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Title: The Telephone
An Account of the Phenomena of Electricity, Magnetism, and
Sound, as Involved in Its Action
Author: A. E. Dolbear
Release Date: July 14, 2010 [EBook #33154]
Language: English
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THE TELEPHONE
By Professor A. E. Dolbear
_THE TELEPHONE_
With directions for making a Speaking Telephone Illustrated 50 cents
_THE ART OF PROJECTING_
A Manual of Experimentation in Physics, Chemistry, and Natural History,
with the Porte Lumière and Magic Lantern New Edition Revised Illustrated
$2.00
_MATTER, ETHER, AND MOTION_
The Factors and Relations of Physical Science Illustrated $1.75
Lee and Shepard Publishers Boston
THE TELEPHONE:
AN ACCOUNT OF THE
_Phenomena of Electricity, Magnetism, and Sound,_
AS INVOLVED IN ITS ACTION.
WITH DIRECTIONS FOR MAKING
A SPEAKING TELEPHONE.
BY
PROF. A. E. DOLBEAR,
TUFTS COLLEGE,
AUTHOR OF "THE ART OF PROJECTING," ETC.
BOSTON:
LEE & SHEPARD, PUBLISHERS.
COPYRIGHT,
1877,
BY A. E. DOLBEAR.
PREFACE.
THE popular exhibitions of the speaking-telephone during the past six
months, together with numerous newspaper articles, have created a
widespread interest in the instrument; and it has been thought that a
small book explanatory of its action would meet a public want.
It has seemed to be necessary to call attention to the various phenomena
and inter-actions of the forces involved; and hence the author has
attempted to make plain and intelligible the phenomena of electricity,
magnetism, and sound. Cuts have been inserted where they could be useful
in making the mechanical conditions more intelligible; and a table of
tone-composition has been devised, which shows at a glance the
constituents of the sounds of various musical instruments.
As the speaking-telephone, in which magneto-electric currents were
utilized for the transmission of speech and other kinds of sounds, was
invented by me, I have described at some length my first instrument, and
have also given explicit directions for making a speaking-telephone
which I know, by trial, to be as efficient as any hitherto made; but
nothing in the book is to be taken as a dedication of the invention to
the public, as steps have already been taken to secure letters-patent
according to the laws of the United States.
A. E. DOLBEAR.
COLLEGE HILL, MASS.
THE TELEPHONE.
ELECTRICITY.
SOME of the phenomena of electricity are manifested upon so large a
scale as to be thrust upon the attention of everybody. Thus lightning,
which accompanies so many showers in warm weather in almost every
latitude, has always excited in some individuals a superstitious awe, as
being an exhibition of supernatural agency; and probably every one feels
more or less dread of it during a thunder-shower, and this for the
reason that it affects so many of the senses at the same time. The flash
may be blinding to the eyes if near to us; the thunder may be deafening
to the ears, and so powerful as to shake the foundations of the hills,
and make the ground upon which we stand to sensibly move: these with the
remembered destructive effects that have been witnessed, of buildings
demolished and large trees torn to splinters in an instant, are quite
sufficient to raise a feeling of dread in the strongest mind. In the
polar regions, both north and south, where thunder-storms are less
frequent, the atmospheric electricity assumes the form called the aurora
borealis, or the aurora australis, according as it is seen north or
south of the equator.
More than two thousand years ago it was noticed by the Greeks that a
certain kind of a mineral which was thrown up on the shores of the
Mediterranean Sea, when rubbed would attract light bodies, such as
shreds of silk or linen and bits of paper. To this substance they gave
the name of Elektron, and the property developed thus by friction was
afterwards called electricity. In 1600 Dr. Gilbert, physician to Queen
Elizabeth, published a book in which he described numerous experiments
demonstrating that electricity could be developed by friction upon a
great variety of substances, such as stones, gems, and resins. The first
machine for developing electricity was made by Otto von Guericke of
Magdeburg, about 1680. His machine consisted of a ball of sulphur about
six inches in diameter, which could be rotated. If the dry hand were
held against the sulphur while it was being turned in a dark room, the
sphere appeared to emit light: it also gave out a peculiar hissing or
crackling sound. Newton experimented a little with electricity, and
noticed that the rubber was an important element in developing
electricity. He does not seem to have given to the subject the same
attention that he gave to some other departments of science. Had he done
so, it is probable that he would have advanced the study a hundred
years; that is to say, he would probably have left it at the place where
it actually was in 1790. So great were his abilities that in one
lifetime he made greater additions to human knowledge than all the rest
of mankind had made during the preceding thousand years. In the month of
June, 1752, Franklin made that memorable experiment which immortalized
him. He flew his kite to the thunder-cloud, practically asking the
question of the lightning whether or not it was identical with
electricity. The lightning came down the wetted twine to his hand, and
proclaimed its identity.
For the next forty years the natural philosophers in both Europe and
America only rung the changes upon what was known. They flew kites to
the clouds; they made and charged Leyden jars, and discharged them
through wires and chains and circuits of clasped hands, and studied the
attractions and repulsions manifested by electrified bodies; but they
added nothing of importance in the way of experiments.
In 1791 Galvani, a professor of anatomy at Bologna, announced a
manifestation of electricity that was new and of a remarkable character,
having its origin in the muscles of animals, and so was called animal
electricity. He had some frogs' legs prepared for eating; by chance they
were placed near an electrical machine with which Galvani was
experimenting, so that a spark would occasionally pass to the legs, when
they would contract as often as a spark passed to them. The motion was
first observed by his wife, who called his attention to the phenomenon;
and he very soon discovered that the thighs of a frog, skinned and
suspended, made a very good electroscope. While experimenting in this
way he made another and more important discovery; namely, that, when
the muscles and nerves of the frog's leg were touched by pieces of two
different metals, the leg would contract as before. Alexander Volta,
another Italian professor, who had invented the electrophorus, and was
possessed of great experimental skill, now turned his attention to the
experiment of Galvani, and very soon discovered that the origin of the
electricity that moved the frogs' legs was not in the legs themselves,
but in the metals used. The first form of the galvanic battery was the
result of Volta's investigations, and was called the Voltaic pile. This
pile consisted of alternate disks of zinc, flannel, and copper, piled
one on top of the other in constant succession in that order. The
flannel was moistened with salt and water, or with diluted sulphuric
acid. When the first zinc was connected with the last copper by means of
a wire, a powerful current of electricity was obtained. This form of
battery is not in use at all now, as much more efficient means are known
for producing electricity; but this in 1800, when it was first made
known in England, was very startling, and was one of those surprises
which have been so frequent since then in the history of electricity.
Surprising things were done by Sir Humphry Davy, with a large Voltaic
battery. Water was decomposed, and the metals potassium and sodium were
first separated from their compounds with oxygen. Bonaparte had offered
a prize of sixty thousand francs "to the person who by his experiments
and discoveries should advance the knowledge of electricity and
galvanism as much as Franklin and Volta did," and of "three thousand
francs for the best experiments which should be made in each year on the
galvanic fluid." This latter prize was awarded to Davy.
After Davy's successes in 1806, there was nothing of importance in an
experimental way added to the knowledge of electricity, until 1820, when
Oersted of Copenhagen announced that "the conducting wire of a Voltaic
circuit acts upon a magnetic needle," and that the needle tends to set
itself at right angles to the wire. This was a kind of action altogether
unexpected. This observation was of the utmost importance; and at once
the philosophers in Europe and America set themselves to inquire into
the new phenomenon. The laws of the motion of the magnetic needle when
acted upon by a current of electricity traversing a wire were
successfully investigated by M. Ampère of the French Academy. He
observed that whenever a wire through which a current of electricity was
passing was held over and parallel with a magnetic needle which was free
to move, and therefore pointed to the north, if the current was moving
_towards_ the north, the north pole was deflected to the west; if the
current was moving towards the south, the south pole of the magnet was
deflected towards the west; and that in all cases the magnet tended to
set itself at right angles to the current; also that this angular
displacement depended upon the strength of the current. Thus originated
the _galvanometer_, an instrument that not only detects the existence of
an electric current, but enables us to determine its direction and its
strength. Our present knowledge of electrical laws is due, in a very
large measure, to observations made with this instrument. Of course it
has been very much modified, and made almost incredibly sensitive: yet,
in all galvanometers, the fundamental principle involved in their
structure is that of the action of a current of electricity upon a
magnet, which was first noticed by Oersted.
MAGNETS.
It is related by Nicander that among the shepherds who tended their
flocks upon the sides of Mount Ida was one named Magnes, who noticed,
that, while taking his herds to pasture, his shepherd's crook adhered to
some of the rocks. From this man's name some have supposed the name
_magnet_ to have been derived. It is, however, generally believed to
have received its name from the ancient city of Magnesia in Asia Minor,
near which the loadstone or magnetic substance was found. This rock,
which possesses the remarkable property of attracting and holding to
itself small pieces of iron or steel, is now known to be one of the ores
of iron, and is called magnetite by mineralogists. The iron is
chemically combined with oxygen, and forms 72.5 per cent of its weight.
There is another ore of iron, known as hematite, which contains seventy
per cent of iron; but the difference of two and a half per cent of iron
in the ore is enough to make the difference between a magnetically inert
substance, and one which may be able to lift a mass of iron equal to
many times its own weight.
Sir Isaac Newton is said to have worn in a finger-ring a small loadstone
weighing three grains, which would lift seven hundred and fifty grains,
which is equal to two hundred and fifty times its own weight. The most
powerful magnet now known is owned by M. Obelliane of Paris. It can lift
forty times its own weight. Large pieces, however, do not support
proportionally greater weights, seldom more than one or two times their
own weight.
There are in many places in the world immense beds of magnetic iron-ore.
Such are to be found in the Adirondack region in Northern New York, and
in Chester County, Pennsylvania. The celebrated iron-mines of Sweden
consist of it, and in Lapland there are several large mountains of it.
It must not be inferred, that, because the mineral is called magnetite,
all specimens possess the property called magnetism. The large masses
seldom manifest any such force, any more than ordinary pieces of iron or
steel manifest it: yet any of it will be attracted by a magnet in the
same way as iron will be. The most powerful native magnets are found in
Siberia, and in the Hartz, a range of mountains in Northern Germany.
When a piece of this magnetically endowed ore is placed in a mass of
iron-filings, it will be seen that the filings adhere to it in greatest
quantity upon two opposite ends or sides, and these are named the poles
of the magnet. If the piece be suspended by a string so as to turn
freely, it will invariably come to rest with the same pole turned
towards the north; and this pole is therefore called the north pole of
the magnet, and the action is called the directive action. This
directive action was known to the Chinese more than three thousand years
ago. In traversing those vast steppes of Tartary they employed magnetic
cars, in which was the figure of a man, whose movable, outstretched arm
always pointed to the south. Dr. Gilbert affirms that the compass was
brought from China to Italy in 1260, by a traveller named Paulus
Venetus.
When a piece of hardened steel is rubbed upon a natural magnet, it
acquires the same directive property; and, as the steel could be easily
shaped into a convenient form for use, a steel needle has generally been
used for the needle of a compass. The directive power of the magnet has
been and still is of incalculable value to all civilized nations. Ocean
navigation would be impossible without it, and territorial boundaries
are fixed by means of it; but there are other properties and relations
of a magnet, which have been discovered within the last fifty years,
which are destined to be as important to mankind as that of the compass
has been.
In 1825 William Sturgeon of Woolwich, Eng., discovered that if a copper
wire were wound around a piece of soft iron, and a current of
electricity sent through the wire, the soft iron would become a magnet,
but would retain its magnetism no longer than while the current of
electricity was passing through the coil. The magnetism developed in
this way was called electro-magnetism, and the iron so wound was called
an electro-magnet. The first electro-magnet was made by winding bare
wire upon the soft iron. This method will not produce very strong
magnets. In 1830 Prof. Henry insulated the wire by covering it with
silk, and was the first to produce powerful magnets.
On a soft iron bar of fifty-nine pounds weight he used twenty-six coils
of wire, thirteen on each leg, all joined to a common conductor by their
opposite ends, and having an aggregate length of seven hundred and
twenty-eight feet. This apparatus was found able to sustain a weight of
twenty-five hundred pounds. This electro-magnet is now owned by Yale
College.
The power of the electro-magnet is enormously greater than that of any
permanent magnet. A permanent magnet made by Jamin of Paris, which is
made up of many strips of thin steel bound together, and weighing four
pounds, is able to support a weight of one hundred pounds; but Dr. Joule
made an electro-magnet, by arranging the coils to advantage, that would
support thirty-five hundred times its own weight, or one hundred and
forty times the proportionate load of Sir Isaac Newton's ring magnet.
THE GALVANIC BATTERY.
