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Title: The Story Of Electricity
Author: John Munro
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THE STORY OF ELECTRICITY
BY JOHN MUNRO
AUTHOR OF ELECTRICITY AND ITS USES, PIONEERS OF ELECTRICITY,
HEROES OF THE TELEGRAPH, ETC., AND JOINT AUTHOR OF MUNRO AND
JAMIESON'S POCKET-BOOK OF ELECTRICAL RULES AND TABLES
PREFACE.
A work on electricity needs little recommendation to stimulate the
interest of the general reader. Electricity in its manifold
applications is so large a factor in the comfort and convenience
of our daily life, so essential to the industrial organization
which embraces every dweller in a civilized land, so important in
the development and extension of civilization itself, that a
knowledge of its principles and the means through which they are
directed to the service of mankind should be a part of the mental
equipment of everyone who pretends to education in its truest
sense. Let anyone stop to consider how he individually would be
affected if all electrical service were suddenly to cease, and he
cannot fail to appreciate the claims of electricity to attentive
study.
The purpose of this little book is to present the essential facts
of electrical science in a popular and interesting way, as befits
the scheme of the series to which it belongs. Electrical phenomena
have been observed since the first man viewed one of the most
spectacular and magnificent of them all in the thunderstorm, but
the services of electricity which we enjoy are the product solely
of scientific achievement in the nineteenth century. It is to
these services that the main part of the following discussion is
devoted. The introductory chapters deal with various sources of
electrical energy, in friction, chemical action, heat and
magnetism. The rest of the book describes the applications of
electricity in electroplating, communication by telegraph,
telephone, and wireless telegraphy, the production of light and
heat, the transmission of power, transportation over rails and in
vehicles, and the multitude of other uses.
July, 1915.
PUBLISHERS' NOTE.
For our edition of this work the terminology has been altered to
conform with American usage, some new matter has been added, and a
few of the cuts have been changed and some new ones introduced, in
order to adapt the book fully to the practical requirements of
American readers.
CONTENTS.
I. THE ELECTRICITY OF FRICTION
II. THE ELECTRICITY OF CHEMISTRY
III. THE ELECTRICITY OF HEAT
IV. THE ELECTRICITY OF MAGNETISM
V. ELECTROLYSIS
VI. THE TELEGRAPH AND TELEPHONE
VII. ELECTRIC LIGHT AND HEAT
VIII. ELECTRIC POWER
IX. MINOR USES OF ELECTRICITY
X. THE WIRELESS TELEGRAPH
XI. ELECTRO-CHEMISTRY AND ELECTRO-METALLURGY
XII. ELECTRIC RAILWAYS
APPENDIX
THE STORY OF ELECTRICITY.
CHAPTER I.
THE ELECTRICITY OF FRICTION.
A schoolboy who rubs a stick of sealing-wax on the sleeve of his
jacket, then holds it over dusty shreds or bits of straw to see
them fly up and cling to the wax, repeats without knowing it the
fundamental experiment of electricity. In rubbing the wax on his
coat he has electrified it, and the dry dust or bits of wool are
attracted to it by reason of a mysterious process which is called
"induction."
Electricity, like fire, was probably discovered by some primeval
savage. According to Humboldt, the Indians of the Orinoco
sometimes amuse themselves by rubbing certain beans to make them
attract wisps of the wild cotton, and the custom is doubtless very
old. Certainly the ancient Greeks knew that a piece of amber had
when rubbed the property of attracting light bodies. Thales of
Miletus, wisest of the Seven Sages, and father of Greek
philosophy, explained this curious effect by the presence of a
"soul" in the amber, whatever he meant by that. Thales flourished
600 years before the Christian era, while Croesus reigned in
Lydia, and Cyrus the Great, in Persia, when the renowned Solon
gave his laws to Athens, and Necos, King of Egypt, made war on
Josiah, King of Judah, and after defeating him at Megiddo,
dedicated the corslet he had worn during the battle to Apollo
Didymaeus in the temple of Branchidas, near Miletus.
Amber, the fossil resin of a pine tree, was found in Sicily, the
shores of the Baltic, and other parts of Europe. It was a precious
stone then as now, and an article of trade with the Phoenicians,
those early merchants of the Mediterranean. The attractive power
might enhance the value of the gem in the eyes of the
superstitious ancients, but they do not seem to have investigated
it, and beyond the speculation of Thales, they have told us
nothing more about it.
Towards the end of the sixteenth century Dr. Gilbert of
Colchester, physician to Queen Elizabeth, made this property the
subject of experiment, and showed that, far from being peculiar to
amber, it was possessed by sulphur, wax, glass, and many other
bodies which he called electrics, from the Greek word elektron,
signifying amber. This great discovery was the starting-point of
the modern science of electricity. That feeble and mysterious
force which had been the wonder of the simple and the amusement of
the vain could not be slighted any longer as a curious freak of
nature, but assuredly none dreamt that a day was dawning in which
it would transform the world.
Otto von Guericke, burgomaster of Magdeburg, was the first to
invent a machine for exciting the electric power in larger
quantities by simply turning a ball of sulphur between the bare
hands. Improved by Sir Isaac Newton and others, who employed glass
rubbed with silk, it created sparks several inches long. The
ordinary frictional machine as now made is illustrated in figure
i, where P is a disc of plate glass mounted on a spindle and
turned by hand. Rubbers of silk R, smeared with an amalgam of
mercury and tin, to increase their efficiency, press the rim of
the plate between them as it revolves, and a brass conductor C,
insulated on glass posts, is fitted with points like the teeth of
a comb, which, as the electrified surface of the plate passes by,
collect the electricity and charge the conductor with positive
electricity. Machines of this sort have been made with plates 7
feet in diameter, and yielding sparks nearly 2 feet long.
The properties of the "electric fire," as it was now called, were
chiefly investigated by Dufay. To refine on the primitive
experiment let us replace the shreds by a pithball hung from a
support by a silk thread, as in figure 2. If we rub the glass rod
vigorously with a silk handkerchief and hold it near, the ball
will fly toward the rod. Similarly we may rub a stick of sealing
wax, a bar of sulphur, indeed, a great variety of substances, and
by this easy test we shall find them electrified. Glass rubbed
with glass will not show any sign of electrification, nor will wax
rubbed on wax; but when the rubber is of a different material to
the thing rubbed, we shall find, on using proper precautions, that
electricity is developed. In fact, the property which was once
thought peculiar to amber is found to belong to all bodies. ANY
SUBSTANCE, WHEN RUBBED WITH A DIFFERENT SUBSTANCE, BECOMES
ELECTRIFIED.
The electricity thus produced is termed frictional electricity. Of
course there are some materials, such as amber, glass, and wax,
which display the effect much better than others, and hence its
original discovery.
In dry frosty weather the friction of a tortoise-shell comb will
electrify the hair and make it cling to the teeth. Sometimes
persons emit sparks in pulling off their flannels or silk
stockings. The fur of a cat, or even of a garment, stroked in the
dark with a warm dry hand will be seen to glow, and perhaps heard
to crackle. During winter a person can electrify himself by
shuffling in his slippers over the carpet, and light the gas with
a spark from his finger. Glass and sealing-wax are, however, the
most convenient means for investigating the electricity of
friction.
A glass rod when rubbed with a silk handkerchief becomes, as we
have seen, highly electric, and will attract a pithball (fig. 2).
Moreover, if we substitute the handkerchief for the rod it will
also attract the ball (fig. 3). Clearly, then, the handkerchief
which rubbed the rod as well as the rod itself is electrified. At
first we might suppose that the handkerchief had merely rubbed off
some of the electricity from the rod, but a little investigation
will soon show that is not the case. If we allow the pithball to
touch the glass rod it will steal some of the electricity on the
rod, and we shall now find the ball REPELLED by the rod, as
illustrated in figure 4. Then, if we withdraw the rod and bring
forward the handkerchief, we shall find the ball ATTRACTED by it.
Evidently, therefore, the electricity of the handkerchief is of a
different kind from that of the rod.
Again, if we allow the ball to touch the handkerchief and rub off
some of its electricity, the ball will be REPELLED by the
handkerchief and ATTRACTED by the rod. Thus we arrive at the
conclusion that whereas the glass rod is charged with one kind of
electricity, the handkerchief which rubbed it is charged with
another kind, and, judging by their contrary effects on the
charged ball or indicator, they are of opposite kinds. To
distinguish the two sorts, one is called POSITIVE and the other
NEGATIVE electricity.
Further experiments with other substances will show that sometimes
the rod is negative while the rubber is positive. Thus, if we rub
the glass rod with cat's fur instead of silk, we shall find the
glass negative and the fur positive. Again, if we rub a stick of
sealing-wax with the silk handkerchief, we shall find the wax
negative and the silk positive. But in every case one is the
opposite of the other, and moreover, an equal quantity of both
sorts of electricity is developed, one kind on the rod and the
other on the rubber. Hence we conclude that EQUAL AND OPPOSITE
QUANTITIES OF ELECTRICITY ARE SIMULTANEOUSLY DEVELOPED BY
FRICTION.
If any two of the following materials be rubbed together, that
higher in the list becomes positively and the other negatively
electrified:--
POSITIVE (+).
Cats' fur.
Polished glass.
Wool.
Cork, at ordinary temperature.
Coarse brown paper.
Cork, heated.
White silk.
Black silk.
Shellac.
Rough glass.
NEGATIVE (-).
The list shows that quality, as well as kind, of material affects
the production of electricity. Thus polished glass when rubbed
with silk is positive, whereas rough glass is negative. Cork at
ordinary temperature is positive when rubbed with hot cork. Black
silk is negative to white silk, and it has been observed that the
best radiator and absorber of light and heat is the most negative.
Black cloth, for instance, is a better radiator than white, hence
in the Arctic regions, where the body is much warmer than the
surrounding air, many wild animals get a white coat in winter, and
in the tropics, where the sunshine is hotter than the body, the
European dons a white suit.
The experiments of figures 1, 2, and 3 have also shown us that
when the pithball is charged with the positive electricity of the
glass rod it is REPELLED by the like charge upon the rod, and
ATTRACTED by the negative or unlike charge on the handkerchief.
Again, when it is charged with the negative electricity of the
handkerchief it is REPELLED by the like charge on the handkerchief
and ATTRACTED by the positive or unlike charge on the rod.
Therefore it is usual to say that LIKE ELECTRICITIES REPEL AND
UNLIKE ELECTRICITIES ATTRACT EACH OTHER.
