack?
25. What is an answering jack?
26. Make a diagram showing the principle of multiple switchboards.
27. What is the busy signal?
28. What determines the size of a multiple switchboard?
29. What is the use of the intermediate distributing frame?
30. Make diagram of the series magneto multiple switchboard and describe
its operation.
31. What are the defects of this system?
32. Give a diagram of the branch terminal magneto multiple switchboard.
33. Give a diagram and a short description of the Monarch magneto
multiple switchboard.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 69--134
* * * * *
1. Sketch and describe the line circuit of the common-battery multiple
switchboard of the Bell companies.
2. Make a diagram of the cord circuit of the Western Electric standard
multiple common-battery switchboard.
3. Describe the busy test in this system.
4. What is the function of the order-wire circuits?
5. What is jumper wire?
6. Give a short description of the relay mounting in the standard No. 1
relay board of the Western Electric Company.
7. What is the ultimate capacity of the No. 1 Western Electric
switchboard?
8. What is the capacity of the No. 10 Western Electric switchboard?
9. How does this switchboard No. 10 differ from No. 1?
10. Give a diagram of the two-wire line circuit of the Kellogg Company.
11. What is the capacity of the condenser of the cord circuit in the
foregoing system?
12. Give a complete diagram of the Kellogg two-wire board.
13. Describe the busy test in this system.
14. Give diagram of the Stromberg-Carlson multiple-board circuit.
15. What is the most important piece of apparatus in a multiple
switchboard?
16. What is the spacing of the multiple jacks in the No. 1 Western
Electric switchboard?
17. How do the relays of the Western Electric Company differ from those
of other companies?
18. Describe the relay construction of the Monarch Telephone Company.
19. What is meant by inter-office trunking?
20. What is the present practice in America as to the capacity of
multiple hoards?
21. What is the tendency in Europe regarding the capacity of multiple
boards?
22. Discuss the preferences in American practice.
23. State the different methods of trunking between exchanges.
24. When are two-way trunks employed?
25. Make diagram of the Western Electric inter-office connection system.
26. Describe the standard four-party line trunk ringing key of the
Western Electric Company.
27. Sketch and describe a keyless trunk.
28. Give diagram of the inter-office connection of the Kellogg system.
29. How does this system differ from the Western Electric in regard to
the ringing?
30. Why are the A and B switchboards in large exchanges entirely
separated?
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 135--226
* * * * *
1. What is the general object of automatic telephone systems?
2. What are the common arguments against these systems and how are they
met?
3. Give the operations that the calling subscriber has to go through in
any one of the successful systems.
4. During calling what is happening at the central office?
5. Describe the action of the Strowger or Automatic Electric Company
selecting switch.
6. What is the function of a line switch?
7. Describe the Strowger scheme of trunking and illustrate its action by
diagram.
8. Make a diagram of the sub-station apparatus and connections.
9. Make a diagram of the line switch unit.
10. Describe the action of the various guarding features necessary to
protect a busy line.
11. Make a simple diagram of the circuits of the first selector.
12. Give the functions and operations of the connector.
13. Give a diagram of connecting circuits.
14. Tell all you can regarding the battery supply to the connected
subscriber.
15. How are subscribers disconnected after they are through talking?
16. Describe a multi-office system.
17. Give a diagram of circuits of the trunk repeater.
18. Make a complete diagram of the connections between a calling and a
called subscriber in an automatic system.
19. What is the rotary connector?
20. Describe the sub-station equipment of the Lorimer automatic system.
21. Describe the Lorimer central-office apparatus.
22. Give a description of the progress of a call from its institution to
the final disconnection in the Lorimer system.
23. What is the automanual system?
24. Give general features of the operation in the automanual system.
25. Describe the automanual system subscribers' apparatus.
26. Give a description of the automanual central-office equipment.
REVIEW QUESTIONS
ON SUBJECT OF TELEPHONY
PAGES 227--270
* * * * *
1. What kinds of currents are employed?
2. What types of power plants are used?
3. Describe the sources of current supplied for the operator's
transmitter current and ringing current.