The original form of the galvanic battery as devised by Volta, and
modified but little during thirty years, consisted of a cell to contain
a fluid, which was usually dilute sulphuric acid, in which two plates of
different metals were immersed: the metals used were generally plates of
zinc and copper, or zinc and silver. Such plates, when first placed in
the liquid, will give a very good current of electricity; but it will
not last long. The reason of this is easy to understand. Whenever a
current of electricity is generated by chemical action of a liquid upon
two different metals, there is always some decomposition of the liquid,
and this decomposition takes place upon the plates themselves; and the
liberated gases _adhere to the plates, and prevent further contact with
the acid_; at the same time, the gases themselves act upon the plates,
and generate a current of electricity in the opposite direction. This
will of course interfere with the first current; and very soon the
battery is useless until the plates have been withdrawn from the liquid.
This physico-chemical process that takes place in such a battery is
called the _polarization of the plates_.
[Illustration: FIG. 1.]
The accompanying figure will help one to understand the actions going on
in a battery cell of the kind mentioned. Let Pt represent a plate of
platinum, and Zn a plate of zinc, both placed in a vessel containing
hydrochloric acid, which is also represented by the symbols HCl. As such
molecules are extremely minute, there will of course be an immense
number of them between the plates. The plates are now to be connected by
a wire running between them through the air. As soon as these conditions
are fulfilled, a hissing sound will be heard coming from the cell, and
bubbles of gas will be seen to rise from the platinum plate: these
bubbles prove upon analysis to be bubbles of hydrogen. At the same time
the zinc will begin to dissolve, forming what proves by analysis to be
the chloride of zinc; and at the same time a current of electricity
travels through the wire from the platinum to the zinc. The quantity of
electricity that is thus generated is strictly proportionate to the
quantity of hydrogen liberated, which is also proportionate to the
weight of zinc dissolved; and this, in turn, is proportionate to the
surface of the metals exposed to the action of the acid. Now, it happens
under such circumstances as the above, that the liberated hydrogen
adheres very strongly to the platinum, as there is nothing for it to
unite with chemically; and therefore the plate will very soon be visibly
covered with bubbles, which may be scraped off with a feather or a swab,
but only to have the same thing repeated.
This coating of bubbles will prevent the acid from touching the plate,
and so practically diminishes the surface of it; but the quantity of
electricity generated being proportionate to the surface exposed to the
chemical action, it will be understood at once how such polarization of
the plates must soon bring the battery to a standstill.
In 1836 Prof. J. F. Daniell of London contrived a battery, which has
been called the Daniell Cell, in which the metal (copper) that had the
hydrogen liberated upon it was separated by a porous cell from the zinc.
The zinc was immersed in dilute sulphuric acid, and the copper in an
acid solution of blue vitriol (copper sulphate). The porous cup did not
prevent the electricity from passing, nor the decomposition from taking
place; but the hydrogen, which in this case would have been liberated at
the copper plate, at once united with oxygen there, which it got by
decomposing the copper sulphate: hence water was formed, and copper was
deposited upon the copper plate; and, being an excellent conductor, the
battery would keep up a strong action for a long time.
Mr. Grove, also of London, in 1839 invented a battery which still goes
by his name, in which the hydrogen plate is of platinum immersed in
strong nitric acid, enclosed also in a porous earthen cell; and this, in
turn, is plunged into a vessel containing dilute sulphuric acid and the
zinc. In this case the liberated hydrogen immediately decomposes the
nitric acid, which readily parts with its oxygen; water is the product,
as in the other case, and the nitric acid loses strength. Strips of
carbon have been substituted for the platinum, and this is called the
Bunsen battery. It is otherwise like the Grove battery; it gives a very
powerful and constant current and it is by the use of one or the other
of these batteries, that most of the experiments in electricity are
performed in institutions of learning, and, until lately, most in use
for telegraphic purposes.
OTHER MEANS FOR GENERATING ELECTRICITY.
THERMO-ELECTRICITY.
IF two strips of different metals, such as silver and iron, be soldered
together at one end, and the other ends be connected with a
galvanometer, on heating the soldered junction of the metals it will be
found that a current of electricity traverses the circuit from the iron
to the silver. If other metals be used, having the same size, and the
same degree of heat be applied, the current of electricity thus
generated will give a greater or a less deflection, which will be
constant for the metals employed. The two metals generally employed are
bismuth and antimony, in bars about an inch long and an eighth of an
inch square. These are soldered together in series so as to present for
faces the ends of the bars, and these often number as many as fifty
pairs. Such a series is called a thermo-pile. This method of generating
electricity was discovered by Seebeck of Berlin in 1821, but the
thermo-pile so much in use now in heat investigations was invented by
Nobili in 1835. The strength of this current is not very great, a single
Daniell cell being equal to nine pairs of the strongest combination yet
discovered, namely, the artificial sulphuret of copper with German
silver.
MAGNETO-ELECTRICITY.
[Illustration: FIG. 2.]
It has already been mentioned, that Oersted found that a magnet when
free to turn tended to set itself at right angles to a wire in which a
current of electricity was passing, thus demonstrating some inter-action
between electricity and magnetism; but it remained for Faraday to
discover the converse fact, namely, that a magnet moving across a wire,
the ends of which were connected with a galvanometer or otherwise
closed, originated a current of electricity in the wire, the direction
of which depended upon the direction of the movement of the magnet. If
the wire was coiled into a hollow helix, the magnet in moving through
the helix moved across, that is, at right angles to all the turns of
the helix; and each complete turn added to the intensity of the current.
This will be understood by reference to the diagram, Fig. 2. Let G be a
galvanometer connected with the wires from a helix; N S, a permanent bar
magnet. If the magnet be thrust into the coil, a current of electricity
will traverse the helix, wire, and galvanometer, and the needle will
indicate its direction. If the magnet be now withdrawn, a current will
move in the opposite direction through the whole circuit. The
electricity that is thus originated is said to be induced. The quantity
of electricity that can be induced thus is almost unlimited, depending
upon the size and strength of the magnet, the size of the wire, and the
length of wire in the coil. There are now many forms of machines for
developing electricity from the motion of coils of wire in front of the
poles of permanent magnets. They are generally called magneto-electric
machines. The action involved in these machines is so important in its
bearing upon telephony as to necessitate a fuller description of them.
MAGNETIC INDUCTION.
[Illustration: FIG. 3.]
Let N S, Fig. 3, be a bar of hardened steel rendered permanently
magnetic. If now there be brought near to it a board-nail, the latter
will become a magnet through the _inductive_ action of the first magnet.
This induced magnetism may be demonstrated by bringing a tack or other
bit of iron to the end that is farthest from the permanent magnet; the
tack will adhere to the nail, but will fall off when the nail is removed
from the neighborhood of the magnet. By testing the polarity of the
nail, it will be found that the end nearest the magnet will be a south
pole if the magnet has its north pole towards it, in all cases having a
polarity opposite to that of the pole acting upon it. The strength of
this induced magnetism thus developed depends upon the distance apart of
the magnet and the iron, being at its maximum when the two touch. But
the tack itself is also made a magnet, and will attract another tack,
and that one still another, the number which can be thus supported being
dependent upon the strength of the first or inducing magnet.
Suppose now that we should wind a few feet of wire about the nail, and
fasten the two ends of the wire to an ordinary galvanometer, and then
make the nail to approach the permanent magnet. The galvanometer needle
would be seen to move as the nail approached; and, if the latter were
allowed to touch the magnet, the movement of the needle would suddenly
be much hastened, but would directly come to rest, showing that, so
long as there is no motion of the nail towards or away from the magnet,
no electricity is moving in the wire, although the nail is a strong
magnet while it is in contact with the permanent magnet. If the nail be
now withdrawn, the two phenomena happen as before: that is to say, as
the nail recedes it loses its magnetism; and the giving-up of its
magnetism induces a current of electricity through the wire in the
opposite direction to that it had when the nail approached. The current
of electricity in the opposite direction is indicated by the
galvanometer needle, which moves according to Ampère's law mentioned on
a preceding page.
It may be noted here that we have an effect quite analogous to that
already mentioned on page 21 as the experiment of Faraday. In one case a
permanent magnet is thrust into a coil of wire, and in the other a piece
of iron is made a magnet while enclosed in a coil. In each case there is
generated a current of electricity _which lasts no longer than the
mechanical motion of the parts lasts_.
MAGNETO-ELECTRIC MACHINES.
Such transient currents are practically useless, and several devices
have been invented to make the flow continuous. The common form of
machine for doing this may be understood by reference to the diagram.
[Illustration: FIG. 4.]
N S, Fig. 4, is the permanent magnet, which is bent into a U form in
order to utilize both poles. N´ and S´ are short rods of soft iron
fastened into a yoke-piece Y, also of soft iron. Coils of wire surround
each of the rods as represented, the ends of the wires connecting with
each other and with what is called a pole-changer. The whole of this
part is capable of revolving upon an axis P Y by a pulley at P. The
action is as follows: From their position, the soft-iron rods N´ S´
must be magnets through the inductive action of the permanent magnet,
just as the nail was made a magnet in like position. So long as the
parts have the relative position shown in the figure, and there is no
motion, no electricity can be developed; but, if the axis P Y be turned,
S´, which represents the polarity of the rod opposite N, will be losing
its induced magnetism; and, when half a revolution has been made, that
same pole will be where N´ now is; but it will then have N´ polarity
instead of S´; that is, it has been losing south polarity as it receded
from N, and gaining north polarity as it approached S: hence a current
of electricity has steadily been flowing through the coil in one
direction. At the same time, the other rod N´ has passed through similar
phases; and its enveloping coil has had a current of electricity induced
in it in the same direction as in the first coil. This doubles the
intensity of the current; and the whole is conducted by the
connecting-wires where the current is wanted. Machines have been built
upon this plan, that contained fifty or sixty powerful compound
permanent magnets, and as many wire coils, needing a steam-engine of
eight or ten horse-power to run them.
A less cumbersome and much more efficient magneto-electric machine has
been made by changing the form of the soft iron armature to something
like a shuttle, and winding the wire inside of it. This is called the
"Siemen's Armature." The latest pattern of such machines is known as the
_Gramme_; and its peculiarity consists in the substitution of a broad
ring of soft iron for the armature. About this ring a good many coils,
of equal lengths, of insulated copper wire are wound in such a manner
that one-half of any turn in the wire goes through the inside of the
ring, making the coils longitudinal. The whole of the armature thus
prepared is fixed upon a shaft, so as to permit rotation, and fixed
between the poles of a powerful Jamin magnet. The ends of the coils are
connected with conductors upon the axis; and, when the armature thus
constructed is rotated, a very constant and powerful current of
electricity flows in a single direction, unlike the other forms. It is
stated, that, with one-horse power, a light can be obtained equal to
that from a battery of fifty Grove cells.
SECONDARY CURRENTS.
So long ago as 1836 it was noticed by Prof. Page of Salem, that,
whenever a current of electricity was made to flow in a coil of wire,
another current in the opposite direction was induced in a coil that was
parallel with the first; and also, when the current in the first was
broken, another current in the second coil would flow in the opposite
direction to the former one. These currents, which are called secondary
currents, are very transient. No current at all flows save at the
instant of making or breaking the current. In this respect, we are
reminded of the behavior of the soft iron within the coil, which gives
origin to a current of electricity when it is made to approach a magnet
or recede from it, but gives no current so long as it is still.
These secondary currents were investigated by Prof. Henry, resulting in
the discovery of many curious and interesting phenomena. It will be
sufficient here for me to refer to what are called induction coils,
which are developments of the principles involved in electro-magnetism
and electro-induction. Imagine a rod of soft iron of any size to be
wound with a coil of wire, the ends of the wire to be so left that they
may be connected with a galvanic battery. Around this coil let another
coil be wound of very fine and well-insulated wire; the terminal wires
of it to be left adjustable to any distance from each other. Now, upon
making connection with a battery to the primary coil, there will be two
results produced simultaneously. First, the soft iron will be rendered
magnetic; and, second, a current of electricity will be generated in the
secondary coil; and the strength of this secondary current is very much
increased by the inductive action of the soft iron that has been made a
magnet. When the battery current is broken, the iron loses its
magnetism, and a current of electricity is again started in the
secondary coil in the opposite direction. The energy of this derived
current is so great that it will jump some distance through the air, and
thus is apparently unlike the electricity that originates in a battery.
An induction coil made by Mr. Ritchie for the Stevens Institute at
Hoboken, N.J., has a primary coil of 195 feet of No. 6 wire. The
secondary coil is over fifty miles in length, and is made of No. 36
wire, which is but .005 of an inch in diameter. This instrument has
given a spark twenty-one inches in length, with three large cells of a
bichromate battery.