We have said that all bodies yield electricity under the friction
of dissimilar bodies; but this cannot be proved for every body by
simply holding it in one hand and rubbing it with the excitor, as
may be done in the case of glass. For instance, if we take a brass
rod in the hand and apply the rubber vigorously, it will fail to
attract the pithball, for there is no trace of electricity upon
it. This is because the metal differs from the glass in another
electrical property, and they must therefore be differently
treated. Brass, in fact, is a conductor of electricity and glass
is not. In other words, electricity is conducted or led away by
brass, so that, as soon as it is generated by the friction, it
flows through the hand and body of the experimenter, which are
also conductors, and is lost in the ground. Glass on the other
hand, is an INSULATOR, and the electricity remains on the surface
of it. If, however, we attach a glass handle to the rod and hold
it by that whilst rubbing it, the electricity cannot then escape
to the earth, and the brass rod will attract the pith-ball.
All bodies are conductors of electricity in some degree, but they
vary so enormously in this respect that it has been found
convenient to divide them into two extreme classes--conductors and
insulators. These run into each other through an intermediate
group, which are neither good conductors nor good insulators. The
following are the chief examples of these classes:--
CONDUCTORS.--All the metals, carbon.
INTERMEDIATE (bad conductors and bad insulators).--Water, aqueous
solutions, moist bodies; wood, cotton, hemp, and paper in any but
a dry atmosphere; liquid acids, rarefied gases.
INSULATORS.--Paraffin (solid or liquid), ozokerit, turpentine,
silk, resin, sealing-wax or shellac, india-rubber, gutta-percha,
ebonite, ivory, dry wood, dry glass or porcelain, mica, ice, air
at ordinary pressures.
It is remarkable that the best conductors of electricity, that is
to say, the substances which offer least resistance to its
passage, for instance the metals, are also the best conductors of
heat, and that insulators made red hot become conductors. Air is
an excellent insulator, and hence we are able to perform our
experiments on frictional electricity in it. We can also run bare
telegraph wires through it, by taking care to insulate them with
glass or porcelain from the wooden poles which support them above
the ground. Water, on the other hand, is a partial conductor, and
a great enemy to the storage or conveyance of electricity, from
its habit of soaking into porous metals, or depositing in a film
of dew on the cold surfaces of insulators such as glass,
porcelain, or ebonite. The remedy is to exclude it, or keep the
insulators warm and dry, or coat them with shellac varnish, wax,
or paraffin. Submarine telegraph wires running under the sea are
usually insulated from the surrounding water by india-rubber or
gutta-percha.
The distinction between conductors and non-conductors or
insulators was first observed by Stephen Gray, a pensioner of the
Charter-house. Gray actually transmitted a charge of electricity
along a pack-thread insulated with silk, to a distance of several
hundred yards, and thus took an important step in the direction of
the electric telegraph.
It has since been found that FRICTIONAL ELECTRICITY APPEARS ONLY
ON THE EXTERNAL SURFACE OF CONDUCTORS.
This is well shown by a device of Faraday resembling a small
butterfly net insulated by a glass handle (fig. 5). If the net be
charged it is found that the electrification is only outside, and
if it be suddenly drawn outside in, as shown by the dotted line,
the electrification is still found outside, proving that the
charge has shifted from the inner to the outer surface. In the
same way if a hollow conductor is charged with electricity, none
is discoverable in the interior. Moreover, its distribution on the
exterior is influenced by the shape of the outer surface. On a
sphere or ball it is evenly distributed all round, but it
accumulates on sharp edges or corners, and most of all on points,
from which it is easily discharged.
A neutral body can, as we have seen (fig. 4), be charged by
CONTACT with an electrified body: but it can also be charged by
INDUCTION, or the influence of the electrified body at a distance.
Thus if we electrify a glass rod positively (+) and bring it near
a neutral or unelectrified brass ball, insulated on a glass
support, as in figure 6, we shall find the side of the ball next
the rod no longer neutral but negatively electrified (-), and the
side away from the rod positively electrified (+).
If we take away the rod again the ball will return to its neutral
or non-electric state, showing that the charge was temporarily
induced by the presence of the electrified rod. Again, if, as in
figure 7, we have two insulated balls touching each other, and
bring the rod up, that nearest the rod will become negative and
that farthest from it positive. It appears from these facts that
electricity has the power of disturbing or decomposing the neutral
state of a neighbouring conductor, and attracting the unlike while
it repels the like induced charge. Hence, too, it is that the
electrified amber or sealing-wax is able to attract a light straw
or pithball. The effect supplies a simple way of developing a
large amount of electricity from a small initial charge. For if in
figure 6 the positive side of the ball be connected for a moment
to earth by a conductor, its positive charge will escape, leaving
the negative on the ball, and as there is no longer an equal
positive charge to recombine with it when the exciting rod is
withdrawn, it remains as a negative charge on the ball. Similarly,
if we separate the two balls in figure 7, we gain two equal
charges--one positive, the other negative. These processes have
only to be repeated by a machine in order to develop very strong
charges from a feeble source.
Faraday saw that the intervening air played a part in this action
at a distance, and proved conclusively that the value of the
induction depended on the nature of the medium between the induced
and the inducing charge. He showed, for example, that the
induction through an intervening cake of sulphur is greater than
through an equal thickness of air. This property of the medium is
termed its INDUCTIVE CAPACITY.
The Electrophorus, or carrier of electricity, is a simple device
for developing and conveying a charge on the principle of
induction. It consists, as shown in figure 8, of a metal plate B
having an insulating handle of glass H, and a flat cake of resin
or ebonite R. If the resin is laid on a table and briskly rubbed
with cat's fur it becomes negatively electrified. The brass plate
is then lifted by the handle and laid upon the cake. It touches
the electrified surface at a few points, takes a minute charge
from these by contact. The rest of it, however, is insulated from
the resin by the air. In the main, therefore, the negative charge
of the resin is free to induce an opposite or positive charge on
the lower surface and a negative charge on the upper surface of
the plate. By touching this upper surface with the finger, as
shown in figure 8, the negative charge will escape through the
body to the ground or "earth," as it is technically called, and
the positive charge will remain on the plate. We can withdraw it
by lifting the plate, and prove its existence by drawing a spark
from it with the knuckle. The process can be repeated as long as
the negative charge continues on the resin.
These tiny sparks from the electrophorus, or the bigger discharges
of an electrical machine, can be stored in a simple apparatus
called a Leyden jar, which was discovered by accident. One day
Cuneus, a pupil of Muschenbroeck, professor in the University of
Leyden, was trying to charge some water in a glass bottle by
connecting it with a chain to the sparkling knob of an electrical
machine. Holding the bottle in one hand, he undid the chain with
the other, and received a violent shock which cast the bottle on
the floor. Muschenbroeck, eager to verify the phenomenon, repeated
the experiment, with a still more lively and convincing result.
His. nerves were shaken for two days, and he afterwards protested
that he would not suffer another shock for the whole kingdom of
France.
The Leyden jar is illustrated in figure 9, and consists in general
of a glass bottle partly coated inside and out with tinfoil F, and
having a brass knob K connecting with its internal coat. When the
charged plate or conductor of the electrophorus touches the knob
the inner foil takes a positive charge, which induces a negative
charge in the outer foil through the glass. The corresponding
positive charge induced at the same time escapes through the hand
to the ground or "earth." The inner coating is now positively and
the outer coating negatively electrified, and these two opposite
charges bind or hold each other by mutual attraction. The bottle
will therefore continue charged for a long time; in short, until
it is purposely discharged or the two electricities combine by
leakage over the surface of the glass.
To discharge the jar we need only connect the two foils by a
conductor, and thus allow the separated charges to combine. This
should be done by joining the OUTER to the INNER coat with a stout
wire, or, better still, the discharging tongs T, as shown in the
figure. Otherwise, if the tongs are first applied to the inner
coat, the operator will receive the charge through his arms and
chest in the manner of Cuneus and Muschenbroeck.
Leyden jars can be connected together in "batteries," so as to
give very powerful effects. One method is to join the inner coat
of one to the outer coat of the next. This is known as connecting
in "series," and gives a very long spark. Another method is to
join the inner coat of one to the inner coat of the next, and
similarly all the outer coats together. This is called connecting
"in parallel," or quantity, and gives a big, but not a long spark.
Of late years the principle of induction, which is the secret of
the Leyden jar and electrophorus, has been applied in constructing
"influence" machines for generating electricity. Perhaps the most
effective of these is the Wimshurst, which we illustrate in figure
10, where PP are two circular glass plates which rotate in
opposite directions on turning the handle. On the outer rim of
each is cemented a row of radial slips of metal at equal
intervals. The slips at opposite ends of a diameter are connected
together twice during each revolution of the plates by wire
brushes S, and collecting combs TT serve to charge the positive
and negative conductors CC, which yield very powerful sparks at
the knobs K above. The given theory of this machine may be open to
question, but there can be no doubt of its wonderful performance.
A small one produces a violent spark 8 or 10 inches long after a
few turns of the handle.
The electricity of friction is so unmanageable that it has not
been applied in practice to any great extent. In 1753 Mr. Charles
Morrison, of Greenock, published the first plan of an electric
telegraph in the Scots Magazine, and proposed to charge an
insulated wire at the near end so as to make it attract printed
letters of the alphabet at the far end. Sir Francis Ronalds also
invented a telegraph actuated by this kind of electricity, but
neither of these came into use. Morrison, an obscure genius, was
before his age, and Ronalds was politely informed by the
Government of his day that "telegraphs of any kind were wholly
unnecessary." Little instruments for lighting gas by means of the
spark are, however, made, and the noxious fumes of chemical and
lead works are condensed and laid by the discharge from the
Wimshurst machine. The electricity shed in the air causes the dust
and smoke to adhere by induction and settle in flakes upon the
sides of the flues. Perhaps the old remark that "smuts" or
"blacks" falling to the ground on a sultry day are a sign of
thunder is traceable to a similar action.
The most important practical result of the early experiments with
frictional electricity was Benjamin Franklin's great discovery of
the identity of lightning and the electric spark. One day in June,
1792, he went to the common at Philadelphia and flew a kite
beneath a thundercloud, taking care to insulate his body from the
cord. After a shower had wetted the string and made it a
conductor, he was able to draw sparks from it with a key and to
charge a Leyden jar. The man who had "robbed Jupiter of his
thunderbolts" became celebrated throughout the world, and
lightning rods or conductors for the protection of life and
property were soon brought out. These, in their simplest form, are
tapes or stranded wires of iron or copper attached to the walls of
the building. The lower end of the conductor is soldered to a
copper plate buried in the moist subsoil, or, if the ground is
rather dry, in a pit containing coke. Sometimes it is merely
soldered to the water mains of the house. The upper end rises
above the highest chimney, turret, or spire of the edifice, and
branches into points tipped with incorrosive metal, such as
platinum. It is usual to connect all the outside metal of the
house, such as the gutters and finials to the rod by means of
soldered joints, so as to form one continuous metallic network or
artery for the discharge.