4. Make a diagram of the Warner pole changer.
5. Make a diagram of pole changers for harmonic ringing.
6. What is a multi-cyclic generator set?
7. Make a diagram of governor for harmonic ringing generators.
8. Describe the various primary sources of power.
9. Make a diagram of the mercury-arc-rectifier circuits.
10. What provision against breakdown is made?
11. Tell all you can about the storage battery--its construction and its
operation.
12. What is a pilot cell?
13. Describe the switches, meters, and protective devices used on the
power switchboard.
14. Give a diagram showing a typical example of a common-battery manual
switchboard equipment and circuits.
15. Give the main points concerning the construction of a central-office
building.
16. What provision should be made for cable runways?
17. Make a sketch of a small central-office floor plan.
18. Describe the Western Electric main and intermediate frames. Give
diagrams.
19. Give principal points regarding small office terminal apparatus.
20. Give types of line circuits.
21. Describe the typical equipment of a large manual office. Give floor
plans.
22. Give floor plan of an automatic office.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 271--320
* * * * *
1. What is a private-branch exchange?
2. What does "P. B. X." mean?
3. What is the function of the private-branch exchange operator?
4. Describe the key type of a small private-branch exchange switchboard.
5. Describe the different methods of supervision of private-branch
connections.
6. Describe the automatic equipment of the common-battery type in
private-branch exchanges.
7. How is secrecy of individual lines obtained in a private-exchange
equipment?
8. What is an intercommunicating system?
9. Sketch a magneto intercommunicating system.
10. Sketch and describe a plug type common-battery intercommunicating
system.
11. Sketch and describe the action of the push button in the Monarch
system and in the Western Electric system.
12. Sketch and describe the Monarch intercommunicating system.
13. What is the office of the junction box in this system?
14. What is a long-distance message?
15. What is the function of the repeating coil in the long-distance
line?
16. Which is the simplest form of long-distance switch?
17. What is a phantom circuit?
18. Under what control is the ringing of the subscriber in
long-distance calls?
19. What is meant by ticket passing?
20. What particular advantage has a common-battery set on long-distance
lines?
21. Give a typical load curve for telephone traffic.
22. Why is traffic a study of importance?
23. State the function of the intermediate distributing frame.
24. State the different methods of traffic study.
25. What is the trunking factor?
26. Define _trunking efficiency_.
27. Enumerate some of the elements upon which the quality of service in
a manual system depends.
28. What is team work?
29. How does the cost of telephone service vary?
30. What two general methods of charging for telephone service are in
use?
31. Describe a calculagraph and how is it used?
32. How are toll connections timed by the Monarch Telephone Company?
33. Sketch and describe the Western Electric Company line circuit and
service meter.
REVIEW QUESTIONS
ON THE SUBJECT OF TELEPHONY
PAGES 321--358
* * * * *
1. Describe a phantom circuit with diagram.
2. Explain how two phantoms may be joined by a physical circuit.
3. Which are the better, phantom or physical circuits, and why?
4. Explain how the simplex circuit differs from the phantom telephone
circuit.
5. Why are not telegraph wires as serviceable for telephone work as
telephone wires are for telegraph work?
6. Give the names of the different parts of a railway composite set and
explain method of operating.
7. State the causes of the introduction of the telephone into the train
dispatching field and explain the advantages it has over the telegraph
for this work.
8. In transmitting orders for train dispatching, how are mistakes
avoided?
9. Describe the Western Electric selector and explain its use.
10. In what way does the Gill selector differ from the Western Electric?
11. What special feature does the multiple coil selector possess?
12. What special arrangement is provided for the train dispatcher in
noisy locations?
13. How can a man on a wrecking train get connection with the train
dispatcher?
14. What is the usual limit in length of a telephone train dispatching
circuit and what is the largest number of stations at present existing
on such a circuit?
15. What is the voltage of the sending battery for a train dispatcher's
circuit and upon what is it dependent?
16. For what purpose is a repeater circuit used?
17. How is the noise caused by a high voltage battery absorbed so that
the dispatcher may talk and signal simultaneously?
18. Draw a diagram showing the circuit arrangement for the dispatcher's
outfit of the Gill system.