Mr. Spottiswood of London has just had completed for him the largest
induction coil ever made. It has two primary coils, one containing
sixty-seven pounds of wire, and the other eighty-four pounds, the wire
being .096 inch in diameter. The secondary coil is two hundred and
eighty miles long, and has 381,850 turns. This coil is made in three
parts, the diameter of the wire in the first part being .0095 inch; of
the second part, .015; and the third part, .011. With five Grove cells
this induction coil has given a spark forty-two inches long, and has
perforated glass three inches thick.
The electricity thus developed in secondary coils is of the same
character as that developed by friction; and all of the experiments
usually performed with the latter may be repeated with the former, many
of them being greatly heightened in beauty and interest. Such, for
instance, are the discharges in vacuo in Geisler tubes, exhibiting
stratifications, fluorescence, phosphorescence, the production of ozone
in great quantity, decomposition of chemical compounds, &c.
The electricity developed by friction upon glass, wax, resin, and other
so-called non-conductors, has heretofore been called static electricity,
for the reason that when it was once originated upon a surface it would
remain upon it for an indefinite time, or until some conducting body
touched it, and thus gave it a way of escape. Thus, a cake of wax if
rubbed with a piece of flannel, or struck with a cat-skin or a fox-tail
becomes highly electrified, and in a dry atmosphere will remain so for
months. Common air has, however, always a notable quantity of moisture
in it; and, as water is a conductor of electricity, such damp air moving
over the electrified surface will carry off very soon all the
electricity.
Again, the electricity developed through chemical action in a battery
and through the inter-action of magnets and coils of wire has been
called dynamic electricity, inasmuch as it never appeared to exist save
when it was in motion in a completed circuit. This, however, is not
true; for if one of the wires from a galvanic battery be connected with
the earth, and the other wire be attached to a delicate electrometer, it
will be found that the latter gives evidence of electrical excitement in
the same manner as it does for the electricity developed by friction in
another body. This is sometimes called _tension_, and is very slight for
a single cell; but in a series of cells it becomes noticeable in other
ways. Thus when the terminals of a single cell are taken in the hands,
no effect is perceived: if, however, the terminals of a battery
consisting of forty or fifty cells be thus taken, a decided shock is
felt, not to be compared though with the shock that would be felt from
the discharge of a very small Leyden jar. The shock from several hundred
cells would be very dangerous.
It was formerly doubted that the electricity would pass between the
terminals of a battery without actual contact of the terminals. Gassiot
first showed that the spark would jump between the wires of a battery of
a large number of cells before actual contact was made. Latterly Mr. De
La Rue has been measuring the distance across which the spark would
jump, using a battery of a large number of cells.
I give his table as taken from the "Proceedings of the Royal Society:"--
Cells. Striking distance.
600 .0033 inch.
1,200 .0130 "
1,800 .0345 "
2,400 .0535 "
This table shows that the striking distance is very nearly as the square
of the number of cells. Thus, with 600 cells the spark jumped .0033
inch; and with double the number of cells, 1,200, the spark jumped .0130
inch, or within .0002 of an inch as far as four times the first
distance.
This leads one to ask how big a battery would be needed to give a spark
of any given length, say like a flash of lightning. One cell would give
a spark .00000001 inch long, and a hundred thousand would give a spark
92 inches long. A million cells would give a spark 764 feet long, a
veritable flash of lightning. It is hardly probable that so many as a
million cells will ever be made into one connected battery, but it is
not improbable that a hundred thousand cells may be. De La Rue has since
completed 8,040 cells, and finds that the striking distance of that
number is 0.345 inch, a little more than one-third of an inch. He also
states that the striking distance increases faster than the above
indicated ratio, as determined by experimenting with a still larger
number of cells.
These experiments and many others show that there is no essential
difference between the so-called static and dynamic electricity. In the
one case it is developed upon a surface which has such a molecular
character that it cannot be conducted away, every surface molecule being
practically a little battery cell with one terminal free in the air, so
that when a proper conductor approaches the surface it receives the
electricity from millions of cells, and therefore becomes strongly
electrified so that a spark may at once be drawn from it.
WHAT IS ELECTRICITY?
THEORIES.
NUMEROUS attempts have been made to explain the phenomena of
electricity. As a general thing, these phenomena are so utterly unlike
other phenomena that have been explained and are easily intelligible,
that it has quite generally been taken for granted, until lately, that
something very different from ordinary matter and the laws of forces
applicable to it must be involved in the phenomena themselves.
Consequently the term _imponderable_ was applied to it,--something that
was matter minus some of the essentials of matter; and as it was
apparent that, whatever it was, it moved, apparently flowed, from one
place to another, the term _fluid_ was applied to it, a term descriptive
of a certain form of matter. Imponderable fluid was the descriptive name
applied to electricity. Newton supposed that an excited body emitted
such a fluid that could penetrate glass. When the two facts of
electrical attraction and repulsion had to be accounted for, two
theories were propounded,--one by Benjamin Franklin, the other by Dufay.
Franklin supposed that electricity was a subtle, imponderable fluid, of
which all bodies contained a certain normal quantity. By friction or
otherwise this normal quantity was disturbed. If a body received more
than its due share, it was said to be positively electrified: if it had
less than its normal quantity, it was said to be negatively electrified.
Franklin supposed this electric fluid to be highly self-repulsive, and
that it powerfully attracted the particles of matter.
According to Dufay, there are two electric fluids, opposite in tendency
but equal in amount. When associated together in equal quantities, they
neutralize each other completely. A portion of this neutral compound
fluid pervades all matter in its unexcited state. By friction or
otherwise this compound fluid is decomposed, the rubber and the body
rubbed exchanging equal quantities of opposite kinds with each other,
leaving one of them positively, the other negatively electrified. These
two fluids were supposed to be self-repulsive, but to attract each
other: so that, if two bodies be charged with either positive or
negative electricity, such bodies would mutually repel each other; but
if one was charged with positive, while the other was charged with
negative electricity, the two bodies would mutually attract each other.
Either of these two theories may be used to illustrate the phenomena,
and so have done good service in systematizing the facts. It is evident
that both of them cannot be true, and it is in the highest degree
probable that neither of them is true.
Some have supposed that there was a kind of electric atmosphere about
every atom of matter; and still another theory, now advocated by Edlund
of Stockholm, assumes that electricity is identical with the ether by
which radiant energy, light and heat, is transmitted.
Before a correct judgment can be formed of the nature of any force, it
is necessary to know what it can do, what kind of phenomena it can
produce. Let us, then, take a brief survey of what electricity can do.
1st, It can directly produce _motion_, through the attractions and
repulsions of electrified bodies,--as indicated by electrometers, the
rotation of the fly-wheel, the deflection of the galvanometer needle. It
has been proved by the mathematical labors of Clausius, and confirmed by
experiment, that, when electricity performs any mechanical work, so much
electricity is lost, annihilated as electricity.
2d, It can directly produce _heat_, as shown by passing a sufficient
quantity of electricity through a fine platinum wire: the wire becomes
heated, and glows, and it may even be fused by the intensity of the
heat. The heat developed in the so-called electric arc is so great as to
fuse the most refractory substances. If a current of electricity from a
battery be sent through a thermo-pile, one of the faces of the pile will
be heated. The heat of the spark from a Leyden jar may be made to ignite
gunpowder, and dissipate gold into vapor. The heat produced by lightning
is seen when a live tree is struck by a powerful flash: the sap of the
tree is instantly converted into steam of so high a tension as to
explode the tree, scattering it in small fragments over a wide area. The
tips of lightning-rods often exhibit this heating effect, being fused by
the passage of too great a quantity of electricity.
In the early part of the present century it was demonstrated by Count
Rumford, and also by Sir Humphry Davy, that heat was but a form of
molecular motion. Since then the exact relations between the motion of a
mass of matter and the equivalent heat have been experimentally
determined by Joule, so that the unit of heat may be expressed in the
motion of a mass of matter. This is deducible from a more general law,
known as the conservation of energy. The application in this place is,
that whenever heat appears through electric action, as in the
above-mentioned places, we know that it still is only _motion_ that is
the product, only that this motion is now among the molecules of the
body, instead of the motion of the whole body in space, as when a
pith-ball moves, or a galvanometer-needle turns.
3d, It can directly produce _light_. This is seen in every spark from an
electric machine, in the flash of lightning, and in the electric light.
It has been shown in numberless ways, that there is no essential
difference between light and heat, and that what we call light is only
the active relation which certain rays of radiant energy have to the
eyes. In order to make this plain, suppose that a beam of light, say
from the sun, be permitted to fall upon a triangular prism of glass: at
once it is seen that the beam is deflected, and instead of appearing a
spot of white light, as it did before it was deflected, it now appears
as a brilliant band of colors, which is called the solar spectrum. If
now this spectrum be examined as to the distribution of heat, by moving
a thermo-pile through it from the blue end towards the red end, it will
be noticed that the galvanometer-needle will be but slightly deflected
at the blue end; but, as the thermo-pile is moved, the deflections are
greater until it is past the red end, where the heat is greatest. On
this account it has been customary to say that the red end of the
spectrum was the heating end. With various pieces of mechanism the rays
may be separated from each other, and measured; and then it appears that
a red ray of light has a wave length of about 1/37000 in., and the
violet ray about 1/60000 in. The rays beyond the red have also been
measured, and found to be greater in length uniformly as one recedes
from the visible part of the spectrum.
In like manner, beyond the blue end the wave lengths become shorter and
shorter; and in each of these directions the spectrum that is invisible
is much longer than the visible one. Now, it has also been found that
where a prism of glass or other material is used to produce a spectrum,
it distributes the rays very unevenly; that is, towards the red end of
the spectrum they are very much crowded, while towards the blue end they
are more dispersed. Hence, if one were measuring the heating power of
such a spectrum, many more rays would fall upon an equal surface of the
thermo-pile at the red end than at the blue end; therefore the
indications of the galvanometer would be fallacious. Before any thing
definite could be known about the matter, it would plainly be necessary
to work with an equal dispersion of all the rays. This was effected a
few years ago by Dr. Draper of New York. He took the spectrum produced
by diffraction instead of refraction, and measured that. In that way it
was found that the heating power of the spectrum is equal in every part
of it; and hence the pictures in treatises on physics that represent the
heating power of the spectrum to be concentrated at the red end is not
true save where the spectrum is irregularly produced. As for vision, the
mechanical structure of the eye is such that radiant vibrations having a
wave length between 1/37000 in. and 1/60000 in. can affect it, while
longer or shorter wave lengths can not. Such waves we call light, but it
is not at all improbable that some animals and insects have eyes adapted
to either longer or shorter wave-lengths; in which case, what would be
perfectly dark to us would be light to them. It is a familiar enough
fact, that many animals, such as dogs, cats, rats, and mice, can see in
the night. Some horses may be trusted to keep in the road in a dark
night, when the driver cannot see even the horse itself. This has
usually been accounted for by saying that their eyes are constructed so
as to collect a greater number of luminous rays. It is much better
explained by supposing their eyes to be constructed to respond to
wave-lengths either greater or less than those of mankind.
A ray of light, then, consists of a single line of undulations of a
definite wave length, such that if it falls upon the eye it will produce
sight; if it falls upon a thermo-pile it heats it by just the same
quantity that another wave-length would heat it; if it falls upon matter
in unstable chemical relations, it will do chemical work, depending upon
the kinds of matter. A red ray is as effective for some substances as a
violet ray is for others. The statement, then, so often lately made to
do certain analogical work, namely, that a ray of light consists of
three distinct parts, which may be separated from each other, and are
called heat, light, and chemical properties, is simply untrue. What a
ray will do, depends upon what kind of a structure it falls on; and when
it has done that work, of whatever kind it may be, it ceases to exist as
a ray.
If, therefore, electricity can directly produce light, it is simply
producing _motion_, as in the case of heat, the motion being of such a
sort that the eyes of men are affected by it.
4th, It can produce _magnetism_. A current of electricity passing
through a coil of wire makes such a coil a magnet, which will set itself
in the direction of the magnetic meridian of the earth; and, if a bar of
soft iron be placed in the coil, it becomes the familiar electro-magnet;
and, if hardened steel be put in it, it becomes a permanent magnet.