When a thundercloud charged with electricity passes over the
ground, it induces a charge of an opposite kind upon it. The cloud
and earth with air between are analogous to the charged foils of
the Leyden jar separated by the glass. The two electricities of
the jar, we know, attract each other, and if the insulating glass
is too weak to hold them asunder, the spark will pierce it.
Similarly, if the insulating air cannot resist the attraction
between the thundercloud and the earth, it will be ruptured by a
flash of lightning. The metal rod, however, tends to allow the two
charges of the cloud and earth to combine quietly or to shunt the
discharge past the house.
CHAPTER II.
THE ELECTRICITY OF CHEMISTRY.
A more tractable kind of electricity than that of friction was
discovered at the beginning of the present century. The story goes
that some edible frogs were skinned to make a soup for Madame
Galvani, wife of the professor of anatomy in the University of
Bologna, who was in delicate health. As the frogs were lying in
the laboratory of the professor they were observed to twitch each
time a spark was drawn from an electrical machine that stood by. A
similar twitching was also noticed when the limbs were hung by
copper skewers from an iron rail. Galvani thought the spasms were
due to electricity in the animal, and produced them at will by
touching the nerve of a limb with a rod of zinc, and the muscle
with a rod of copper in contact with the zinc. It was proved,
however, by Alessanjra Volta, professor of physics in the
University of Pavia, that the electricity was not in the animal
but generated by the contact of the two dissimilar metals and the
moisture of the flesh. Going a step further, in the year 1800 he
invented a new source of electricity on this principle, which is
known as "Volta's pile." It consists of plates or discs of zinc
and copper separated by a wafer of cloth moistened with acidulated
water. When the zinc and copper are joined externally by a wire, a
CURRENT of electricity is found in the wire One pair of plates
with the liquid between makes a "couple" or element; and two or
more, built one above another in the same order of zinc, copper,
zinc, copper, make the pile. The extreme zinc and copper plates,
when joined by a wire, are found to deliver a current.
This form of the voltaic, or, as it is sometimes called, galvanic
battery, has given place to the "cell" shown in figure II, where
the two plates Z C are immersed in acidulated water within the
vessel, and connected outside by the wire W. The zinc plate has a
positive and the copper a negative charge. The positive current
flows from the zinc to the copper inside the cell and from the
copper to the zinc outside the cell, as shown by the arrows. It
thus makes a complete round, which is called the voltaic
"circuit," and if the circuit is broken anywhere it will not flow
at all. The positive electricity of the zinc appears to traverse
the liquid to the copper, from which it flows through the wire to
the zinc. The effect is that the end of the wire attached to the
copper is positive (+), and called the positive "pole" or
electrode, while the end attached to the zinc is negative (-), and
called the negative pole or electrode. "A simple and easy way to
avoid confusion as to the direction of the current, is to remember
that the POSITIVE current flows FROM the COPPER TO the ZINC at the
point of METALLIC contact." The generation of this current is
accompanied by chemical action in the cell. Experiment shows that
the mere CONTACT of dissimilar materials, such as copper and zinc,
electrifies them--zinc being positive and copper negative; but
contact alone does not yield a continuous current of electricity.
When we plunge the two metals, still in contact, either directly
or through a wire, into water preferably acidulated, a chemical
action is set up, the water is decomposed, and the zinc is
consumed. Water, as is well known, consists of oxygen and
hydrogen. The oxygen combines with the zinc to form oxide of zinc,
and the hydrogen is set free as gas at the surface of the copper
plate. So long as this process goes on, that is to say, as long as
there is zinc and water left, we get an electric current in the
circuit. The existence of such a current may be proved by a very
simple experiment. Place a penny above and a dime below the tip of
the tongue, then bring their edges into contact, and you will feel
an acid taste in the mouth.
Figure 12 illustrates the supposed chemical action in the cell. On
the left hand are the zinc and copper plates (Z C) disconnected in
the liquid. The atoms of zinc are shown by small circles; the
molecules of water, that is, oxygen, and hydrogen (H2O) by
lozenges of unequal size. On the right hand the plates are
connected by a wire outside the cell; the current starts, and the
chemical action begins. An atom of zinc unites with an atom of
oxygen, leaving two atoms of hydrogen thus set free to combine
with another atom of oxygen, which in turn frees two atoms of
hydrogen. This interchange of atoms goes on until the two atoms of
hydrogen which are freed last abide on the surface of the copper.
The "contact electricity" of the zinc and copper probably begins
the process, and the chemical action keeps it up. Oxygen, being an
"electro-negative" element in chemistry, is attracted to the zinc,
and hydrogen, being "electro-positive," is attracted to the
copper.
The difference of electrical condition or "potential" between the
plates by which the current is started has been called the
electromotive force, or force which puts the electricity in
motion. The obstruction or hindrance which the electricity
overcomes in passing through its conductor is known as the
RESISTANCE. Obviously the higher the electromotive force and the
lower the resistance, the stronger will be the current in the
conductor. Hence it is desirable to have a cell which will give a
high electromotive force and a low internal resistance.
Voltaic cells are grouped together in the mode of Leyden jars.
Figure 13 shows how they are joined "in series," the zinc or
negative pole of one being connected by wire to the copper or
positive pole of the next. This arrangement multiplies alike the
electromotive force and the resistance. The electromotive force of
the battery is the sum of the electromotive forces of all the
cells, and the resistance of the battery is the sum of the
resistances of all the cells. High electromotive forces or
"pressures" capable of overcoming high resistances outside the
battery can be obtained in this way.
Figure 14 shows how the zincs are joined "in parallel," the zinc
or negative pole of one being connected by wire to the zinc or
negative pole of the rest, and all the copper or positive poles
together. This arrangement does not increase the electromotive
force, but diminishes the resistance. In fact, the battery is
equivalent to a single cell having plates equal in area to the
total area of all the plates. Although unable to overcome a high
resistance, it can produce a large volume or quantity of
electricity.
Numerous voltaic combinations and varieties of cell have been
found out. In general, where-ever two metals in contact are placed
in a liquid which acts with more chemical energy on one than on
the other, as sulphuric acid does on zinc in preference to copper,
there is a development of electricity. Readers may have seen how
an iron fence post corrodes at its junction with the lead that
fixes it in the stone. This decay is owing to the wet forming a
voltaic couple with the two dissimilar metals and rusting the
iron. In the following list of materials, when any two in contact
are plunged in dilute acid, that which is higher in the order
becomes the positive plate or negative pole to that which is
lower:--
POSITIVE Iron Silver
Zinc Nickel Gold
Cadmium Bismuth Platinum
Tin Antimony Graphite
Lead Copper NEGATIVE
There being no chemical union between the hydrogen and copper in
the zinc and copper couple, that gas accumulates on the surface of
the copper plate, or is liberated in bubbles. Now, hydrogen is
positive compared with copper, hence they tend to oppose each
other in the combination. The hydrogen diminishes the value of the
copper, the current grows weaker, and the cell is said to
"polarise." It follows that a simple water cell is not a good
arrangement for the supply of a steady current.
The Daniell cell is one of the best, and gives a very constant
current. In this battery the copper plate is surrounded by a
solution of sulphate of copper (Cu SO4), which the hydrogen
decomposes, forming sulphuric acid (H2SO4), thus taking itself out
of the way, and leaving pure copper (Cu) to be deposited as a
fresh surface on the copper plate. A further improvement is made
in the cell by surrounding the zinc plate with a solution of
sulphate of zinc (Zn SO4), which is a good conductor. Now, when
the oxide of zinc is formed by the oxygen uniting with the zinc,
the free sulphuric acid combines with it, forming more sulphate of
zinc, and maintaining the CONDUCTIVITY of the cell. It is only
necessary to keep up the supply of zinc, water, and sulphate of
copper to procure a steady current of electricity.
The Daniell cell is constructed in various ways. In the earlier
models the two plates with their solutions were separated by a
porous jar or partition, which allowed the solutions to meet
without mixing, and the current to pass. Sawdust moistened with
the solutions is sometimes used for this porous separator, for
instance, on board ships for laying submarine cables, where the
rolling of the waves would blend the liquids.
In the "gravity" Daniell the solutions are kept apart by their
specific gravities, yet mingle by slow diffusion. Figure 15
illustrates this common type of cell, where Z is the zinc plate in
a solution of sulphate of zinc, and C is the copper plate in a
solution of sulphate of copper, fed by crystals of the "blue
vitriol." The wires to connect the plates are shown at WW. It
should be noticed that the zinc is cast like a wheel to expose a
larger surface to oxidation, and to reduce the resistance of the
cell, thus increasing the yield of current. The extent of surface
is not so important in the case of the copper plate, which is not
acted on, and in this case is merely a spiral of wire, helping to
keep the solutions apart and the crystals down. The Daniell cell
is much employed in telegraphy. The Bunsen cell consists of a zinc
plate in sulphuric acid, and at carbon plate in nitric acid, with
a porous separator between the liquids. During the action of the
cell, hydrogen, which is liberated at the carbon plate, is removed
by combining with the nitric acid. The Grove cell is a
modification of the Bunsen, with platinum instead of carbon. The
Smee cell is a zinc plate side by side with a "platinised" silver
plate in dilute sulphuric acid. The silver is coated with rough
platinum to increase the surface and help to dislodge the hydrogen
as bubbles and keep it from polarising the cell. The Bunsen,
Grove, and Smee batteries are, however, more used in the
laboratory than elsewhere.
The Leclanche is a fairly constant cell, which requires little
attention. It "polarises" in action but soon regains its normal
strength when allowed to rest, and hence it is useful for working
electric bells and telephones. As shown in figure 16, it consists
of a zinc rod with its connecting wire Z, and a carbon plate C
with its binding screw, between two cakes M M of a mixture of
black oxide of manganese, sulphur, and carbon, plunged in a
solution of sal-ammoniac. The oxide of manganese relieves the
carbon plate of its hydrogen. The strength of the solution is
maintained by spare crystals of sal-ammoniac lying on the bottom
of the cell, which is closed to prevent evaporation, but has a
venthole for the escape of gas.
The Bichromate of Potash cell polarises more than the Leclanche,
but yields a more powerful current for a short time. It consists,
as shown in figure 17, of a zinc plate Z between two carbon plates
C C immersed in a solution of bichromate of potash, sulphuric acid
(vitriol), and water. The zinc is always lifted out of the
solution when the cell is not in use. The gas which collects in
the carbons, and weakens the cell, can be set free by raising the
plates out of the liquid when the cell is not wanted. Stirring the
solution has a similar effect, and sometimes the constancy of the
cell is maintained by a circulation of the liquid. In Fuller's
bichromate cell the zinc is amalgamated with mercury, which is
kept in a pool beside it by means of a porous pot.