19. Explain fully the purpose of the retardation coil in connection with
a waystation set.
20. In case of accident to a train wire between two stations, how can
the connection be patched if the road is also equipped with a message
circuit in addition to the train wire?
21. Why do some railroads have block wires in addition to train wires
and message circuits?
22. If a waystation on a block wire is to be cut out for any length of
time, by what method can the two adjacent blocks be connected,
eliminating the station between?
23. What are some of the methods used for dispatching on electric
railways where the traffic is not especially heavy?
24. On an electric road in case a car approaches a semaphore set at
"danger," what must the crew of the car do?
INDEX
_The page numbers of this volume will be found at the bottom of the
pages; the numbers at the top refer only to the section._
A
Automanual system 218
automatic distribution of calls 223
automatic switching equipment 222
building up a connection 224
characteristics of 218
operation 219
operator's equipment 220
setting up a connection 224
speed in handling calls 224
subscriber's apparatus 219
Automatic desk stand 158
Automatic Electric Company's telephone system 149
automatic sub-offices 201
connector 185
function of 185
location of 186
operation of 186
first selector operation 179
function of line switch 152
line switch 153, 163
bridge cut-off 173
circuit operations 167
guarding functions 173
line and trunk contacts 164
locking segment 172
master switch 171
relation of, to connectors 174
structure of 166
summary of operation 174
trunk ratio 165
trunk selection 165
multi-office system 196
party lines 202
release after conversation 196
rotary connector 202
second selector operation 182
selecting switches 153, 175
release mechanism 178
side switch 175
subdivision of subscribers' lines 152
subscribers' station apparatus 158
operation 160
bell and transmitter springs 160
ground springs 160
impulse springs 161
release springs 163
ringing springs 163
salient points 163
trunking 154
connector action 157
first selector action 156
line switch action 154
second selector action 156
two-wire automatic systems 203
two-wire and three-wire systems 157
underlying feature of trunking system 153
Automatic telephone systems 135
arguments against 135
attitude of public 141
complexity 136
expense 140
flexibility 140
subscriber's station equipment 142
automatic vs. manual 143
comparative costs 142
definition 135
methods of operation 143
fundamental idea 147
grouping of subscribers 145
local and inter-office trunks 148
Lorimer system 144
magnet vs. power-driven switches 144
Automatic telephone systems
methods of operation
multiple vs. trunking 145
outline of action 146
Strowger system 143
testing 148
trunking between groups 145
Automatic wall set 158
B
Blocking sets 355
Busy test 48
busy-test faults 50
potential of test thimbles 49
principle 49
C
Circuits 321
applications 322
composite 326
phantom 321
transmission over 324
transpositions 323
railway composite 327
ringing 327
simplex 324
Common-battery multiple switchboard 69
assembly 106
Dean multiple board 93
cord circuit 94
line circuit 93
listening key 94
ringing keys 94
test 94
Kellogg two-wire multiple board 84
battery feed 88
busy test 90
complete cord and line circuit 88
cord circuit 86
line circuit 85
summary of operation 91
supervisory signals 87
wiring of line circuit 92
multiple switchboard apparatus 97
jacks 99
lamp jacks 100
relays 101
Stromberg-Carlson multiple board 96
cord circuit 96
supervisory signals 97
test 97
Western Electric No. 1 relay board 69
capacity range 80
cord circuit 71
functions of distributing frames 77
line circuit 69
modified relay windings 79
operation 72
operator's circuit detail 75
order-wire circuits 78
pilot signals 79
relay mounting 80
testing--called line busy 75
testing--called line idle 74
wiring of line circuit 76
Western Electric No. 10 board 80
circuits 81
economy 84
operation 83
test 83
Common-battery switchboard 11
advantages of operation 11
common battery vs. magneto 12
cord circuit 20
battery supply 20
complete circuit 21
supervisory signals 21
cycle of operations 23
jacks 30
lamps 24
mounting 25
line signals 14
direct-line lamp 14