This leads to the inquiry as to what magnetism is. We know that it can
produce motion by its moving at a distance a piece of iron or another
magnet. It will also sustain a mass of matter against gravity or some
other contrary force. Through such mechanism as magneto-electric
machines it produces electricity in great abundance, which again can be
used to produce any of the effects of electricity,--moving bodies by
attraction or repulsion, generating heat or light, or again making a
magnet. But as all of these are but varied forms of motion, either of a
mass as a whole, or molecular, can it be doubted for an instant, that
what we call magnetism is but some form of motion? Must it not be either
some form of matter, or some form of motion? If it were a form of
matter, then a magnet would only be permanent so long as it was not
used; for use implies consumption of the force; and, if this be matter
in any form, then in a given mass of matter there can be but a definite
quantity of such magnetic matter, and consumption must lessen that
quantity. As a matter of fact, there is no perceptible lessening of the
power of a magnet when it is properly used. It is also a matter of fact,
that neither motion of a mass, nor electrical effects, nor any other,
can be produced by the action of a magnet alone. It is only when some
form of motion has been added to its own property, that we get any kind
of an effect from it: hence all effects due to its action are
_resultants_ of two forces, one of them being common motion of a mass of
matter, and the other the energy of the magnet. Hence we infer that a
magnet is a mechanism of such a structure as to change the direction and
character of the motion which acts upon it. When the wheel of a common
electrical machine is turned, the product is electricity,--a force very
different from that which originates it. Ordinary mechanical motion
_goes in_; electricity _comes out_, the latter being a modified motion
due to the physical structure of the machine. In like manner, a magnet
may be considered as a machine by means of which mechanical motion may
be converted into some other form of motion. It is evident that
molecular structure is chiefly concerned in this. If a bar of iron that
exhibits no evidence of magnetism whatever be subjected to torsion, it
will immediately become a magnet with poles dependent upon the direction
of the twist. This developed magnetism will re-act upon a coil of wire,
and so move a galvanometer needle. If the bar be permitted to recover
its original condition, it will lose its magnetism, which will at once
re-appear upon twisting the rod again. Now, when the rod is twisted, it
is evident that there is a molecular strain in certain directions
throughout the mass. The converse experiment illustrates the same thing.
It has been found, that when a rod of iron is made magnetic by the
action of a current of electricity circulating about it, and at the same
time passing longitudinally through it, the rod is slightly lengthened
and twisted in a direction that depends upon the direction of the
current. Moreover, if a permanent magnet be heated to a red heat, its
magnetism is destroyed; for such a heat allows the molecules to freely
arrange themselves without any external constraint. Also, if a permanent
magnet be suspended so as to give out a musical sound when it is struck,
the magnetism will be much weakened by making it thus to vibrate. In
this case, as in the other, the vibrations affect every molecule, and so
enable them to re-adjust themselves to the positions they held before
being magnetized. The same thing happens when a bar of iron is made
magnetic through the inductive action of the earth. When this bar is
held in the direction of the magnetic dip, it becomes but very slightly
magnetized; but, if it be so held that when it is struck with a hammer
it will ring, that is, give out a musical sound, it will at once become
decidedly magnetic. Evidently the earth's action tends to set the
molecules of the mass in a new position, but cohesion prevents them from
assuming it. When the molecules are made to vibrate, they can assume
such new positions more readily. The molecules of a magnet, then, are
differently arranged from those in an unmagnetized piece of iron or
steel; and, for every new arrangement of the molecules of a mass of any
kind, we always have some new physical property developed. The same
identical substance may appear as charcoal, coke, plumbago, anthracite
coal, and diamond. Hence a magnet is a machine in which other forces
acting upon it are transformed in character, and re-appear as
attractions and repulsions of other kinds of matter: this transformation
cannot take place, and hence magnetism cannot become apparent, only upon
the condition of another force acting in concert with it; and, if at any
time it may seem to be acting without such external force, it is done at
the expense of the heat it has absorbed, and therefore the magnet must
at such time be losing temperature proportional to the work done. This I
have discovered to be true by making a magnet to exert its force in
front of a thermo-pile, which uniformly exhibits a cooled face under
such conditions. What the particular form of the motion may be that we
call magnetism, is not yet made out; but that it is some form of motion,
is very evident. The following experiments may throw some light upon it.
Last August Mr. Kerr read a paper before the British Association of
Science, in which was detailed the following experiment: The pole of an
electro-magnet was nicely polished so as to reflect light like a mirror.
A beam of sunlight was permitted to fall upon it, and be reflected to a
convenient place for examination. A current of electricity was sent
through the coil, which of course rendered the iron magnetic; and it was
noticed that the light that was reflected from the pole was circularly
polarized: that is, the motion of a ray, instead of being a simple
undulatory movement, was now made to assume such a motion as the water
from a garden-hose has when the nozzle is swung round in a circle while
the water is escaping from it. After reading the account of it, it
occurred to me that the converse experiment might be tried; that is to
say, the effect of a circularly polarized beam of light upon a piece of
steel. By concentrating a large beam of ordinary plane polarized light
with a quartz lens, and passing it through a quarter wave-plate at the
proper angle, a powerful beam of circularly polarized light was
obtained. At the focus of this beam a fine cambric needle without
magnetism was placed so that the light passed it longitudinally. Ten
minutes' exposure was sufficient to make it decidedly magnetic. Hence I
infer that the motions which we call magnetic attractions and repulsions
may be quite analogous to such helical motions; also, that these motions
exist in ether, and evidently may be either right-handed or left-handed.
Wind up on a pencil a piece of wire twelve or fifteen inches long,
making a loose spiral. Bring the two ends of the spiral together; and
note first that one is twisted to the right, the other to the left. If
they be twisted into each other, they will advance very easily; but if a
right-handed spiral were to be interlocked with another like it, and
both turned in the direction of their spiral, they would separate
rapidly. Applying this conception to a magnet, we might suppose that
such spiral motions will be set up in the ether by the magnet, and that
such motions re-acting upon ordinary matter affect it as attraction and
repulsion; and thus we should have at least a conceivable mechanical
explanation of the phenomenon.
[Illustration: FIG. 5.]
There are numberless experiments which might be given to further exhibit
the relation of mass motion to magnetism, but a single one more must
suffice. No rotation of a magnet upon its own axis can produce any
effects upon a current that is exterior to it; but if a loop of wire be
kept stationary adjacent to a magnet, as in Fig. 5, while the magnet
revolves, a current of electricity is produced; and if the magnet be
kept stationary, and the loop revolves, a current will also be produced,
but in the opposite direction. Here, as in all the other cases, no
electricity is originated, save when motion is imparted to one or other
of the parts. This experiment is due to Faraday.
From all these cases we can come to but one conclusion, that both
electricity and magnetism are but forms of motion; electricity being a
form of motion in ordinary matter, for it cannot be made to pass through
a vacuum, while magnetism must be a form of motion induced in the ether,
for it is as effective in a vacuum as out of it; electricity always
needing some material conductor, magnetism needing no more than do
radiant heat and light.
VELOCITY.
Measurements have been made of the velocity of electricity; both that of
high tension, such as the spark from a Leyden jar, and also that from a
battery. The former was found to have a velocity over 200,000 miles a
second, while the electricity from a battery may move as slowly as
15,000 or 20,000 miles a second; but this is very largely a matter of
conductors. Its velocity is seldom above 30,000 miles a second on
ordinary telegraphic lines. If the electricity be used to give signals,
as in ordinary telegraphy, the time required varies nearly as the length
of the line, and in any case is a much greater quantity. Prescott in his
work on the telegraph states that "the time required to produce a signal
on the electro-magnet at the extremity of a line of 300 miles of No. 8
iron wire is about .01 seconds, and that this time increases in a
greater proportion than the length of the line; for example, on a line
600 miles in length it amounts to about .03 seconds." He also states
that it varies much with the kind of magnet used, some forms of magnets
being much more sensitive than others for this work.
Wheatstone proved a good many years ago that the duration of the
electric spark was less than one millionth of a second. When a swiftly
moving body can only be seen by an electric spark, or flash of
lightning, it looks as if it were quiescent. Thus a train of cars
rushing along at the rate of forty or fifty miles per hour appears
sharply defined,--even the driving-wheels of the locomotive can be seen
in detail, which is impossible in continuous light,--and all seems to
be standing still. In like manner will the sails of a windmill, which
may be turning at a rapid rate, be seen apparently at rest. This is
because in the short time during which they are illuminated they do not
appreciably move.
I am not aware that any attempt has been made to measure the velocity of
magnetism. If, however, it be a form of motion in ether, it is probable
that the velocity is comparable to the velocity of radiant energy,
light, which is equal to about 186,000 miles a second.
SOUND.
BEFORE explaining the relation that sound has to telephony, it will be
necessary to make quite plain what sound is, and how it affects the
substance of the body through which it moves. If I strike my pencil upon
the table, I hear a snap that appears to the ear to be simultaneous with
the stroke: if, however, I see a man upon a somewhat distant hill strike
a tree with an axe, the sound does not reach me until some appreciable
time has passed; and it is noted, that, the farther away the place where
a so-called sound originates, the longer time does it take to reach any
listener. Hence sound has in air a certain velocity which has been very
accurately measured, and found to be 1,093 feet per second when the
temperature of the air is at the freezing point of water. As the
temperature increases, the velocity of sound will increase a little more
than one foot for every Fahrenheit degree; so that at 60° the velocity
is 1,125 feet per second. This is the velocity in air. In water the
velocity is about four times greater, in steel sixteen times, in
pine-wood about ten times.
CONSTITUTION OF A SINGLE SOUND-WAVE.
If a person stands at the distance of fifteen or twenty rods from a
cannon that is fired, he will first see the flash, then the cloud of
smoke that rushes from the cannon's mouth, then the ground will be felt
to tremble, and lastly the sound will reach his ear at the same time
that a strong puff of air will be felt. This puff of air is the
sound-wave itself, travelling at the rate of eleven hundred feet or more
per second. At the instant of explosion of the gunpowder, the air in
front of the cannon is very much compressed; and this compression at
once begins to move outwards in every direction, so as to be a kind of a
spherical shell of air constantly increasing in diameter; and, whenever
it reaches an ear, the sound is perceived. Whenever such a sound-wave
strikes upon a solid surface, as upon a cliff or a building, it is
turned back, and the reflected wave may be heard; in which case we call
it an echo. When a cannon is fired, we generally hear the sound
repeated, so that it apparently lasts for a second or more; but when, as
in the first case, we hear the sound of a pencil struck upon the table,
but a single short report is noticed, and this, as may be supposed,
consists of a single wave of condensed air.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
Imagine a tuning-fork that is made to vibrate. Each of the prongs beats
the air in opposite directions at the same time. Look at the physical
condition of the air in front of one of these prongs. As the latter
strikes outwards, the air in front of it will be driven outwards,
condensed; and, on account of the elasticity of the air, the
condensation will at once start to travel outwards in every
direction,--a wave of denser air; but directly the prong recedes,
beating the air back in the contrary direction, which will obviously
rarefy the air on the first side. But the disturbance we call
rarefaction moves in air with the same velocity as a condensation. We
must therefore remember, that just behind the wave of condensation is
the wave of rarefaction, both travelling with the same velocity, and
therefore always maintaining the same relative position to each other.
Now, the fork vibrates a great many times in a second, and will
consequently generate as many of these waves, all of them constituted
alike, and having the same length; by length meaning the sum of the
thicknesses of the condensation and the rarefaction. Suppose a fork to
make one hundred vibrations per second: at the end of the second, the
wave generated by the vibration at the beginning of the second would
have travelled, say, eleven hundred feet; and evenly distributed between
the fork and the outer limit, would be ranged the intermediate waves
occupying the whole distance: that is to say, in eleven hundred feet
there would be one hundred sound-waves, each of them evidently being
eleven feet long. If the fork made eleven hundred vibrations per second,
each of these waves would be one foot long; for sound-waves of all
lengths travel in air with the same rapidity. Some late experiments seem
to show that the actual amplitude of motion of the air, when moved by
such a high sound as that from a small whistle, is less than the
millionth of an inch.
PITCH.
The pitch of a sound depends wholly upon the number of vibrations per
second that produce it; and if one of two sounds consists of twice as
many vibrations per second as the other one, they differ in pitch by the
interval called in music an octave, this latter term merely signifying
the number of intervals into which the larger interval is divided for
the ordinary musical scale. The difference between a high and a low
sound is simply in the number of vibrations of the air reaching the ear
in a given time. The smaller intervals into which the octave is divided
stand in mathematical relations to each other when they are properly
produced, and are represented by the following fractions:--
C D E F G A B C
1 9/8 5/4 4/3 3/2 5/3 15/8 2
[Illustration]
These numbers are to be interpreted thus: Suppose that we have a
tuning-fork giving 256 vibrations per second: the sound will be that of
the standard or concert pitch for the C on the added line as shown on
the staff. Now, D when properly tuned will make 9 vibrations while C
makes but 8; but, as C in this case makes 256, D must make 256×9/8=288.
In like manner G is produced by 256×3/2=384, and C above by 256×2=512,
and so on for any of the others. If other sounds are used in the octave
above or below this one, the number of vibrations of any given note may
be found by either doubling or halving the number for the corresponding
note in the given octave. Thus G below will consist of 384/2=192, and G
above of 384×2=768.