De la Rue's chloride of silver cell (fig. 18) is, from its
constancy and small size, well adapted for medical and testing
purposes. The "plates" are a little rod or pencil of zinc Z, and a
strip or wire of silver S, coated with chloride of silver and
sheathed in parchment paper. They are plunged in a solution of
ammonium chloride A, contained in a glass phial or beaker, which
is closed to suppress evaporation. A tray form of the cell is also
made by laying a sheet of silver foil on the bottom of the shallow
jar, and strewing it with dry chloride of silver, on which is laid
a jelly to support the zinc plate. The jelly is prepared by mixing
a solution of chloride of ammonium with "agar-agar," or Ceylon
moss. This type permits the use of larger plates, and adapts the
battery for lighting small electric lamps. Skrivanoff has modified
the De la Rue cell by substituting a solution of caustic potash
for the ammonium chloride, and his battery has been used for
"star" lights, that is to say, the tiny electric lamps of the
ballet. The Schanschieff battery, consisting of zinc and carbon
plates in a solution of basic sulphate of mercury, is suitable for
reading, mining, and other portable lamps.
The Latimer Clark "standard" cell is used by electricians in
testing, as a constant electromotive force. It consists of a pure
zinc plate separated from a pool of mercury by a paste of
mercurous proto-sulphate and saturated solution of sulphate of
zinc. Platinum wires connect with the zinc and mercury and form
the poles of the battery, and the mouth of the glass cell is
plugged with solid paraffin. As it is apt to polarise, the cell
must not be employed to yield a current, and otherwise much care
should be taken of it.
Dry cells are more cleanly and portable than wet, they require
little or no attention, and are well suited for household or
medical purposes. The zinc plate forms the vessel containing the
carbon plate and chemical reagents. Figure 19 represents a section
of the "E. C. C." variety, where Z is the zinc standing on an
insulating sole I, and fitted with a connecting wire or terminal T
(-), which is the negative pole. The carbon C is embedded in black
paste M, chiefly composed of manganese dioxide, and has a binding
screw or terminal T (+), which is the positive pole. The black
paste is surrounded by a white paste Z, consisting mainly of lime
and sal-ammoniac. There is a layer of silicate cotton S C above
the paste, and the mouth is sealed with black pitch P, through
which a waste-tube W T allows the gas to escape.
The Hellesen dry cell is like the "E. C. C.," but contains a
hollow carbon, and is packed with sawdust in a millboard case. The
Leclanche-Barbier dry cell is a modification of the Leclanche wet
cell, having a paste of sal-ammoniac instead of a solution.
All the foregoing cells are called "primary," because they are
generators of electricity. There are, however, batteries known as
"secondary," which store the current as the Leyden jar stores up
the discharge from an electrical machine.
In the action of a primary cell, as we have seen, water is split
into its constituent gases, oxygen and hydrogen. Moreover, it was
discovered by Carlisle and Nicholson in the year 1800 that the
current of a battery could decompose water in the outer part of
the circuit. Their experiment is usually performed by the.
apparatus shown in figure 20, which is termed a voltameter, and
consists of a glass vessel V, containing water acidulated with a
little sulphuric acid to render it a better conductor, and two
glass test-tubes OH inverted over two platinum strips or
electrodes, which rise up from the bottom of the vessel and are
connected underneath it to wires from the positive and negative
poles of the battery C Z. It will be understood that the current
enters the water by the positive electrode, and leaves it by the
negative electrode.
When the power of the battery is sufficient the water in the
vessel is decomposed, and oxygen being the negative element,
collects at the positive foil or electrode, which is covered by
the tube O. The hydrogen, on the other hand, being positive,
collects at the negative foil under the tube H. These facts can be
proved by dipping a red-hot wick or taper into the gas of the tube
O and seeing it blaze in presence of the oxygen which feeds the
combustion, then dipping the lighted taper into the gas of the
tube H and watching it burn with the blue flame of hydrogen. The
volume of gas at the CATHODE or negative electrode is always twice
that at the ANODE or positive electrode, as it should be according
to the known composition of water.
Now, if we disconnect the battery and join the two platinum
electrodes of the voltameter by a wire, we shall find a current
flowing out of the voltameter as though it were a battery, but in
the reverse direction to the original current which decomposed the
water. This "secondary" or reacting current is evidently due to
the polarisation of the foils--that is to say, the electro-
positive and electro-negative gases collected on them.
Professor Groves constructed a gas battery on this principle, the
plates being of platinum and the two gases surrounding them oxygen
and hydrogen, but the most useful development of it is the
accumulator or storage battery.
The first practicable secondary battery of Gaston Plante was made
of sheet lead plates or electrodes, kept apart by linen cloth
soaked in dilute sulphuric acid, after the manner of Volta's pile.
It was "charged" by connecting the plates to a primary battery,
and peroxide of lead (PbO2) was formed on one plate and spongy
lead (Pb) on the other. When the charging current was cut off the
peroxide plate became the positive and the spongy plate the
negative pole of the secondary cell.
Faure improved the Plante cell by adding a paste of red lead or
minium (Pb204) and dilute sulphuric acid (H2SO4), by which a large
quantity of peroxide and spongy lead could be formed on the
plates. Sellon and Volckmar increased its efficiency by putting
the paste into holes cast in the lead. The "E. P. S." accumulator
of the Electrical Power Storage Company is illustrated in figure
21, and consists of a glass or teak box containing two sets of
leaden grids perforated with holes, which are primed with the
paste and steeped in dilute sulphuric acid. Alternate grids are
joined to the poles of a charging battery or generator, those
connected to the positive pole being converted into peroxide of
lead and the others into spongy lead. The terminal of the peroxide
plates, being the positive pole of the accumulator, is painted
red, and that of the spongy plates or negative pole black.
Accumulators of this kind are highly useful as reservoirs of
electricity for maintaining the electric light, or working
electric motors in tramcars, boats, and other carriages.
CHAPTER III.
THE ELECTRICITY OF HEAT.
In the year 1821 Professor Seebeck, of Berlin, discovered a third
source of electricity. Volta had found that two dissimilar metals
in contact will produce a current by chemical action, and Seebeck
showed that heat might take the place of chemical action. Thus, if
a bar of antimony A (fig. 22) and a bar of bismuth S are in
contact at one end, and the junction is heated by a spirit lamp to
a higher temperature than the rest of the bars, a difference in
their electric state or potential will be set up, and if the other
ends are joined by a wire W, a current will flow through the wire.
The direction of the current, indicated by the arrow, is from the
bismuth to the antimony across the joint, and from the antimony to
the bismuth through the external wire. This combination, which is
called a "thermo-electric couple," is clearly analogous to the
voltaic couple, with heat in place of chemical affinity. The
direction of the current within and without the couple shows that
the bismuth is positive to the antimony. This property of
generating a current of electricity by contact under the influence
of heat is not confined to bismuth and antimony, or even to the
metals, but is common to all dissimilar substances in their
degree. In the following list of bodies each is positive to those
beneath it, negative to those above it, and the further apart any
two are in the scale the greater the effect. Thus bismuth and
antimony give a much stronger current with the same heating than
copper and iron. Bismuth and selenium produce the best result, but
selenium is expensive and not easy to manipulate. Copper and
German silver will make a cheap experimental couple:--
POSITIVE
Bismuth
Cobalt
Potassium
Nickel
Sodium
Lead
Tin
Copper
Platinum
Silver
Zinc
Cadmium
Arsenic
Iron
Red phosphorus
Antimony
Tellurium
Selenium
NEGATIVE
Other things being equal, the hotter the joint in comparison with
the free ends of the bars the stronger the current of electricity.
Within certain limits the current is, in fact, proportional to
this difference of temperature. It always flows in the same
direction if the joint is not overheated, or, in other words,
raised above a certain temperature.
The electromotive force and current of a thermo-electric couple is
very much smaller than that given by an ordinary voltaic cell. We
can, however, multiply the effect by connecting a number of pairs
together, and so forming a pile or battery. Thus figure 23 shows
three couples joined "in series," the positive pole of one being
connected to the negative pole of the next. Now, if all the
junctions on the left are hot and those on the right are cool, we
will get the united effect of the whole, and the total current
will flow through the wire W, joining the extreme bars or positive
and negative poles of the battery. It must be borne in mind that
although the bismuth and antimony of this thermo-electric battery,
like the zinc and copper of the voltaic or chemico-electric
battery, are respectively positive and negative to each other, the
poles or wires attached to these metals are, on the contrary,
negative and positive. This peculiarity arises from the current
starting between the bismuth and antimony at the heated junction.
The internal resistance of a "thermo-electric pile" is, of course,
very slight, the metals being good conductors, and this fact gives
it a certain advantage over the voltaic battery. Moreover, it is
cleaner and less troublesome than the chemical battery, for it is
only necessary to keep at the required difference of temperature
between the hot and cold junctions in order to get a steady
current. No solutions or salts are required, and there appears to
be little or no waste of the metals. It is important, however, to
avoid sudden heating and cooling of the joints, as this tends to
destroy them.
Clammond, Gulcher, and others have constructed useful thermo-piles
for practical purposes. Figure 24 illustrates a Clammond thermo-
pile of 75 couples or elements. The metals forming these pairs are
an alloy of bismuth and antimony for one and iron for the other.
Prisms of the alloy are cast on strips of iron to form the
junctions. They are bent in rings, the junctions in a series
making a zig-zag round the circle. The rings are built one over the
other in a cylinder of couples, and the inner junctions are heated
by a Bunsen gas-burner in the hollow core of the battery. A gas-
pipe seen in front leads to the burner, and the wires WW connected
to the extreme bars or poles are the electrodes of the pile.
Thermo-piles are interesting from a scientific point of view as a
direct means of transforming heat into electricity. A sensitive
pile is also a delicate detector of heat by virtue of the current
set up, which can be measured with a galvanometer or current
meter. Piles of antimony and bismuth are made which can indicate
the heat of a lighted match at a distance of several yards, and
even the radiation from certain of the stars.
Thermo-batteries have been used in France for working telegraphs,
and they are capable of supplying small installations of the
electric light or electric motors for domestic purposes.
The action of the thermo-pile, like that of a voltaic cell, can be
reversed. By sending a current through the couple from the
antimony to the bismuth we shall find the junction cooled. This
"Peltier effect," as it is termed, after its discoverer, has been
known to freeze water, but no practical application has been made
of it.