direct-line lamp with ballast 15
line lamp with relay 17
pilot signals 17
mechanical signals 27
Kellogg 28
Monarch 28
Western Electric 27
relays 28
switchboard assembly 31
Composite circuits 326
Connector 185
Cord circuit 20
Cord circuit
battery supply 20
complete circuit 21
supervisory signals 21
Cord-rack connectors 66
Cummings-Wray selector 342
D
Dean multiple board 93
Dispatchers' keys 339
Dispatching on electric railways 356
G
Gill selector 341
H
Housing central-office equipment 249
arrangement of apparatus in small manual offices 252
combined main and intermediate frames 253
floor plans for 252
types of line circuits 255
automatic offices 267
typical automatic office 270
central-office building 249
fire hazard 249
provision for cable runways 251
provision for employes 251
size of building 250
strength of building 250
large manual office 256
I
Intercommunicating systems 282
common-battery systems 283
Kellogg plug type 284
Kellogg push-button type 285
Monarch system 287
Western Electric system 285
definition 282
limitations 282
for private-branch exchanges 290
simple magneto system 282
J
Jacks 30
K
Kellogg mechanical signal 28
Kellogg trunk circuits 125
Kellogg two-wire multiple board 84
Keyboard wiring 67
L
Lamp mounting 25
Lamps 24
Line signals 14
direct-line lamp 14
direct-line lamp with ballast 15
line lamp with relay 17
pilot signals 17
Line switch 163
Long-distance switching 293
definitions 293
center-checking 297
operators' orders 294
by call circuits 294
by telegraph 294
particular party calls 295
switching through local board 293
ticket passing 296
trunking 295
high-voltage toll trunks 295
through ringing 295
two-number calls 294
use of repeating coil 293
waystations 297
Lorimer automatic system 144, 205
central-office apparatus 208
connective division 210
sectional apparatus 209
switches 213
interconnector 214
interconnector selector 214
primary connector 213
rotary switch 213
secondary connector 214
signal transmitter controller 214
operation 215
subscriber's station equipment 206
M
Magneto multiple switchboard 53
branch-terminal multiple board 58
arrangement of apparatus 61
magnet windings 61
operation 60
field of utility 53
modern magneto multiple board 63
assembly 66
cord circuit 64
test 62
Magneto multiple switchboard
series-multiple board 54
defects 57
operation 56
Measured service 310
local service 316
meter method 316
prepayment method 318
ticket method 316
rates 310
toll service 311
long haul 311
short haul 311
timing toll connections 312
units of charging 311
Mechanical signals 27
Kellogg 28
Monarch 28
Western Electric 27
Mercury-arc rectifier circuits 237
Monarch visual signal 28
Multi-office exchanges, necessity for 109
Multiple switchboard 43
busy test 48
cord circuits 46
diagram showing principle of 47
double connections 46
field of each operator 51
field of utility 43
influence of traffic 52
line signals 45
multiple feature 43
P
Phantom circuit 321
Pilot signals 17
Plug-seat switch 38
Pole changers for harmonic ringing 231
Power plants 227
auxiliary signaling currents 233
currents employed 227
alternating current 227
direct current 227
operator's transmitter supply 228
power plant circuit 248
power switchboard 246
meters 246
protective devices 248
switches 246
primary sources 234
charging from direct-current mains 234
charging dynamos 235
mercury-arc rectifiers 236
rotary converters 234
provision against breakdown 237
capacity of power units 238
duplicate charging machines 238
duplicate primary sources 238
duplicate ringing machines 238
ringing-current supply 229
magneto generators 229
pole changers 229
ringing dynamos 232
storage battery 239
initial charge 241
installation 240
low cells 244
operation 242
overcharge 243
pilot cell 243
regular charge 244
replacing batteries 245
sediment 245
types 227
common-battery systems 228
magneto systems 228
Power switchboard 246
Private-branch exchanges 271
with automatic offices 278
secrecy 279
battery supply 279
circuits, key-type board 276
definitions 271
desirable features 281
functions of the private-branch exchange operator 272
marking of apparatus 281
private-branch switchboards 273
common-battery type 273
cord type 275
key type 275
magneto type 273
ringing current 280
supervision of private-branch connections 277