During the past century there has been a quite steady rise in the
standard pitch, and this has been brought about in a very curious and
unsuspected way. The tuning-fork has been the instrument to preserve the
pitch, as it is the best available instrument for such a purpose, it
being convenient to use, and does not vary as most other musical
instruments do. But a tuning-fork is brought to its pitch with a file,
which warms it somewhat, so that at the moment when it is in tune with
the standard that is being duplicated it is above its normal
temperature; and when it cools its tone rises. When another is made of
like pitch with this one, the same thing is repeated; and so it has
continued until the standard pitch has risen nearly a tone higher than
it was in Händel's time.
The common A and C tuning-forks to be had in music stores, often vary a
great deal from the accepted concert pitch. Such as the writer has
measured have been generally too high; sometimes being ten or more
vibrations per second beyond the proper number. The tuning-forks made by
M. Köenig of Paris are accurate within the tenth of one vibration, the C
making 256 vibrations in one second.
LIMITS OF AUDIBILITY.
Numerous experiments have been made to determine the limits of audible
sounds; and here it is found that there is a very great difference in
individuals in their ability to perceive sounds. Helmholtz states that
about 23 vibrations per second is the fewest in number that can be heard
as continuous sound; if they are fewer in number than that, the
vibrations are heard as separate distinct noises, as when one knocks
upon a door four or five times a second. If one could knock evenly 23
times per second, he would be making a continuous musical sound of a
very low pitch. But this limit of 23 is not the limit for all: some can
hear a continuous sound with as few as 16 or 18 vibrations per second,
while others are as far above the medium as this is below it. The
limits of sound in musical instruments are about all included in the
range of a 7-octave pianoforte from F to F, say from 42 to 5,460
vibrations per second. But this high number is not anywhere near the
upper limit of audible sounds for man.
Very many of the familiar sounds of insects, such as crickets and
mosquitoes, have a much higher pitch. Helmholtz puts this upper limit at
38,000 vibrations per second, and Despraetz at 36,850. The discrepancy
of results is due solely to the marked difference in individuals as to
acoustic perception.
For the production of high musical tones, Köenig of Paris makes a set of
steel rods. A steel rod of a certain length, diameter, and temper, will
give a musical sound which may be determined. The proper length for
other rods for giving higher tones may be determined by the rule that
the number of vibrations is inversely proportional to the square of the
length of the rod.
The dimensions of these rods when made 2 c. m. in diameter are as
follows:--
Length. Vibrations.
66.2 m. m. 20,000
59.1 " " 25,000
53.8 " " 30,000
50.1 " " 35,000
47.5 " " 40,000
These rods need to be suspended upon loops of silk, and they are struck
with a piece of steel so short as to be wholly beyond the ability of any
ear to hear its ring. Nothing but a short thud is to be heard from it
when it strikes, while from the others comes a distinct ringing sound.
In experimenting with such a set of steel rods I have not found any one
yet who could hear as many as 25,000 per second, my own limit being
about 21,000. But it has been experimentally found that children and
youth have a perceptive power for high sounds considerably above adults.
Dr. Clarence Blake of Boston reports a case in his aural practice, of a
woman whose hearing had been gradually diminishing for some years until
she could not hear at all with one ear, and the ticking of a watch could
only be heard with the other when the watch was held against the ear.
After treatment it was discovered that the sensibility to high sounds
was very great, and that she could hear the steel rod having a tone of
40,000 vibrations.
Last year Mr. F. Galton, F.R.S., exhibited before the Science Conference
an instrument in the shape of a very small whistle, which he had devised
for producing a very high sound. The whistle had a diameter less than
the one twenty-fifth of an inch. The length could be varied by moving a
plug at the end of the whistle. It was easy to make a sound upon such an
instrument that was altogether out of hearing-range of any person. Mr.
Galton tried some very interesting experiments upon animals, by using
these whistles. He went through the Zoölogical Gardens, and produced
such high sounds near the ears of all the animals. Some of them would
prick up their ears, showing that they heard the sound; while others
apparently could not hear it. He declares that among all the animals the
cat was found to hear the sharpest sound. Small dogs can also hear very
shrill notes, while larger ones can not. Cattle were found to hear
higher sounds than horses. The squeak of bats and of mice cannot be
heard by many persons who can hear ordinary sounds as well as any;
sharpness of hearing having nothing to do with the limits of hearing.
EFFECTS OF SOUND UPON OTHER BODIES.
If a vibrating tuning-fork be held close to a delicately suspended body,
the latter will approach the fork, as if impelled by some attractive
force. The experiment can be made by fastening a bit of paper about an
inch square to a straw five or six inches long, and then suspending the
straw to a thread, so that it is balanced horizontally. Bring the
vibrating tuning-fork within a quarter of an inch of the paper. In this
case the motion of approach is due to the fact that the pressure of the
air is less close to a vibrating body than at a distance from it; there
is therefore a slightly greater pressure on the side of the paper away
from the fork than on the side next to it.
If a vibrating tuning-fork be held near to the ear, and turned around,
there may be found four places in one rotation where the sound will be
heard but very faintly, while in every other position it can be heard
plainly enough. The extinction of the sound is due to what is called
interference. Each of the prongs of the fork is giving out a sound-wave
at the same time, but in opposite directions, each wave advancing
outwards in every direction. Where the rarefied part of one wave exactly
balances the condensed part of the other, there of course the sound will
be extinguished; and these lines of interference are found to be
hyperbolas, or, if considered with reference to both entire waves, two
hyperbolic surfaces.
SYMPATHETIC VIBRATIONS.
When it is once understood that a musical sound is caused by the
vibrations more or less frequent which only make the difference we call
pitch, it might at once be inferred, that if we have a body that is
capable of vibrating say a hundred times a second, and it receives a
hundred pulses or pushes a second, it would in this way be made to
vibrate. Suppose, then, that we take two tuning-forks, each capable of
vibrating 256 times a second: if one be struck while the other is left
free, the former one will be giving to the air 256 impulses per second,
which will reach the other fork, each pulse tending to move it a little,
the cumulative result being to make it move perceptibly, that is, to
give out a sound. The principle is just the same as that employed in the
common swing. One push makes the swing to move a little, upon its return
another is given, in like manner a third, and so on until a person may
be swung many feet high. If a glass tumbler be struck, it gives out a
musical sound of a certain pitch, which will set a piano-string sounding
that is tuned to the same pitch, provided that the damper be raised. It
is said that some persons' voices have broken tumblers by singing
powerfully near them the same note which the tumblers could give out,
the vibrations of the tumblers being so great as to overcome cohesion of
the molecules.
There are very many interesting effects due to sympathetic vibrations.
Large trees are sometimes uprooted by wind that comes in gusts timed to
the rate of vibration of the tree. When troops of soldiers are to cross
a bridge, the music ceases, and the ranks are broken, lest the
accumulated strain of timed vibrations should break the structure;
indeed, such accidents have several times occurred. There is not so much
danger to a bridge when it is heavily loaded with men or with cattle, as
when a few men go marching over it. "When the iron bridge at Colebrooke
Dale was building, a fiddler came along, and said to the workmen that he
could fiddle their bridge down. The builders thought this boast a
fiddle-de-dee, and invited the musician to fiddle away to his heart's
content. One note after another was struck upon the strings, until one
was found with which the bridge was in sympathy. When the bridge began
to shake violently, the workmen were alarmed at the unexpected result,
and ordered the fiddler to stop."
Some halls and churches are wretchedly adapted to hear either speaking
or singing in. If wires be stretched across such halls, between the
speaker's stand and the opposite end, they will absorb the passing
sound-waves, and will be made to sympathetically vibrate, thus
preventing in a good degree the interfering echoes. The wire should be
rather fine piano-wire, and it should be stretched so tightly as to give
out a low musical sound when plucked with the fingers. In a large hall
there should be twenty or more such wires.
RESONANCE.
When a tuning-fork is struck, and held out in the air, the vibrations
can be felt for a time by the fingers; but the sound is hardly audible
unless the fork be placed close to the ear. Let the stem of the fork
rest upon the table, a chair, or any solid body of considerable size,
and the sound is so much increased in loudness as to be heard in every
part of a large room. The reason appears to be, that in the first case
the vibrations are so slight that the air is not much affected. Most of
the force of the vibration is absorbed by the hand that holds it; but
when the stem rests upon a hard body of considerable extent, the
vibrations are given up to it, and every part of its surface is giving
off the vibrations to the air. In other words, it is a much larger body
that is now vibrating, and consequently the air is receiving the
amplified sound-waves.
If the stem of the fork had been made to rest upon a bit of rubber, the
sound would not only not have been re-enforced in such a way, but the
fork would very soon have been brought to rest; for India rubber
_absorbs_ sound vibrations, and converts them into heat vibrations, as
is proved by placing such a combination upon the face of a thermo-pile.
If one will but put his hand upon a table or a chair-back in any room
where a piano or an organ is being played, or where voices are singing,
especially in church, he cannot fail to feel the sound; and if he
notices carefully he will perceive that some sounds make such table or
seat to shake much more vigorously than others,--a genuine case of
sympathetic vibrations.
It is for this reason that special materials and shapes are given to
parts of musical instruments, so that they may respond to the various
vibrations of the strings or reeds. For instance, the piano has an
extensive thin board of spruce underneath all the strings, which is
called the sounding-board. This board takes up the vibrations of the
strings; but, unlike the rubber, gives them all out to the air, greatly
re-enforcing their strength, and changing somewhat their quality. But
the air itself may act in like manner. In almost any room or hall not
more than fifteen or twenty feet long, a person can find some tone of
the voice that will seem to meet some response from the room. Some short
tunnels will from certain positions yield very powerful, responsive,
resonant tones. There is certainly one such in Central Park, New York.
It is forty or fifty feet long. To a person standing in the middle of
this, and speaking or making any kind of a noise on a certain pitch, the
resonance is almost deafening. It is easy to understand. When a column
of air enclosed in a tube is made to vibrate by any sound whose
wave-length is twice the length of the tube, we have such column of air
now filled with the condensed part of the wave, and now with the
rarefied part; and as these motions cannot be conducted laterally, but
must move in the direction of the length of the tube, the air has a
very great amplitude of motion, and the sound is very loud. If one end
of the tube be closed, then the length must be but one-fourth of the
wave-length of the sound. Take a tuning-fork of any convenient pitch,
say a C of 512 vibrations per second: hold it while vibrating over a
vertical test-tube about eight inches long. No response will be heard;
but, if a little water be carefully poured into the tube to the depth of
about two inches, the tube will respond loudly, so that it might be
heard over a large hall. In this case the length of the air-column that
was responding, being one-fourth the wave-length, would give twenty-four
inches as the wave-length of that fork.
It is easy in this way to measure approximately the number of vibrations
made by a fork.
Letting _l_ = depth of tube,
_d_ = diameter of tube,
_v_ = velocity of sound reduced for temperature,
_N_ = number of vibrations,
Then _N_ = _v_
------------
(4(_l_+_d_)).
When a vibrating tuning-fork is placed opposite the embouchure of an
organ-pipe of the same pitch, the pipe will resound to it, giving quite
a volume of sound. In 1872 it occurred to me, that the action of an
organ-pipe might be quite like that of a vibrating reed in front of the
embouchure. As the air is driven past it from the bellows, the form of
the escaping air will evidently be like a thin, elastic strip; and,
having considerable velocity, it will carry off by friction a little of
the air in the tube: this will of course rarefy the air in the tube
somewhat, and a wave of condensation will travel down the tube. At the
bottom, being suddenly stopped, its re-action will be partly outwards,
and so will drive the strip of air away from the tube. After this will
follow, for a like reason, the other phase of the wave, the rarefaction,
which will swing the strip of air towards the tube. This theory I
verified by filling the bellows with smoke, and watching the motion of
the escaping air and smoke with a stroboscope. This view is now
advocated by an organ-builder in England, Herman Smith; but whether he
discovered it before or after me, I do not know.
When a membrane vibrates, its motion is generally perceptible to the
eye; and it may have a very great amplitude of motion, as in the case of
the drum; and various instruments have been devised for the study of
vibrations, using membranes like rubber, gold-beater's skin, or even
tissue paper, to receive the vibrations. One of the musical instruments
of a former generation of boys was the comb. A strip of paper was placed
in front of it, and placed at the mouth, and sung through, the paper
responding to the pitch with a loose nasal sound. Köenig fixed a
membrane across a small capsule, one side of which was connected by a
tube to any source of sound, and the other side to a gas-pipe and a
small burner. A sound made in the tube would shake the flame, and a
mirror moving in front of the flame would show a zigzag outline
corresponding to the sound vibrations.