A very feeble thermo-electric effect can be produced by heating
the junction of two different pieces of the same substance, or
even by making one part of the same conductor hotter than another.
Thus a sensitive galvanometer will show a weak current if a copper
wire connected in circuit with it be warmed at one point.
Moreover, it has been found by Lord Kelvin that if an iron wire is
heated at any point, and an electric current be passed through it,
the hot point will shift along the wire in a direction contrary to
that of the current.
CHAPTER IV.
THE ELECTRICITY OF MAGNETISM.
We have already seen how electricity was first produced by the
simple method of rubbing one body on another, then by the less
obvious means of chemical union, and next by the finer agency of
heat. In all these, it will be observed, a substantial contact is
necessary. We have now to consider a still more subtle process of
generation, not requiring actual contact, which, as might be
expected, was discovered later, that, mainly through the medium of
magnetism.
The curious mineral which has the property of attracting iron was
known to the Chinese several thousand years ago, and certainly to
the Greeks in the times of Thales, who, as in the case of the
rubbed amber, ascribed the property to its possession of a soul.
Lodestone, a magnetic oxide of iron (FE3O4), is found in various
parts of China, especially at T'szchou in Southern Chihli, which
was formerly known as the "City of the Magnet." It was called by
the Chinese the love-stone or thsu-chy, and the stone that
snatches iron or ny-thy-chy, and perchance its property of
pointing out the north and south direction was discovered by
dropping a light piece of the stone, if not a sewing needle made
of it, on the surface of still water. At all events, we read in
Pere Du Halde's Description de la Chine, that sometime in or about
the year 2635 B.C. the great Emperor Hoang-ti, having lost his way
in a fog whilst pursuing the rebellious Prince Tchiyeou on the
plains of Tchou-lou, constructed a chariot which showed the
cardinal points, thus enabling him to overtake and put the prince
to death.
A magnetic car preceded the Emperors of China in ceremonies of
state during the fourth century of our era. It contained a genius
in a feather dress who pointed to the south, and was doubtless
moved by a magnet floating in water or turning on a pivot. This
rude appliance was afterwards refined into the needle compass for
guiding mariners on the sea, and assisting the professors of feng-
shui or geomancy in their magic rites.
Magnetite was also found at Heraclea in Lydia, and at Magnesium on
the Meander or Magnesium at Sipylos, all in Asia Minor. It was
called the "Heraclean Stone" by the people, but came at length to
bear the name of "Magnet" after the city of Magnesia or the
mythical shepherd Magnes, who was said to have discovered it by
the attraction of his iron crook.
The ancients knew that it had the power of communicating its
attractive property to iron, for we read in Plato's "Ion" that a
number of iron rings can be supported in a chain by the Heraclean
Stone. Lucretius also describes an experiment in which iron
filings are made to rise up and "rave" in a brass basin by a
magnet held underneath. We are told by other writers that images
of the gods and goddesses were suspended in the air by lodestone
in the ceilings of the temples of Diana of Ephesus, of Serapis at
Alexandria, and others. It is surprising, however, that neither
the Greeks nor Romans, with all their philosophy, would seem to
have discovered its directive property.
During the dark ages pieces of Lodestone mounted as magnets were
employed in the "black arts." A small natural magnet of this kind
is shown in figure 25, where L is the stone shod with two iron
"pole-pieces," which are joined by a "keeper" A or separable
bridge of iron carrying a hook for supporting weights.
Apparently it was not until the twelfth century that the compass
found its way into Europe from the East. In the Landnammabok of
Ari Frode, the Norse historian, we read that Flocke Vildergersen,
a renowned viking, sailed from Norway to discover Iceland in the
year 868, and took with him two ravens as guides, for in those
days the "seamen had no lodestone (that is, no lidar stein, or
leading stone) in the northern countries." The Bible, a poem of
Guiot de Provins, minstrel at the court of Barbarossa, which was
written in or about the year 890, contains the first mention of
the magnet in the West. Guiot relates how mariners have an "art
which cannot deceive" of finding the position of the polestar,
that does not move. After touching a needle with the magnet, "an
ugly brown stone which draws iron to itself," he says they put the
needle on a straw and float it on water so that its point turns to
the hidden star, and enables them to keep their course. Arab
traders had probably borrowed the floating needle from the
Chinese, for Bailak Kibdjaki, author of the Merchant's Treasure,
written in the thirteenth century, speaks of its use in the Syrian
sea. The first Crusaders were probably instrumental in bringing it
to France, at all events Jacobus de Vitry (1204-15) and Vincent de
Beauvais (1250) mention its use, De Beauvais calling the poles of
the needle by the Arab words aphron and zohran.
Ere long the needle was mounted on a pivot and provided with a
moving card showing the principal directions. The variation of the
needle from the true north and south was certainly known in China
during the twelfth, and in Europe during the thirteenth century.
Columbus also found that the variation changed its value as he
sailed towards America on his memorable voyage of 1492. Moreover,
in 1576, Norman, a compass maker in London, showed that the north-
seeking end of the needle dipped below the horizontal.
In these early days it was supposed that lodestone in the pole-
star, that is to say, the "lodestar" of the poets or in mountains
of the far north, attracted the trembling needle; but in the year
1600, Dr. Gilbert, the founder of electric science, demonstrated
beyond a doubt that the whole earth was a great magnet. A magnet,
as is well known, has, like an electric battery, always two poles
or centres of attraction, which are situated near its extremities.
Sometimes, indeed, when the magnet is imperfect, there are
"consequent poles" of weaker force between them. One of the poles
is called the "north," and the other the "south," because if the
magnet were freely pivotted like a compass needle, the former
would turn to the north and the latter to the south.
Either pole will attract iron, but soft or annealed iron does not
retain the magnetism nearly so well as steel. Hence a boy's test
for the steel of his knife is only efficacious when the blade
itself becomes magnetic after being touched with the magnet. A
piece of steel is readily magnetised by stroking it from end to
end in one direction with the pole of a magnet, and in this way
compass needles and powerful bar magnets can be made.
The poles attract iron at a distance by "induction," just as a
charge of electricity, be it positive or negative, will attract a
neutral pith ball; and Dr. Gilbert showed that a north pole always
repels another north pole and attracts a south pole, while, on the
other hand, a south pole always repels a south pole and attracts a
north pole. This can be proved by suspending a magnetic needle
like a pithball, and approaching another towards it, as
illustrated in figure 26, where the north pole N attracts the
south S. Obviously there are two opposite kinds of magnetic poles,
as of electricity, which always appear together, and like magnetic
poles repel, unlike magnetic poles attract each other.
It follows that the magnetic pole of the compass needle which
turns to the north must be unlike the north and like the south
magnetic pole of the earth. Instead of calling it the "north," it
would be less confusing to call it the "north-seeking" pole of the
needle.
Gilbert made a "terella," or miniature of the earth, as a magnet,
and not only demonstrated how the compass needle sets along the
lines joining the north and south magnetic poles, but explained
the variation and the dip. He imagined that the magnetic poles
coincided with the geographical poles, but, as a matter of fact,
they do not, and, moreover, they are slowly moving round the
geographical poles, hence the declination of the needle, that is
to say its angle of divergence from the true meridian or north and
south line, is gradually changing. The north magnetic pole of the
earth was actually discovered by Sir John Ross north of British
America, on the coast of Boothia (latitude 70 degrees 5' N,
longitude 96 degrees 46' W), where, as foreseen, the needle
entirely lost its directive property and stood upright, or, so to
speak, on its head. The south magnetic pole lies in the Prince
Albert range of Victona Land, and was almost reached by Sir James
Clark Ross.
The magnetism of the earth is such as might be produced by a
powerful magnet inside, but its origin is unknown, although there
is reason to believe that masses of lodestone or magnetic iron
exist in the crust. Coulomb found that not only iron, but all
substances are more or less magnetic, and Faraday showed in 1845
that while some are attracted by a magnet others are repelled. The
former he called paramagnetic and the latter diamagnetic bodies.
The following is a list of these.--
Paramagnetic Diamagnetic
Iron Bismuth
Nickel Phosphorus
Cobalt Antimony
Aluminium Zinc
Manganese Mercury
Chromium Lead
Cerium Silver
Titanium Copper
Platinum Water
Many ores and Alcohol
salts of the Tellurium
above metals Selenium
Oxygen Sulphur
Thallium
Hydrogen
Air
We have theories of magnetism that reduce it to a phenomenon of
electricity, though we are ignorant of the real nature of both. If
we take a thin bar magnet and break it in two, we find that we
have now two shorter magnets, each with its "north" and "south"
poles, that is to say, poles of the same kind as the south and
north--magnetic poles of the earth. If we break each of these
again, we get four smaller magnets, and we can repeat the process
as often as we like. It is supposed, therefore, that every atom of
the bar is a little magnet in itself having its two opposite
poles, and that in magnetising the bar we have merely partially
turned all these atoms in one direction, that is to say, with
their north poles pointing one way and their south poles the other
way, as shown in figure 27. The polarity of the bar only shows
itself at the ends, where the molecular poles are, so to speak,
free.
There are many experiments which support this view. For example,
if we heat a magnet red hot it loses its magnetism, perhaps
because the heat has disarranged the particles and set the
molecular poles in all directions. Again, if we magnetise a piece
of soft iron we can destroy its magnetism by striking it so as to
agitate its atoms and throw them out of line. In steel, which is
iron with a small admixture of carbon, the atoms are not so free
as in soft iron, and hence, while iron easily loses its magnetism,
steel retains it, even under a shock, but not under a cherry red-
heat. Nevertheless, if we put the atoms of soft iron under a
strain by bending it, we shall find it retain its magnetism more
like a bit of steel.
It has been found, too, that the atoms show an indisposition to be
moved by the magnetising force which is known as HYSTERESIS. They
have a certain inertia, which can be overcome by a slight shock,
as though they had a difficulty of turning in the ranks to take up
their new positions. Even if this molecular theory is true,
however, it does not help us to explain why a molecule of matter
is a tiny magnet. We have only pushed the mystery back to the
atom. Something more is wanted, and electricians look for it in
the constitution of the atom, and in the luminiferous ether which
is believed to surround the atoms of matter, and to propagate not
merely the waves of light, but induction from one electrified body
to another.