R
Relays 28
Rotary connector 202
S
Selecting switches 175
Selector 175
Simplex circuits 324
Storage battery 239
Storage cell 240
Stromberg-Carlson multiple board 96
Strowger automatic system 143
Subscribers' board 259-261
Switchboard assembly 31
T
Table
automanual system time data 225
automatic systems, messages per trunk in 305
calling rates 302
long-distance groups, messages per trunk in 305
manual system, messages per trunk in 304
out-trunking, effect of, on operator's capacity 303
subscribers' waiting time 226
Telephone traffic 298
importance of traffic study 300
methods of traffic study 301
observation of service 308
quality of service 305
accuracy and promptness 307
answering time 306
busy and don't answer calls 307
courtesy and form 307
disconnecting time 306
enunciation 308
team work 308
rates of calling 300
representative traffic data 302
calling rates 302
operators' loads 302
toll traffic 304
trunk efficiency 303
trunking factor 303
traffic variations 298
busy hour ratio 299
unit of traffic 298
Telephone train dispatching 333
advantages 335
apparatus 338
Cummings-Wray selector 342
dispatcher's transmitter 343
Gill selector 341
portable train sets 345
siding telephones 345
waystation telephones 344
Western Electric selector 338
blocking sets 355
causes of its introduction 334
Cummings-Wray circuits 350
on electric railways 356
Gill circuits 349
railroad conditions 337
rapid growth 333
test boards 353
transmitting orders 337
waystation circuits 348
Western Electric circuits 347
Telephone train-dispatching circuit
Cummings-Wray 350
Gill 349
waystation 348
Western Electric 347
Test boards 353
Transfer switchboard 34
field of usefulness 41
handling transfers 38
limitations 40
plug-seat switch 38
transfer lines 35
jack-ended trunk 35
plug-ended trunk 37
Trunking in multi-office systems 109
classification 112
one-way trunks 103
two-way trunks 112
Kellogg trunk circuits 125
necessity for exchanges 109
Western Electric trunk circuits 116
W
Warner pole changer 230
Waystation telephones 344
Western Electric
mechanical signal 27
selector 338
trunk circuits 116
Transcriber's Notes.
Spelling variants where it wasn't possible to determine the author's
intent were left as is. These include: "clockwork" and "clock-work;"
"doorkeeper" and "door-keeper;" "interrelation" and "inter-relation;"
"multicyclic" and "multi-cyclic;" "redesign" and "re-design," along with
derivatives.
Added closing double quote in Steinmetz entry in list of authorities:
"Theoretical Elements of Electrical Engineering."
Changed "switch-hook" to "switch hook" on page 88: "the subscriber's
switch hook."
Page 107 says there is room for 300 banks of 100 multiple jacks, but
then says this allows for 3,000 multiple jacks in all, rather than
30,000. Based on the figure, 300 banks should be 30 banks, which would
correct the arithmetic. However, I did not change this.
Changed "bi-paths" to "by-paths" on page 185: "circuits or by-paths."
Changed "appararus" to "apparatus" on page 209: "The sectional
apparatus."
Changed "two number" to "two-number" on page 312: "the two-number calls
are ticketed."
On page 333, a paragraph begins with "It has been only within the past
three few." Perhaps the author meant "It has been only within the past
three years" or "It has been only within the past few years." But since
I didn't know, I left is as is.
Changed "them ain" to "the main" on page 333: "on the main line."
Changed "weatherproof" to "weather-proof" on page 357: "iron
weather-proof sets."
Changed "interoffice" to "inter-office" three times on page 364, to
match the spelling in the body of the document: "meant by inter-office
trunking;" "inter-office connection system;" "of the inter-office
connection."
Changed "break-down" to "breakdown" on page 367: "provision against
breakdown."
Changed "way-station" to "waystation" twice on page 372: "with a
waystation set;" and "a waystation on a block wire."
Changed "way stations" to "waystations" on page 375, in the entry for
Long-distance switching.
Each page of the Index repeated this text: "Note.--For page numbers see
foot of pages." They were removed.
End of the Project Gutenberg EBook of Cyclopedia of Telephony and
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