In like manner if a thin rubber be stretched over the end of a tube one
or two inches in diameter and four or five inches long, and a bit of
looking-glass one-fourth of an inch square be made fast to the middle of
the membrane, the motions of the latter can be seen by letting a beam
of sunlight fall upon the mirror so as to be reflected upon a white wall
or screen a few feet away. (Fig. 8.)
[Illustration: FIG. 8.]
When a sound is made in this tube, the spot of light will at once assume
some peculiar form,--either a straight line with some knots of light in
it, or some curve simple or compound, and such as are known as Lissajous
curves. If, while some of these forms are upon the screen, the
instrument be moved sideways, the forms will change to undulating lines
with or without loops, varying with the pitch and intensity, but being
alike for the same pitch and intensity. (Fig. 9)
This instrument I called the opeidoscope.
[Illustration: FIG. 9.]
The vibration of a membrane and that of a solid differ chiefly in the
amplitude of such vibration. The scratch of a pin at one end of a long
log can be heard by an ear applied to the other end of the log; but
every molecule in the log must move slightly; and there are all degrees
of movement between that visible to the eye, which we call mass motion,
and that called molecular simply because we cannot measure the amplitude
of the motion. We may, then, roughly divide all bodies into two classes,
as to their relations to sound,--such as re-enforce it, and such as
distribute it: the first depending upon the form of the body, as related
to a particular sound; the second independent of form, and responding to
all orders of vibrations. Air, wood, and metals belong in this latter
class. The common toy-string telegraph, or _lovers' telegraph_, is an
example of this class. Two tin boxes are connected by a string passing
through the middle of the bottom of each. When the string is stretched,
and a person speaks in one box, what is said can be heard by an ear
applied at the other. If the speaking-tubes be made about four inches in
diameter, and about four inches deep, they are capable of doing much
more service than is generally supposed to be possible. I know of two
lines, one of five hundred feet and the other of a thousand feet in
length, over which one can talk, and be heard with distinctness. In the
line of a thousand feet, the end of the tube is made of sheepskin
tightly stretched, and the line is made of No. 8 cotton thread. The
greater the tension, the better is the sound transmitted. The thread is
supported at intervals by running through a loop on the ends of cords
not less than three feet long, attached to supports. The thread pierces
the membrane, and is attached to a small button which is in contact with
the membrane. Wind and rain affect this line disadvantageously. The
other line of five hundred feet, between a passenger and a freight
depot, has the tube end covered with stretched calfskin. Instead of
thread, a copper-relay wire is employed (any small uninsulated wire will
do as well). This permits a good tension, and is unaffected by the
weather. One may stand in front of it about three feet, and converse
with ease, and in an ordinary tone. The wire is supported in loops of
string, as in the other.
Musicians have in all times employed various instruments for the
production of musical effects. Whistles made of bone were used by
pre-historic men, some of them having finger-holes so that different
tones could be produced. A stag-horn that was blown like a flageolet,
and having three finger-holes, has also been found; while on the old
monuments of Egypt are pictured harps, pipes with seven finger-holes, a
kind of flute, drums, tambourines, cymbals, and trumpets. In later times
these primeval forms have been modified into the various instruments in
use in the modern orchestra. It seems as if no musician had ever been
interested in the question as to why one instrument should give out a
sound so different from another one, even though it was sounding upon
the same pitch. No one can ever mistake the sound of a violin, or a
horn, or a piano, for any other instrument; and no two persons have
voices alike. This difference in tone, which enables us to identify an
instrument by its sound or a friend by his voice, is called quality of
tone, or _timbre_.
About twenty years ago, that great German physicist Helmholtz undertook
the investigation of this subject, and succeeded in unravelling the
whole mystery of the qualities of sound.
He discovered first, that a musical sound is very rarely a simple tone,
but is made up of several tones, sometimes as many as ten or fifteen,
having different degrees of intensity and pitch. The lowest sound, which
is also the strongest, is called the _fundamental_; and it is this tone
we mean when we speak of the pitch of a sound, as the pitch of middle C
upon a piano, or the pitch of the _A_ string on a violin. The higher
sounds that accompany the fundamental are called sometimes harmonics,
sometimes upper partial tones, but generally _overtones_. The character
or quality of a sound depends altogether upon the number and intensity
of these overtones associated with the fundamental. If a sound can be
made upon a pipe and a violin, that consists wholly of the fundamental
with no overtones, the two instruments sound absolutely alike. It is
exceedingly difficult to do this; and such sound when produced is
smooth, but without character, and unpleasing.
Second, Helmholtz discovered that the overtones always stand in the
simplest mathematical relation to the fundamental tone,--in fact, are
simple multiples of that tone, being two, three, four, and so on, times
the number of vibrations of it.
This will be readily understood by considering the position of such
related sounds when they are written upon the staff.
[Illustration]
If we start with C in the bass as indicated in the staff, calling that
the fundamental, then the notes that will represent the above ratios
are those indicated by smaller notes, which are the overtones up to the
ninth. The first overtone, being produced by twice the number of
vibrations, must be the octave; the second, the fifth of the second
octave; the third will be two octaves from the first, and so on: the
number of vibrations of each of these notes being the number of the
fundamental multiplied by its order in the series.
Taking C with 128 vibrations, we have for this series:--
128 × 1 = 128 = C fundamental.
128 × 2 = 256 = C´.
128 × 3 = 384 = G´.
128 × 4 = 512 = C´´.
128 × 5 = 640 = E´´.
128 × 6 = 768 = G´´.
128 × 7 = 896 = B´´[flat].
128 × 8 = 1,024 = C´´´.
128 × 9 = 1,152 = D´´´.
128 × 10 = 1,280 = E´´´.
This series is continued up to the limits of hearing. Now, it appears
that all instruments do not give the complete series: indeed, it is not
possible to obtain them all upon some instruments. Each of them,
however, when present helps in the general effect which we call quality.
Sometimes the overtones are more prominent than the fundamental, as when
a piano-wire is struck with a nail. It has always been noticed that it
does not give out the sound that is wanted when it is struck in this
way. Hence it is the art of an instrument-maker to so construct the
instrument as to develop and re-enforce such tones as are pleasing, and
to suppress the interfering and disagreeable overtones. Piano-makers
learned by trial where was the proper place to strike the stretched wire
in order to develop the most musical sound upon it; but no reason could
be given until it was observed that striking it at a point about
one-seventh or one-ninth its length from either end prevented the
development of the objectionable overtones, the seventh and the ninth.
Hence they can scarcely be heard in a properly constructed instrument.
These overtones are very discordant with the lower sounds.
Organ-pipes have their specific qualities given to them by making them
wide-mouthed, narrow-mouthed, conical, and so on; shapes which
experience has determined give pleasing sounds with different qualities.
The violin is an instrument that seems to puzzle makers more than almost
any other. Some of the old violins made two hundred years ago by the
Amati family at Cremona are worth many times their weight in gold.
Recent makers have tried in vain to equal them; but, when their
ingenuity and skill have failed, they declare that _age_ has much to do
with such instruments, that age mellows the sounding quality of the
violin. But the Cremona violins were just as extraordinary instruments
when they left the hands of the makers as they are now; and the fame of
the Amati family as violin-makers was over all Europe while they were
living.
A good violin when well played gives an exquisite musical effect, and on
account of its range and quality of tones it is the leading orchestral
instrument, always pleasing and satisfying; but in unskilled hands even
the best _Cremona_ will give forth sounds that make one grieve that it
was ever invented. Overtones of all sorts and with all degrees of
prominence may be easily developed upon it: therefore the skilful player
draws the bow at such a place upon the strings as to develop the
overtones he wants, and suppress the ones not wanted. The usual rule is
to draw the bow about an inch below the bridge; but the place for the
bow depends upon where the fingers are that stop the strings, and also
the pressure upon it. It requires an almost incredible amount of
practice to be able to play a violin very well.
In the accompanying table will be found the component parts of tones
upon a few instruments in common use.
TONE COMPOSITION.
The components of the tones are indicated by lines in the column
underneath the figures representing the series. Thus the narrow-stopped
organ-pipe gives a sound composed of a fundamental, and overtones three,
five, seven, and nine times the number of vibrations of it.
TONE COMPOSITION.
--------------------------+---+---+---+---+---+---+---+---+---+---
INSTRUMENTS. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10
--------------------------+---+---+---+---+---+---+---+---+---+---
/ Wide stopped | / | | | | | | | | |
| +---+---+---+---+---+---+---+---+---+---
| Narrow " | / | | / | | / | | / | | / |
| +---+---+---+---+---+---+---+---+---+---
| Narrow cylinder | / | / | / | / | / | / | | | |
ORGAN < +---+---+---+---+---+---+---+---+---+---
PIPES. | Principal }| / | / | / | | | | | | |
| (Wood) }| | | | | | | | | |
| +---+---+---+---+---+---+---+---+---+---
| Conically }| / | | | | / | / | / | | |
\ narrow at top. }| | | | | | | | | |
+---+---+---+---+---+---+---+---+---+---
Flute | / | / | / | / | | | | | |
+---+---+---+---+---+---+---+---+---+---
Violin | / | / | / | / | / | / | / | / | / | /
+---+---+---+---+---+---+---+---+---+---
Piano | / | / | / | / | / | / | / | / | |
+---+---+---+---+---+---+---+---+---+---
Bell | / | / | / | / | / | / | / | | |
+---+---+---+---+---+---+---+---+---+---
Clarionet | / | | / | | / | | / | | / |
+---+---+---+---+---+---+---+---+---+---
Bassoon | / | / | / | / | / | / | / | | |
+---+---+---+---+---+---+---+---+---+---
Oboe | / | / | / | / | / | / | / | | |
--------------------------+---+---+---+---+---+---+---+---+---+---
It must not be inferred that all of the overtones are of equal strength:
they are very far from that; but these differ in different
instruments, and it is this that constitutes the difference between a
good instrument and a poor one of the same name.
In a few of the spaces very light lines are made for the purpose of
indicating that such overtones are quite weak. For instance: the piano
has the sixth, seventh, and eighth thus marked; these tones being
suppressed by the mechanism, as described on a former page.
Only a few of the many forms of organ-pipes are given; but these are
sufficient to show what a physical difference there is between the
musical tones in such pipes.
As for the human voice, it is very rich in overtones; but no two voices
are alike, therefore it would be impossible to tabulate the components
of it in the manner they are tabulated for musical instruments.
In Helmholtz's experiments in the analysis of sounds, use was made of
the principle of resonance of a body of air enclosed in a vessel. In the
experiment with the tuning-fork to determine the wave-length, p. 78, it
is remarked that no response came until the volume of the air in the
tube was reduced to a certain length, which depended upon the vibration
number of the fork. If instead of a test-tube a bottle had been taken,
the result would have been the same. Every kind of a vessel can respond
to some tone of a definite wave-length, and a sphere has been found to
give the best results. These are made with a hole on one side for the
sound-wave to enter, and a projection on the opposite side, through
which a hole about the one-eighth of an inch is made, this to be placed
in the ear. Any sound that is made in front of the large orifice will
not meet any response, unless it be that particular one which the globe
can naturally re-enforce, when it will be plainly heard. Suppose, then,
one has a series of twenty or more of these, graduated to the proper
size for re-enforcing sounds in the ratio of one, two, three, four, and
so on. Take any instrument, say a flute: have one to blow it upon the
proper pitch to respond to the largest sphere, then take each of the
spheres in their order, applying them to the ear while the flute is
being sounded. When the overtones are present they will be heard
plainly and distinct from the fundamental sound. In like manner any or
all other sounds may be studied.
But Helmholtz did not stop after analyzing sounds of so many kinds: he
invented a method of synthesis, by which the sounds of any kind of an
instrument could be imitated. A tuning-fork, when made to vibrate by an
electric current, gives out a tone without harmonics or overtones. So if
a series of forks with vibration periods equal to the numbers of the
series of overtones given on p. 86 be so arranged that any of them may
be made to vibrate at will, it is evident that the resulting compound
tone would be comparable with that from an instrument having such
overtones. Thus, if with a tuning-fork giving a fundamental C, other
forks giving two, three, and four times the number of the fundamental
were associated, each one giving a simple tone, we should have for a
resultant the tone of a flute, as shown on p. 91. If one, three, five,
seven, and nine, were all sounded, the resulting tone would be that of
the clarionet, and so on. This he actually accomplished, and now makers
of physical apparatus advertise just such instruments.
Helmholtz also contrived a set of tuning-forks, which, when bowed, will
give out the vowel sounds like the voice.