We know in proof of this ethereal action that the space around a
magnet is magnetic. Thus, if we lay a horse-shoe magnet on a table
and sprinkle iron filings round it, they will arrange themselves
in curving lines between the poles, as shown in figure 28. Each
filing has become a little magnet, and these set themselves end to
end as the molecules in the metal are supposed to do. The "field"
about the magnet is replete with these lines, which follow certain
curves depending on the arrangement of the poles. In the horse-
shoe magnet, as seen, they chiefly issue from one pole and sweep
round to the other. They are never broken, and apparently they are
lines of stress in the circumambient ether. A pivoted magnet tends
to range itself along these lines, and thus the compass guides the
sailor on the ocean by keeping itself in the line between the
north and south magnetic poles of the earth. Faraday called them
lines of magnetic force, and said that the stronger the magnet the
more of these lines pass through a given space. Along them
"magnetic induction" is supposed to be propagated, and a magnet is
thus enabled to attract iron or any other magnetic substance. The
pole induces an opposite pole to itself in the nearest part of the
induced body and a like pole in the remote part. Consequently, as
unlike poles attract and like repel, the soft iron is attracted by
the inducing pole much as a pithball is attracted by an electric
charge.
The resemblances of electricity and magnetism did not escape
attention, and the derangement of the compass needle by the
lightning flash, formerly so disastrous at sea, pointed to an
intimate connection between them, which was ultimately disclosed
by Professor Oersted, of Copenhagen, in the year 1820. Oersted was
on the outlook for the required clue, and a happy chance is said
to have rewarded him. His experiment is shown in figure 29, where
a wire conveying a current of electricity flowing in the direction
of the arrow is held over a pivoted magnetic needle so that the
current flows from south to north. The needle will tend to set
itself at right angles to the wire, its north or north-seeking
pole moving towards the west. If the direction of the current is
reversed, the needle is deflected in the opposite direction, its
north pole moving towards the east. Further, if the wire is held
below the needle, in the first place, the north pole will turn
towards the east, and if the current be reversed it will move
towards the west.
The direction of a current can thus be told with the aid of a
compass needle. When the wire is wound many times round the needle
on a bobbin, the whole forms what is called a galvanoscope, as
shown in figure 30, where N is the needle and B the bobbin. When a
proper scale is added to the needle by which its deflections can
be accurately read, the instrument becomes a current measurer or
galvanometer, for within certain limits the deflection of the
needle is proportional to the strength of the current in the wire.
A rule commonly given for remembering the movement of the needle
is as follows:--Imagine yourself laid along the wire so that the
current flows from your feet to your head; then if you face the
needle you will see its north pole go to the left and its south
pole to the right. I find it simpler to recollect that if the
current flows from your head to your feet a north pole will move
round you from left to right in front. Or, again, if a current
flows from north to south, a north pole will move round it like
the sun round the earth.
The influence of the current on the needle implies a magnetic
action, and if we dust iron filings around the wire we shall find
they cling to it in concentric layers, showing that circular lines
of magnetic force enclose it like the water waves caused by a
stone dropped into a pond. Figure 31 represents the section of a
wire carrying a current, with the iron filings arranged in circles
round it. Since a magnetic pole tends to move in the direction of
the lines of force, we now see why a north or south pole tends to
move ROUND a current, and why a compass needle tries to set itself
at right angles to a current, as in the original experiment of
Oersted. The needle, having two opposite poles, is pulled in
opposite directions by the lines, and being pivoted, sets itself
tangentically to them. Were it free and flexible, it would curve
itself along one of the lines. Did it consist of a single pole, it
would revolve round the wire.
Action and re-action are equal and opposite, hence if the needle
is fixed and the wire free the current will move round the magnet;
and if both are free they will circle round each other. Applying
the above rule we shall find that when the north pole moves from
left to right the current moves from right to left. Ampere of
Paris, following Oersted, promptly showed that two parallel wires
carrying currents attracted each other when the currents flowed in
the same direction, and repelled each other when they flowed in
opposite directions. Thus, in figure 32, if A and B are the two
parallel wires, and A is mounted on pivots and free to move in
liquid "contacts" of mercury, it will be attracted or repelled by
B according as the two currents flow in the same or in opposite
directions. If the wires cross each other at right angles there is
no attraction or repulsion. If they cross at an acute angle, they
will tend to become parallel like two compass needles, when the
currents are in one direction, and to open to a right angle and
close up the other way when the currents are in opposite
directions, always tending to arrange themselves parallel and
flowing in the same direction. These effects arise from the
circular lines of force around the wire. When the currents are
similar the lines act as unlike magnetic poles and attract, but
when the currents are dissimilar the lines act as like magnetic
poles and repel each other.
Another important discovery of Ampere is that a circular current
behaves like a magnet; and it has been suggested by him that the
atoms are magnets because each has a circular current flowing
round it. A series of circular currents, such as the spiral S in
figure 33 gives, when connected to a battery C Z, is in fact a
skeleton ELECTRO-MAGNET having its north and south poles at the
extremities. If a rod or core of soft iron I be suspended by
fibres from a support, it will be sucked towards the middle of the
coil as into a vortex, by the circular magnetic lines of every
spire or turn of the coil. Such a combination is sometimes called
a solenoid, and is useful in practice.
When the core gains the interior of the coil it becomes a
veritable electromagnet, as found by Arago, having a north pole at
one end and a south pole at the other. Figure 34 illustrates a
common poker magnetised in the same way, and supporting nails at
both ends. The poker has become the core of the electromagnet. On
reversing the direction of the current through the spiral we
reverse the poles of the core, for the poker being of soft or
wrought iron, does not retain its magnetism like steel. If we stop
the current altogether it ceases to be a magnet, and the nails
will drop away from it.
Ampere's experiment in figure 32 has shown us that two currents,
more or less parallel, influence each other; but in 1831 Professor
Faraday of the Royal Institution, London, also found that when a
current is started and stopped in a wire, it induces a momentary
and opposite current in a parallel wire. Thus, if a current is
STARTED in the wire B (fig. 32) in direction of the arrow, it will
induce or give rise to a momentary current in the wire A, flowing
in a contrary direction to itself. Again, if the current in B be
STOPEED, a momentary current is set up in the wire A in a
direction the same as that of the exciting current in B. While the
current in B is quietly flowing there is no induced current in A;
and it is only at the start or the stoppage of the inducing or
PRIMARY current that the induced or SECONDARY current is set up.
Here again we have the influence of the magnetic field around the
wire conveying a current.
This is the principle of the "induction coil" so much employed in
medical electricity, and of the "transformer" or "converter" used
in electric illumination. It consists essentially, as shown in
figure 35, of two coils of wire, one enclosing the other, and both
parallel or concentric. The inner or primary coil P C is of short
thick wire of low resistance, and is traversed by the inducing
current of a battery B. To increase its inductive effect a core of
soft iron I C occupies its middle. The outer or secondary coil S C
is of long thin wire terminating in two discharging points D1 D2.
An interrupter or hammer "key" interrupts or "makes and breaks"
the circuit of the primary coil very rapidly, so as to excite a
great many induced currents in the secondary coil per second, and
produce energetic sparks between the terminals D1 D2. The
interrupter is actuated automatically by the magnetism of the iron
core I C, for the hammer H has a soft iron head which is attracted
by the core when the latter is magnetised, and being thus drawn
away from the contact screw C S the circuit of the primary is
broken, and the current is stopped. The iron core then ceases to
be a magnet, the hammer H springs back to the contact screw, and
the current again flows in the primary circuit only to be
interrupted again as before. In this way the current in the
primary coil is rapidly started and stopped many times a second,
and this, as we know, induces corresponding currents in the
secondary which appear as sparks at the discharging points. The
effect of the apparatus is enhanced by interpolating a "condenser"
C C in the primary circuit. A condenser is a form of Leyden jar,
suitable for current electricity, and consists of layers of
tinfoil separated from each other by sheets of paraffin paper,
mica, or some other convenient insulator, and alternate foils are
connected together. The wires joining each set of plates are the
poles of the condenser, and when these are connected in the
circuit of a current the condenser is charged. It can be
discharged by joining its two poles with a wire, and letting the
two opposite electricities on its plates rush together. Now, the
sudden discharge of the condenser C C through the primary coil P C
enhances the inductive effect of the current. The battery B, here
shown by the conventional symbol [Electrical Symbol] where the
thick dash is the negative and the thin dash the positive pole, is
connected between the terminals T1 T2, and a COMMUTATOR or pole-
changer R, turned with a handle, permits the direction of the
current to be reversed at will.
Figure 36 represents the exterior of an ordinary induction coil of
the Ruhmkorff pattern, with its two coils, one over the other C,
its commutator R, and its sparkling points D1D2, the whole being
mounted on a mahogany base, which holds the condenser.
The intermittent, or rather alternating, currents from the
secondary coil are often applied to the body in certain nervous
disorders. When sent through glass tubes filled with rarefied
gases, sometimes called "Geissler tubes," they elicit glows of
many colours, vieing in beauty with the fleeting tints of the
aurora polaris, which, indeed, is probably a similar effect of
electrical discharges in the atmosphere.
The action of the induction is reversible. We can not only send a
current of low "pressure" from a generator of weak electromotive
force through the primary coil, and thus excite a current of high
pressure in the secondary coil, but we can send a current of high
pressure through the secondary coil and provoke a current of low
pressure in the primary coil The transformer or converter, a
modified induction coil used in distributing electricity to
electric lamps and motors, can not only transform a low pressure
current into a high, but a high pressure current into a low. As
the high pressure currents are best able to overcome the
resistance of the wire convening them, it is customary to transmit
high pressure currents from the generator to the distant place
where they are wanted by means of small wires, and there transform
them into currents of the pressure required to light the lamps or
drive the motors.
We come now to another consequence of Oersted's great discovery,
which is doubtless the most important of all, namely, the
generation of electricity from magnetism, or, as it is usually
called, magneto-electric induction. In the year 1831 the
illustrious Michael Faraday further succeeded in demonstrating
that when a magnet M is thrust into a hollow coil of wire C, as
shown in figure 37, a current of electricity is set up in the coil
whilst the motion lasts. When the magnet is withdrawn again
another current is induced in the reverse direction to the first.
If the coil be closed through a small galvanometer G the movements
of the needle to one side or the other will indicate these
temporary currents. It follows from the principle of action and
reaction that if the magnet is kept still and the coil thrust over
it similar currents will be induced in the coil. All that is
necessary is for the wires to cut the lines of magnetic force
around the magnet, or, in other words, the lines of force in a
magnetic field We have seen already that a wire conveying a
current can move a magnetic pole, and we are therefore prepared to
find that a magnetic pole moved near a wire can excite a current
in it.
Figure 38 illustrates the conditions of this remarkable effect,
where N and S are two magnetic poles with lines of force between
them, and W is a wire crossing these lines at right angles, which
is the best position. If, now, this wire be moved so as to sink
bodily through the paper away from the reader, an electric current
flowing in the direction of the arrow will be induced in it. If,
on the contrary, the wire be moved across the lines of force
towards the reader, the induced current will flow oppositely to
the arrow. Moreover, if the poles of the magnet N and S exchange
places, the directions of the induced currents will also be
reversed. This is the fundamental principle of the well known
dynamo-electric machine, popularly called a dynamo.