It was remarked upon p. 89 that it has generally been considered that
age has a mellowing effect upon the sound of a violin. Once in
possession of the facts concerning sound that have been alluded to on
the preceding pages, it is easy to see how such an opinion should arise,
and also the fallacy of it. It is proved conclusively that the ability
to hear high sounds decreases as one grows older. As the violin gives a
very great number of overtones, even up to the limits of audibility, it
is plain that if such an instrument should not change in its quality of
tone in the least degree, yet to a man who played upon it for a number
of years it would seem to change by subtracting some of the higher
overtones from the sound; that is, it would seem to become mellower.
There is no evidence that such a physical change takes place in the
instrument. It is not here affirmed that no change does take place. It
may be probable; but all the evidence we have is the opinions of
individuals whose hearing we know does change; and this change is
competent to modify the judgment as to the quality of the sound in the
same direction. Before it can be affirmed that such a physical change
does take place in the violin as to make a perceptible difference in the
quality of its tone, it will be needful to determine accurately the
number and intensity of the overtones at intervals during many years,
and then to compare them. This has not yet been done.
FORM OF A COMPOUND SOUND-WAVE IN AIR.
Upon p. 63 is given a picture of the form of a simple sound-wave in air,
which, as described, consists of two parts, a condensation and a
rarefaction. All simple sound-waves have such a form; but when two or
more sound-waves that stand in some simple ratio to each other, as do
the sounds of musical instruments, are formed in air, the resulting wave
is more or less complex in structure; and where there are many
components, as there are where a number of different kinds of
instruments are all sounding at once, it is well-nigh impossible to
figure even approximately the form of such wave-combinations. It is
generally given in treatises upon sound with ordinates representing the
factors with their relative intensities. When the extremities of the
ordinates are connected, there is drawn a curved line with regularly
recurring loops. This cannot give a correct idea of the form of the
wave, because the motion of a particle of air is not up and down like a
floating body upon waving water, but it is forward and back, in the
direction of the motion of the wave.
In Fig. 10 three simple sound-waves are thus represented at 1, 2, and 3,
these having the wave-length 1, 2, and 3. In 4, the three are combined
into one compound wave, and better show the form of a transverse section
of such a sound-wave in the air. The organ-pipe called the principal
gives out such a compound wave as is seen by referring to the table on
p. 91. The second overtone, however, is quite weak in that pipe, which
would so modify the form as to lessen somewhat the density at _b_, and
increase it at _a_.
[Illustration: FIG. 10.]
In like manner the space in the length of the fundamental sound,
whatever it may be, is divided up into a number of minor condensations
and rarefactions, which may strengthen each other, or so interfere as to
change the position of both; as is seen in the figure at _b_, where the
condensation due to wave 2 interferes with the rarefaction of 3.
CORRELATION.
HAVING treated at some length of the three factors involved in
telephony,--namely, electricity, magnetism, and sound,--it remains to
follow up the various steps that have led to the actual transmission of
musical sounds and speech over an ordinary electric circuit.
It is stated upon p. 31, that, when a current of electricity is passed
through a coil of wire that surrounds a rod of soft iron, the latter is
made a temporary magnet: it loses its magnetic property the instant that
the current ceases. If the rod be of considerable size, say a foot or
more in length, and half an inch or more in diameter, and the current be
strong enough to make a powerful magnet of it, whenever the current from
the battery is broken, the bar may be heard to give out a single
_click_. This will happen as often as the current is broken. This is
occasioned by a molecular movement which results in a _change_ of
_length_ of the bar. When it is made a magnet, it elongates about
1/25000 of its length; and, when it loses its magnetism, it _suddenly_
regains its original length; and this change is accompanied with the
sound. This sound was first noticed by Prof. C. G. Page of Salem, Mass.,
in 1837. If some means be devised for breaking such a circuit more than
fifteen or sixteen times a second, we shall have a continuous sound with
a pitch depending upon the number of clicks per second. Such a device
was first invented by the same man, and was accomplished by fixing the
armature of an electro-magnet to a spring which was in the circuit when
the spring was pressing against a metallic knob, at which time the
current made the circuit in the coil of the electro-magnet. The magnet
attracting the armature away from the button broke the circuit, which of
course destroyed the magnetism of the magnet, and allowed the spring to
fly back against the button, to complete the circuit and reproduce the
same series of changes. The rapidity with which the current may be
broken in this way is only limited by the strength of both spring and
current. The greater the tension of the spring with a given current,
the greater number of vibrations will it make.
[Illustration: FIG. 11.]
Suppose such an intermittent current to pass through the coil
surrounding the soft iron rod, 256 times per second; then the rod would
evidently give 256 clicks per second, which would have the pitch of C.
When these clicks are produced in the rod hold in the hand, the sound is
hardly perceptible, being like that of a sounding tuning-fork when held
thus. In order to strengthen it, it is necessary to place it on some
resonant surface. It is customary to mount it upon an oblong box with
one or two holes in its upper surface, inasmuch as such a form is found
to give a louder response than any other, and is the shape usually given
to Æolian harps. The accompanying cut shows the combination of battery
B, the circuit-breaker, and the rod mounted upon the box. The wire W may
evidently be of any length, the magnetized rod and box responding to the
number of vibrations of the spring S, how long soever the circuit may
be.
HELMHOLTZ' ELECTRIC INTERRUPTOR.
In some of Helmholtz' experiments, it was essential to maintain the
vibrations of a tuning-fork for a considerable time. He effected this by
placing a short electro-magnet between the prongs of the fork, and
affixing a platinum point at the end of one prong in such a manner,
that, as the prong descended in its vibration, the platinum point dipped
into a small cup of mercury that completed the circuit. When the prong
receded, it was of course withdrawn from the mercury, and the current
was broken. As it is not possible for a tuning-fork to vibrate in more
than one period, such an arrangement would evidently make and break the
current as many times per second as the fork vibrated. When, therefore,
such an interruptor is inserted in the circuit with the click-rod on its
resonant box, the latter must give out just such a sound as the fork is
giving. With such a device, it is possible to reproduce at almost any
distance in a telegraphic circuit, a sound of a given pitch. It is
therefore a true telephone.
REISS' TELEPHONE.
The ease with which membranes are thrown into vibrations corresponding
in period to that of the sounding body has already been alluded to on p.
80; and several attempts have been made, at different times, to make
membranes available in telephony. The first of these attempts was made
by Philip Reiss of Friedrichsdorf, Germany, in 1861.
His apparatus consisted of a hollow box, with two apertures: one in
front, in which was inserted a short tube for producing the sound in,
and indicated by the arrow in the cut, Fig. 12; the other on the top.
This was covered with the membrane _m_,--a piece of bladder stretched
tight over it. Upon the middle of the membrane, a thin piece of platinum
was glued; and this piece of platinum was connected by a wire to a
screw-cup from which another wire went to a battery.
[Illustration: FIG. 12.]
A platinum finger, S, rested upon the strip of platinum, but was made
fast at one end to the screw-cup that connected with the other wire from
the battery. Now, when a sound is made in the box, the membrane is made
to vibrate powerfully: this makes the platinum strip to strike as often
upon the platinum finger, and as often to bound away from it, thus
making and breaking the current the same number of times per second. If,
then, a person sings into this box while it is in circuit with the
afore-mentioned click-rod and box, the latter will evidently change its
pitch as often as it is changed by the voice. In this apparatus we have
a telephone with which a melody may be reproduced at a distance with
distinctness. But the sounds are not loud, and they have a tin-trumpet
quality. If one reflects upon the possibilities of such a mechanism, and
upon the conditions necessary to produce a sound of any given quality,
as that of the voice or of a musical instrument as described in
preceding pages, he will understand that it can reproduce only pitch. It
might here be inferred that something more than a single pitch is
transmitted if the sound is like that of a tin trumpet as stated: but
the reason of this is that, whenever a current is passing between two
surfaces that can move only slightly on each other, there is always an
irregularity in the conduction, so as to produce a kind of scratching
sound; and it is this, combined with the other, the true pitch, that
gives the character to the sound of this instrument.
Dr. Wright found that a sound of considerable intensity could be
obtained by passing the interrupted current through the primary wire of
a small induction coil, and placing a conductor made of two sheets of
silvered paper placed back to back in the secondary circuit. The
silvered paper becomes rapidly charged and discharged, making a sound
that can be heard over a large hall, and having the same pitch as the
sending instrument.
GRAY'S TELEPHONES.
In 1873 Mr. Elisha Gray of Chicago discovered that if an induction coil
be made to operate by the current from any automatic circuit-breaker,
and one of the wires from the secondary circuit be held in the hand
while the dry finger of the same hand is rubbed upon a sonorous metallic
plate, the other wire being in connection with the plate, a musical
sound would be given out by the plate, appearing to come from the point
of contact of the finger with the plate. He therefore contrived a
musical instrument with a range of two octaves, in which the reeds were
made to vibrate by electro magnets, the current entering any one by
depressing the appropriate key. This circuit is sent through the primary
wire of an induction coil while one of the terminals of the secondary
coil is connected with the thin sheet metal that forms one head of a
shallow wooden drum about eight inches in diameter, which is so fixed as
to be rotated like a pulley. The other terminal is held in the hand
while one finger of the same hand rests upon the metallic surface. While
the drum is turned with the other hand, the sounds given out have
considerable intensity. The faster the drum is turned, the louder do the
sounds become, though the pitch remains the same.
In this case, as in the case mentioned on p. 105, we have an electric
current passing between two surfaces that are moving upon each other;
the contact not being uniform, the current is varying as well as
intermittent.
Mr. Gray has also invented a musical telephone by means of which many
musical sounds may be simultaneously transmitted and reproduced. The
actual mechanism used is quite complex, and requires considerable
familiarity with electrical science in order to understand it; but the
fundamental principle involved is not difficult to one who has
comprehended the preceding descriptions.
Suppose that we have a series of four steel reeds, each one fixed at one
end to one pole of a short electro-magnet, while the other end is left
free to vibrate over the other pole of the magnet and not quite touching
it. Each of the reeds is to be tuned to a different pitch, say the 1, 3,
5, and 8 of the scale. These electro-magnets with their attached
vibrators are to be attached each to a resonant box (see p. 93), which
can respond to that particular number of vibrations per second. This is
the receiving instrument. The sender consists of a like set of reeds
tuned to the same pitch, which can be made to vibrate at will by
pressing a key which sends the current of electricity through its
electro-magnet, which makes and breaks the current. Imagine one of these
keys to be pressed down so as to make the circuit complete: the sending
instrument then has one of its reeds, let it be the 1 of the scale, set
in vibration; the intermittent current traverses the whole line, going
through all four of the receiving instruments. Now, we know from the
study of the action of sounding bodies, that only one of the four
receivers is competent to vibrate in consonance with this tone, and this
one will respond; that is, the vibrations are truly sympathetic
vibrations. If, instead of making the 1 of the scale in the
sending-instrument, the 3 had been made, the current would have gone
through all of the receiving instruments just the same as before, but
only one of them could take up that vibratory movement: three of them
would remain at rest, the 3 responding loudly. In like manner, any
number of vibrating reeds in the sending instrument can make a
corresponding number of reeds in the receiving instrument to vibrate,
provided the latter be exactly tuned with the former. Each transmitter
is connected with but a part of the battery, so that several tones may
be transmitted at the same time. If the performer plays a piece of music
in its various parts, every part will be reproduced: thus we have a
compound or multiple telephone. This instrument has been used during
the past winter to give concerts in cities when the performer was in a
distant place.
It has also been used as a multiple telegraph; as many as eight
operators sending messages simultaneously over the same wire,--four in
each direction,--without the slightest interference.
BELL'S TELEPHONE.
Prof. A. Graham Bell of Boston independently discovered the same means
for producing multiple effects over the same wire; but it appears he did
not practically work it out as completely as did Mr. Gray. But while the
latter was chiefly employed in perfecting the method as a telegraphic
system, Prof. Bell had set before himself the more difficult problem of
transmitting speech. This he has actually accomplished, as we have so
often been reminded during the past year.
Thoroughly conversant with the acoustic researches of Helmholtz, and
keeping in mind the complex form of the air vibrations produced by the
human voice, he attempted to make these vibrations produce corresponding
pulsations in an electric current in the manner analogous to the
electric interrupter.
Observing that membranes when properly stretched can vibrate to any kind
of a sound, he sought to utilize them for this purpose. So did Reiss;
but Reiss inserted the vibrating membrane into the circuit, and it was
quite evident that such a plan would not answer, therefore the current
must not be broken; but could an electric current be interfered with
without breaking the connections?
The well-known re-actions of magnets upon electrical currents, first
noted by Oersted, and fully developed by Faraday, gave the clew to the
solution. A piece of iron should be made to vibrate by means of sound
vibrations, so as to affect an electro-magnet and induce corresponding
electrical pulsations.
FIRST FORM OF SPEAKING-TELEPHONE.