Again, if we send a current from some external source through the
wire in the direction of the arrow, the wire will move OF ITSELF
across the lines of force away from the reader, that is to say, in
the direction it would need to be moved in order to excite such a
current; and if, on the other hand, the current be sent through it
in the reverse direction to the arrow, it will move towards the
reader. This is the principle of the equally well-known electric
motor. Figure 39 shows a simple method of remembering these
directions.
Let the right hand rest on the north pole of a magnet and the
forefinger be extended in the direction of the lines of force,
then the outstretched thumb will indicate the direction in which
the wire or conductor moves and the bent middle finger the
direction of the current. These three digits, as will be noticed,
are all at right angles to each other, and this relation is the
best for inducing the strongest current in a dynamo or the most
energetic movement of the conductor in an electric motor.
Of course in a dynamo-electric generator the stronger the magnetic
field, the less the resistance of the conductor, and the faster it
is moved across the lines of force, that is to say, the more lines
it cuts in a second the stronger is the current produced.
Similarly in an electric motor, the stronger the current and
magnetic field the faster will the conductor move.
The most convenient motion to give the conductor in practice is
one of rotation, and hence the dynamo usually consists of a coil
or series of coils of insulated wire termed the "armature," which
is mounted on a spindle and rapidly rotated in a strong magnetic
field between the poles of powerful magnets. Currents are
generated in the coils, now in one direction then in another, as
they revolve or cross different parts of the field; and, by means
of a device termed a commutator, these currents can be collected
or sifted at will, and led away by wires to an electric lamp, an
accumulator, or an electric motor, as desired. The character of
the electricity is precisely the same as that generated in the
voltaic battery.
The commutator may only collect the currents as they are
generated, and supply what is called an alternating current, that
is to say, a current which alternates or changes its direction
several hundred times a second, or it may sift the currents as
they are produced and supply what is termed a continuous current,
that is, a current always in the same direction, like that of a
voltaic battery. Some machines are made to supply alternating
currents, others continuous currents. Either class of current will
do for electric lamps, but only continuous currents are used for
electo-plating, or, in general, for electric motors.
In the "magneto-electric" machine the FIELD MAGNETS are simply
steel bars permanently magnetised, but in the ordinary dynamo the
field magnets are electro-magnets excited to a high pitch by means
of the current generated in the moving conductor or armature. In
the "series-wound" machine the whole of the current generated in
the armature also goes through the coils of the field magnets.
Such a machine is sketched in figure 40, where A is the armature,
consisting of an iron core surrounded by coils of wire and
rotating in the field of a powerful electro-magnet NS in the
direction of the arrows. For the sake of simplicity only twelve
coils are represented. They are all in circuit one with another,
and a wire connects the ends of each coil to corresponding metal
bars on the commutator C. These bars are insulated from each other
on the spindle X of the armature. Now, as each coil passes through
the magnetic field in turn, a current is excited in it. Each coil
therefore resembles an individual cell of a voltaic battery,
connected in series. The current is drawn off from the ring by two
copper "brushes" b, be which rub upon the bars of the commutator
at opposite ends of a diameter, as shown. One brush is the
positive pole of the dynamo, the other is the negative, and the
current will flow through any wire or external circuit which may
be connected with these, whether electric lamps, motors,
accumulators, electro-plating baths, or other device. The small
arrows show the movements of the current throughout the machine,
and the terminals are marked (+) positive and (-) negative.
It will be observed that the current excited in the armature also
flows through the coils of the electro-magnets, and thus keeps up
their strength. When the machine is first started the current is
feeble, because the field of the magnets in which the armature
revolves is merely that due to the dregs or "residual magnetism"
left in the soft iron cores of the magnet since the last time the
machine was used. But this feeble current exalts the strength of
the field-magnets, producing a stronger field, which in turn
excites a still stronger current in the armature, and this process
of give and take goes on until the full strength or "saturation"
of the magnets is attained.
Such is the "series" dynamo, of which the well-known Gramme
machine is a type. Figure 41 illustrates this machine as it is
actually made, A being the armature revolving between the poles NS
of the field-magnets M, M, M' M', on a spindle which is driven by
means of a belt on the pulley P from a separate engine The brushes
b b' of the commutator C collect the current, which in this case
is continuous, or constant in its direction.
The current of the series machine varies with the resistance of
the external or working circuit, because that is included in the
circuit of the field magnets and the armature. Thus, if we vary
the number of electric lamps fed by the machine, we shall vary the
current it is capable of yielding. With arc lamps in series, by
adding to the number in circuit we increase the resistance of the
outer circuit, and therefore diminish the strength of the current
yielded by the machine, because the current, weakened by the
increase of resistance, fails to excite the field magnets as
strongly as before. On the other hand, with glow lamps arranged in
parallel, the reverse is the case, and putting more lamps in
circuit increases the power of the machine, by diminishing the
resistance of the outer circuit in providing more cross-cuts for
the current. This, of course, is a drawback to the series machine
in places where the number of lamps to be lighted varies from time
to time. In the "shunt-wound" machine the field magnets are
excited by diverting a small portion of the main current from the
armature through them, by means of a "shunt" or loop circuit. Thus
in figure 42 where C is the commutator and b b' the brushes, M is
a shunt circuit through the magnets, and E is the external or
working circuit of the machine.
The small arrows indicate the directions of the currents. With
this arrangement the addition of more glow lamps to the external
circuit E DIMINISHES the current, because the portion of it which
flows through the by-path M, and excites the magnets, is less now
that the alternative route for the current through E is of lower
resistance than before. When fewer glow lamps are in the external
circuit E, and its resistance therefore higher, the current in the
shunt circuit M is greater than before, the magnets become
stronger, and the electromotive force of the armature is
increased. The Edison machine is of this type, and is illustrated
in figure 43, where M M' are the field magnets with their poles N
S, between which the armature A is revolved by means of the belt
B, and a pulley seen behind. The leading wires W W convey the
current from the brushes of the commutator to the external
circuit. In this machine the conductors of the armature are not
coils of wire, but separate bars of copper.
In shunt machines the variation of current due to a varying number
of lamps in use occasions a rise and fall in the brightness of the
lamps which is undesirable, and hence a third class of dynamo has
been devised, which combines the principles of both the series and
shunt machines. This is the "compound-wound" machine, in which the
magnets are wound partly in shunt and partly in series with the
armature, in such a manner that the strength of the field-magnets
and the electromotive force of the current do not vary much,
whatever be the number of lamps in circuit. In alternate current
machines the electromotive force keeps constant, as the field-
magnets are excited by a separate machine, giving a continuous
current.
We have already seen that the action of the dynamo is reversible,
and that just as a wire moved across a magnetic field supplies an
electric current, so a wire at rest, but conducting a current
across a magnetic field, will move. The electric motor is
therefore essentially a dynamo, which on being traversed by an
electric current from an external source puts itself in motion.
Thus, if a current be sent through the armature of the Gramme
machine, shown in figure 41, the armature will revolve, and the
spindle, by means of a belt on the pulley P, can communicate its
energy to another machine.
Hence the electric motor can be employed to work lathes, hoists,
lifts, drive the screws of boats or the wheels of carriages, and
for many other purposes. There are numerous types of electric
motor as of the dynamo in use, but they are all modifications of
the simple continuous or alternating current dynamo.
Obviously, since mechanical power can be converted into
electricity by the dynamo, and reconverted into mechanical power
by the motor, it is sufficient to connect a dynamo and motor
together by insulated wire in order to transmit mechanical power,
whether it be derived from wind, water, or fuel, to any reasonable
distance.
CHAPTER V.
ELECTROLYSIS.
Having seen how electricity can be generated and stored in
considerable quantity, let us now turn to its practical uses. Of
these by far the most important are based on its property of
developing light and heat as in the electric spark, chemical
action as m the voltameter, and magnetism as in the electromagnet.
The words "current," "pressure," and so on point to a certain
analogy between electricity and water, which helps the imagination
to figure what can neither be seen nor handled, though it must not
be traced too far. 'Water, for example, runs by the force of
gravity from a place of higher to a place of lower level. The
pressure of the stream is greater the more the difference of level
or "head of water" The strength of the current or quantity of
water flowing per second is greater the higher the pressure, and
the less the resistance of its channel. The power of the water or
its rate of doing mechanical work is greater the higher the
pressure and the stronger the current. So, too, electricity flows
by the electromotive force from a place of higher to a place of
lower electric level or potential. The electric pressure is
greater the more the difference of potential or electromotive
force. The strength of the electric current or quantity of
electricity flowing per second is greater the higher the pressure
or electromotive force and the less the resistance of the circuit
The power of the electricity or its rate of doing work is greater
the higher the electromotive force and the stronger the current.
It follows that a small quantity of water or electricity at a high
pressure will give us the same amount of energy as a large
quantity at a low pressure, and our choice of one or the other
will depend on the purpose we have in view. As a rule, however, a
large current at a comparatively low or moderate pressure is found
the more convenient in practice.
The electricity of friction belongs to the former category, and
the electricity of chemistry, heat, and magnetism to the latter.
The spark of a factional or influence machine can be compared to a
highland cataract of lofty height but small volume, which is more
picturesque than useful, and the current from a voltaic battery, a
thermopile, or a dynamo to a lowland river which can be dammed to
turn a mill. It is the difference between a skittish gelding and a
tame carthorse.
Not the spark from an induction coil or Leyden jar, but a strong
and steady current at a low pressure, is adapted for electrolysis
or electrodeposition, and hence the voltaic battery or a special
form of dynamo is usually employed in this work. A flash of
lightning is the very symbol of terrific power, and yet, according
to the illustrious Faraday, it contains a smaller amount of
electricity than the feeble current required to decompose a single
drop of rain.
In our simile of the mill dam and the battery or dynamo, the dam
corresponds to the positive pole and the river or sea below the
mill to the negative pole. The mill-race will stand for the wire
joining the poles, that is to say, the external circuit, and the
mill-wheel for the work to be done in the circuit, whether it be a
chemical for decomposition, a telegraph instrument, an electric
lamp, or any other appliance. As the current in the race depends
on the "head of water," or difference of level between the dam and
the sea as well as on the resistance of the channel, so the
current in the circuit depends on the "electromotive force," or
difference of potential between the positive and negative poles,
as well as on the resistance of the circuit. The relation between
these is expressed by the well-known law of Ohm, which runs: A
current of electricity is directly proportional to the
electromotive force and inversely proportional to the resistance
of the circuit.