A membrane of gold-beater's skin was tightly stretched over the end of a
speaking-tube or funnel; on the middle of this membrane a piece of iron,
N S, Fig. 13, was glued. In front of this piece of iron an
electro-magnet M is so situated that its poles are opposite to it, but
not quite touching it. One of the terminal wires of the electro-magnet
goes to the battery B; the other goes to the receiving instrument R,
which consists of a tubular electro-magnet, the coil being enclosed in a
short tube of soft iron; the wire thence goes to the plate E´, which is
sunk in the earth. On the top of R, at P, is a rather loose, thin disk
of iron, which acts as an armature to the electro-magnet below it.
[Illustration: FIG. 13.]
Supposing that all the parts are thus properly connected, the current of
electricity from the battery makes both M and R magnetic; the
electro-magnet M will inductively make the piece of iron N S, a magnet,
with its poles unlike those of the inducing electro-magnet; and the two
will mutually attract each other. If now this piece of iron N S be made
to move toward M, a current of electricity will be induced in the coils,
which will traverse the whole circuit. This induced electricity will
consist of a single wave or pulse, and its force will depend upon the
velocity of the approach of N S to M. A like pulse of electricity will
be induced in the coils when N S is made to move away from M; but this
current will move through the circuit in the opposite direction, so that
whether the pulsation goes from M to R, or from R to M, depends simply
upon the direction of the motion of N S.
The electricity thus generated in the wire by such vibratory movements
varies in strength proportional to the movement of the armature;
therefore the line wire between two places will be filled with
electrical pulsation exactly like the aërial pulsations in structure.
Fig. 10, p. 98, may be used to illustrate the condition of the wire
through which the currents pass. The dark part may represent the
strongest part of the wave, while the lighter part would show the
weaker part of the wave. The chief difference would be, that electricity
travels so fast, that what is there represented as one wave in air with
a length of two feet would, in an electric wave, be more than fifty
miles long.
These induced electric currents are but very transient (see p. 31); and
their effect upon the receiver R is to either increase or decrease the
power of the magnet there, as they are in one direction or the other,
and consequently to vary the attractive power exercised upon the iron
plate armature.
Let a simple sound be now made in the tube, consisting of 256 vibrations
per second: the membrane carrying the iron will vibrate as many times,
and so many pulses of induced electricity will be _imposed_ upon the
constant current, which will each act upon the receiver, and cause so
many vibrations of the armature upon it; and an ear held at P will hear
the sound with the same pitch as that at the sending instrument. If two
or more sound-waves act simultaneously upon the membrane, its motions
must correspond with such combined motions; that is, its motions will
be the resultant of all the sound-waves, and the corresponding
pulsations in the current must reproduce at R the same effect. Now, when
a person speaks in the tube, the membrane is thrown into vibrations more
complex in structure than those just mentioned, differing only in number
and intensity. The magnet will cause responses from even the minutest
motion; and therefore an ear at R will hear what is said at the tube.
This was the instrument exhibited at the Centennial Exposition at
Philadelphia, and concerning which Sir William Thompson said on his
return to England, "This is the greatest by far of all the marvels of
the electric telegraph."
The popular impression has been, concerning the telephone, that the
_sound_ was in some way conveyed over the wire. It will be obvious to
every one who may read this, that such is very far from being the case.
The fact is, it is a beautiful example of the convertibility of forces
from one form to another. There is first the initial vibratory
mechanical motion of the air, which is imparted to the membrane carrying
the iron. This motion is converted into electricity in the coil of wire
surrounding the electro-magnet, and at the receiving-end is first
effective as magnetism, which is again converted into vibratory motion
of the iron armature, which motion is imparted to the air, and so
becomes again a sound-wave in air like the original one.
This was the first speaking-telephone that was ever constructed, so far
as the writer is aware, but it was not a practicable instrument. Many
sounds were not reproduced at all, and, according to the report of the
judges at the Philadelphia Exposition, one needed to shout himself
hoarse in order that he might be heard at all.
THE AUTHOR'S TELEPHONE.
For several years past my regularly recurring duties have taken me over
the various subjects treated of in this book, and each one has been
extensively illustrated in an experimental way, and a considerable
number of new pieces of apparatus and new experiments to exhibit their
phenomena have been devised by me.
Among these, I would mention the following:--
1. Measurement of the elongation of a magnetized
bar.
2. A magneto-electric telegraph.
3. An electro-magnetic instrument for
demonstrating the rotation of the earth.
4. The permanent magnetism of the magnetic
phantom.
5. The convertibility of sound into electricity.
6. The induction of a vibrating magnet upon an
electric circuit.
7. The origination of electric waves in a circuit
by a sounding magnet.
8. The discovery of the action of the air in a
sounding organ-pipe.
9. Two or three methods for studying the
vibrations of membranes.
10. Lissajous forks for enlarged projections of
sound vibrations.
As soon, therefore, as I gave attention to the subject of telephony, I
was able, with a few preliminary experiments, to determine the proper
conditions for the transmission of speech in an electric circuit; and,
without the slightest knowledge of the mechanism which Prof. Bell had
used, I devised the following arrangement for a speaking-telephone.
[Illustration: FIG. 14.--MY FIRST SPEAKING TELEPHONE.]
[Illustration: FIG. 14.--END VIEW.]
My first speaking-telephone, Fig. 14, consisted of a magnet made out of
half-inch round steel bent into a U form, having the poles about two
inches apart. Over these were slipped two bobbins taken from an old
telegraph register, and were already fitted to a half-inch core. These
bobbins, two inches and a half long, were wound with cotton-covered
copper wire, No. 23, each bobbin containing about 150 feet. This magnet,
with the bobbins slipped upon its poles, was made fast to a post two or
three inches high. The steel was made as strongly magnetic as was
possible, and would hold up three or four times its own weight. In
front of the poles, a sheet of thin steel, one-fiftieth of an inch
thick, was made fast to an upright board having a hole cut through it
three and a half inches in diameter (Fig. 14, end view); the plate was
screwed tightly to this board, so as to cover the hole; and the middle
of the hole was at the same height as the two poles of the magnet. The
wires from the two bobbins were connected, as if to make an
electro-magnet; while the two free terminals were to be connected with
the line-wires. Of course there were two of these instruments, both
alike; and talking and singing were reproduced with these.
A very great number of experiments have been made to determine the best
conditions for each of the essential parts,--the size and strength of
the magnet, the size of the bobbins, as to length and fineness of wire,
the best thickness for the plate for absorbing the vibrations, &c.; and
it is really surprising, how little is the difference between very wide
limits. The following directions will enable any one to construct a
speaking-telephone with which good results may be obtained. The
specifications will be for only one instrument; though of course two
instruments made alike will be necessary for any purposes of speaking or
other signals.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
Procure three common horse-shoe magnets about six inches long, all of
the same size; these retail in the market at about a dollar apiece. They
should be strong enough to hold up several times their own weight each.
Next, have turned out of good hard wood,--such as maple or boxwood,--two
spools not over half an inch long and an inch and a half broad, the
sides cut square both inside and out, as shown at S, Fig. 15; a hole
the third of an inch in diameter is to be made through the spool. Into
this hole is to be fitted a short rod of soft iron, I, about an inch
long, which should be a little rounded at the outer end. The bobbins may
be wound with as much insulated copper wire as they will hold. The wire
may be from the one-fortieth to the one-fiftieth of an inch in diameter,
as is most convenient to obtain, the latter size being preferable. The
resistance of such bobbins will probably be from two to three ohms each.
The soft-iron core I must project backwards far enough to be clamped
between the two outer magnets 1 and 3, while the inner one, 2, is drawn
back. When the bobbins are in their places, and are clamped between the
upper and lower magnets, they will stand as shown in Fig. 16, where the
view is from above; the magnets being buttoned down to the block they
rest on (see Fig. 17), which at the same time holds the soft-iron rods
with the bobbins upon them. The wires on these coils must be connected
in the same way they would be in order to make opposite poles of their
outer ends, if a current of electricity were to be sent through the
coils. An upright board B (Fig. 17) six or seven inches square, having a
round hole four inches in diameter cut out from the middle of it, must
be fixed near the end of the base-board; and over this hole is to be
screwed _tightly_ a piece of thin sheet iron or steel; it may be from
the one-twentieth to the one-fiftieth of an inch in thickness. It does
not seem to make much difference about the thickness of this plate. I
have generally got the best results from a plate one-fiftieth of an inch
thick. The upright board carrying this plate must be very rigid,
otherwise the plate will be kept tight to the magnets all the time; and
one of the conditions of success in working is, that this plate shall be
as close as possible to the magnet-ends, but not to touch: therefore fix
the board tight, and adjust the magnets by means of the button shown on
top of them in the perspective figure.
[Illustration: FIG. 17.]
The sounds to be transmitted, of whatever sort they may be, are to be
made on the side P, Fig. 16; and likewise, when the instrument is used
as a receiver, the ear is to be applied at the same place. A tube about
two inches in diameter may be made fast to the front of the board, in a
line with the centre of the plate; this will aid somewhat in hearing.
When two or three persons are to sing, it will be best to have each one
supplied with a tube to sing through; one end of the tube to be placed
close to the front of the plate. The sound of musical instruments, such
as the flute and the cornet, will be reproduced much louder, if the
front of such instrument be allowed to rest upon the rim of the hole in
the board, just in front of the plate.
It is noticeable that low talking can be heard more distinctly than when
a great effort is made; but the sounds though distinct are not strong at
any time, and other sounds seriously interfere with hearing. It is
probable that some way will hereafter be devised for increasing the
usefulness of the invention by increasing the volume of sound. On
account of the weakness of the sound it becomes necessary to provide a
call to attract the attention of one in the room. This may be
accomplished by having a small electric bell worked by a one or two cell
battery. Another way which I have found to be quite as efficient is to
have a rod of iron or steel about a foot long, and half an inch in
diameter, bent into a U form. When this is held by the bend, and struck
upon the floor or with a stick, it vibrates powerfully; and if one of
its prongs be permitted to strike against the plate P, Fig. 16, the
sound will be reproduced loud enough to hear over a large room. I have
never failed to call with this when any one was in the same room with
the telephone.
Wherever a telephone circuit has been made upon telegraph poles having
other wires upon them, the inductive actions of the currents upon the
other wires has been found to seriously interfere with the action of the
telephones, inasmuch as the latter reproduce every other message. One
skilled in reading by sound in the ordinary way can read through the
telephone what message is travelling in a neighboring wire. Messages may
be thus read upon wires as far distant as ten feet from the telephone
circuit. It there fore seems to be essential that each telephone circuit
should be isolated from every other one, else there can be no secrecy in
messages.
A very interesting effect was noticed one night when there was a bright
aurora display. There was a continuous current through the wires,
accompanied with sounds which increased in intensity as the bright
streamers passed by. This will probably lead to some important results
in science.
In all probability the telephone is as much in its infancy as was
ordinary telegraphy in 1840. Since that time the sciences of electricity
and magnetism have had the most of their growth, and telegraphy has kept
pace with the advancing knowledge until its commercial importance is
second to no other agency. Very many important principles that are
invaluable in telegraphy to-day were wholly unknown in 1840; but it may
here be noted that in the telephone, as it now is, there is not a single
principle that was not well enough known in 1840. This will be apparent
to one who follows out the phenomena from the sender to the receiver.
First, the sound in air causing a corresponding movement in a solid
body, iron. This iron, acting inductively upon a magnet, originates
magneto-electric currents in a wire helix about it; and these travel to
another helix, and, re-acting upon the magnet in it, have
electro-magnetic effects, and increase and decrease the strength of the
magnet; and this variable magnetism affects the plate of iron in front
of that magnet, and makes it to vibrate in a corresponding manner, and
thus to restore to the air in one place the vibrations absorbed from the
air in another place. To some it may seem strange that a simple thing as
the telephone is, involving nothing but principles familiar enough to
every one interested in physical science, should have waited nearly
forty years to be invented. The reason is probably this: Men of science,
as a rule, do not feel called upon to apply the principles which they
may discover. They are content to be _discovering_, not _inventing_.
Now, the schools of the country ought to make the youth quite familiar
with the general principles of physical science, that the inventive
ones--and there are many such--may apply them intelligently. Mechanism
is all that stands between us and aërial navigation; all that is
necessary to reproduce human speech in writing; and all that is needed
to realize completely the prophetic picture of the "Graphic," of the
orator who shall at the same instant address an audience in every city
in the world.
* * * * *
Transcriber's Notes:
The musical flat symbol is represented in the text by [flat].
Page 17, "propererties" changed to "properties" (there are other
properties)
Page 42, "muturally" changed to "mutually" (bodies would mutually)
Page 106, "outby" changed to "out by" (given out by the)
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