In practice electricity is measured by various units or standards
named after celebrated electricians. Thus the unit of quantity is
the coulomb, the unit of current or quantity flowing per second is
the ampere, the unit of electromotive force is the volt, and the
unit of resistance is the ohm.
The quantity of water or any other "electrolyte" decomposed by
electricity is proportional to the strength of the current. One
ampere decomposes .00009324 gramme of water per second, liberating
.000010384 gramme of hydrogen and .00008286 gramme of oxygen.
The quantity in grammes of any other chemical element or ion which
is liberated from an electrolyte or body capable of
electrochemical decomposition in a second by a current of one
ampere is given by what is called the electrochemical equivalent
of the ion. This is found by multiplying its ordinary chemical
equivalent or combining weight by .000010384, which is the
electrochemical equivalent of hydrogen. Thus the weight of metal
deposited from a solution of any of its salts by a current of so
many amperes in so many seconds is equal to the number of amperes
multiplied by the number of seconds, and by the electrochemical
equivalent of the metal.
The deposition of a metal from a solution of its salt is very
easily shown in the case of copper. In fact, we have already seen
that in the Daniell cell the current decomposes a solution of
sulphate of copper and deposits the pure metal on the copper
plate. If we simply make a solution of blue vitriol in a glass
beaker and dip the wires from a voltaic cell into it, we shall
find the wire from the negative pole become freshly coated with
particles of new copper. The sulphate has been broken up, and the
liberated metal, being positive, gathers on the negative
electrode. Moreover, if we examine the positive electrode we shall
find it slightly eaten away, because the sulphuric acid set free
from the sulphate has combined with the particles of that wire to
make new sulphate. Thus the copper is deposited on one electrode,
namely, the cathode, by which the current leaves the bath, and at
the expense of the other electrode, that is to say, the anode, by
which the current enters the bath.
The fact that the weight of metal deposited in this way from its
salts is proportional to the current, has been utilised for
measuring the strength of currents with a fine degree of accuracy.
If, for example, the tubes of the voltameter described on page 38
were graduated, the volume of gas evolved would be a measure of
the current. Usually, however, it is the weight of silver or
copper deposited from their salts in a certain time which gives
the current in amperes.
Electro-plating is the principal application of this chemical
process. In 1805 Brugnatelli took a silver medal and coated it
with gold by making it the cathode in a solution of a salt of
gold, and using a plate of gold for the anode. The shops of our
jewellers are now bright with teapots, salt cellars, spoons, and
other articles of the table made of inferior metals, but
beautified and preserved from rust in this way.
Figure 44 illustrates an electro-plating bath in which a number of
spoons are being plated. A portion of the vat V is cut away to
show the interior, which contains a solution S of the double
cyanide of gold and potassium when gold is to be laid, and the
double cyanide of silver and potassium when silver is to be
deposited. The electrodes are hung from metal rods, the anode A
being a plate of gold or silver G, as the case may be, and the
cathode C the spoons in question. When the current of the battery
or dynamo passes through the bath from the anode to the cathode,
gold or silver is deposited on the spoons, and the bath
recuperates its strength by consuming the gold or silver plate.
Enormous quantities of copper are now deposited in a similar way,
sulphate of copper being the solution and a copper plate the
anode. Large articles of iron, such as the parts of ordnance, are
sometimes copper-plated to preserve them from the action of the
atmosphere. Seamless copper pipes for conveying steam, and wires
of pure copper for conducting electricity, are also deposited, and
it is not unlikely that the kettle of the future will be made by
electrolysis.
Nickel-plating is another extensive branch of the industry, the
white nickel forming a cloak for metals more subject to corrosion.
Nickel is found to deposit best from a solution of the double
sulphate of nickel and ammonia. Aluminium, however, has not yet
been successfully deposited by electricity.
In 1836 De la Rue observed that copper laid in this manner on
another surface took on its under side an accurate impression of
that surface, even to the scratches on it, and three years later
Jacobi, of St. Petersburg, and Jordan, of London, applied the
method to making copies or replicas of medals and woodcuts. Even
non-metallic surfaces could be reproduced in copper by taking a
cast of them in wax and lining the mould with fine plumbago,
which, being a conductor, served as a cathode to receive the layer
of metal. It is by the process of electrotyping or galvano-
plastics that the copper faces for printing woodcuts are prepared,
and copies made of seals or medals.
Natural objects, such as flowers, ferns, leaves, feathers,
insects, and lizards, can be prettily coated with bronze or
copper, not to speak of gold and silver, by a similar process.
They are too delicate to be coated with black lead in order to
receive the skin of metal, but they can be dipped in solutions,
leaving a film which can be reduced to gold or silver. For
instance, they may be soaked in an alcoholic solution of nitrate
of silver, made by shaking 2 parts of the crystals in 100 parts of
alcohol in a stoppered bottle. When dry, the object should be
suspended under a glass shade and exposed to a stream of
sulphuretted hydrogen gas; or it may be immersed in a solution of
1 part of phosphorus in 15 parts of bisulphide of carbon, 1 part
of bees-wax, 1 part of spirits of turpentine, 1 part of asphaltum,
and 1/8 part of caoutchouc dissolved in bisulphide of carbon. This
leaves a superficial film which is metallised by dipping in a
solution of 20 grains of nitrate of silver to a pint of water. On
this metallic film a thicker layer of gold and silver in different
shades can be deposited by the current, and the silver surface may
also be "oxidised" by washing it in a weak solution of platinum
chloride.
Electrolysis is also used to some extent in reducing metals from
their ores, in bleaching fibre, in manufacturing hydrogen and
oxygen from water, and in the chemical treatment of sewage.
CHAPTER VI.
THE TELEGRAPH AND TELEPHONE.
Like the "philosopher's stone," the "elixir of youth," and
"perpetual motion," the telegraph was long a dream of the
imagination. In the sixteenth century, if not before, it was
believed that two magnetic needles could be made sympathetic, so
that when one was moved the other would likewise move, however far
apart they were, and thus enable two distant friends to
communicate their minds to one another.
The idea was prophetic, although the means of giving effect to it
were mistaken. It became practicable, however, when Oersted
discovered that a magnetic needle could be swung to one side or
the other by an electric current passing near it.
The illustrious Laplace was the first to suggest a telegraph on
this principle. A wire connecting the two poles of a battery is
traversed, as we know, by an electric current, which makes the
round of the circuit, and only flows when that circuit is
complete. However long the wire may be, however far it may run
between the poles, the current will follow all its windings, and
finish its course from pole to pole of the battery. You may lead
the wire across the ocean and back, or round the world if you
will, and the current will travel through it.
The moment you break the wire or circuit, however, the current
will stop. By its electromotive force it can overcome the
resistance of the many miles of conductor; but unless it be
unusually strong it cannot leap across even a minute gap of air,
which is one of the best insulators.
If, then, we have a simple device easily manipulated by which we
can interrupt the circuit of the battery, in accordance with a
given code, we shall be able to send a series of currents through
the wire and make sensible signals wherever we choose. These signs
can be produced by the deviation of a magnetic needle, as Laplace
pointed out, or by causing an electro-magnet to attract soft iron,
or by chemical decomposition, or any other sensible effect of the
current.
Ampere developed the idea of Laplace into a definite plan, and in
1830 or thereabout Ritchie, in London, and Baron Schilling, in St.
Petersburg, exhibited experimental models. In 1833 and afterwards
Professors Gauss and Weber installed a private telegraph between
the observatory and the physical cabinet of the University of
Gottingen. Moreover, in 1836 William Fothergill Cooke, a retired
surgeon of the Madras army, attending lectures on anatomy at the
University of Heidelberg, saw an experimental telegraph of
Professor Moncke, which turned all his thoughts to the subject. On
returning to London he made the acquaintance of Professor
Wheatstone, of King's College, who was also experimenting in this
direction, and in 1836 they took out a patent for a needle
telegraph. It was tried successfully between the Euston terminus
and the Camden Town station of the London and North-Western
Railway on the evening of July 25th, 1837, in presence of Mr.
Robert Stephenson, and other eminent engineers. Wheatstone,
sitting in a small room near the booking-office at Euston, sent
the first message to Cooke at Camden Town, who at once replied.
"Never," said Wheatstone, "did I feel such a tumultuous sensation
before, as when, all alone in the still room, I heard the needles
click, and as I spelled the words I felt all the magnitude of the
invention pronounced to be practicable without cavil or dispute."
The importance of the telegraph in working railways was manifest,
and yet the directors of the company were so purblind as to order
the removal of the apparatus, and it was not until two years later
that the Great Western Railway Company adopted it on their line
from Paddington to West Drayton, and subsequently to Slough. This
was the first telegraph for public use, not merely in England, but
the world. The charge for a message was only a shilling,
nevertheless few persons availed themselves of the new invention,
and it was not until its fame was spread abroad by the clever
capture of a murderer named Tawell that it began to prosper.
Tawell had killed a woman at Slough, and on leaving his victim
took the train for Paddington. The police, apprised of the murder,
telegraphed a description of him to London. The original "five
needle instrument," now in the museum of the Post Office, had a
dial in the shape of a diamond, on which were marked the letters
of the alphabet, and each letter of a word was pointed out by the
movements of a pair of needles. The dial had no letter "q," and as
the man was described as a quaker the word was sent "kwaker." When
the tram arrived at Paddington he was shadowed by detectives, and
to his utter astonishment was quietly arrested in a tavern near
Cannon Street.
In Cooke and Wheatstone's early telegraph the wire travelled the
whole round of the circuit, but it was soon found that a "return"
wire in the circuit was unnecessary, since the earth itself could
take the place of it. One wire from the sending station to the
receiving station was sufficient, provided the apparatus at each
end were properly connected to the ground. This use of the earth
not only saved the expense of a return wire, but diminished the
resistance of the circuit, because the earth offered practically
no resistance.
Figure 45 is a diagram of the connections in a simple telegraph
circuit. At each of the stations there is a battery B B', an
interruptor or sending key K K'to make and break the continuity of
the circuit, a receiving instrument R R'to indicate the signal
currents by their sensible effects, and connections with ground or
"earth plates" E E' to engage the earth as a return wire. These
are usually copper plates buried in the moist subsoil or the water
pipes of a city. The line wire is commonly of iron supported on
poles, but insulated from them by earthenware "cups" or
insulators.
At the station on the left the key is in the act of SENDING a
message, and at the post on the right it is conformably in the
position for receiving the message. The key is so constructed that
when it is at rest it puts the line in connection with the earth
through the RECEIVING INSTRUMENT and the earth plate.
The key K consists essentially of a spring-lever, with two
platinum contacts, so placed that when the lever is pressed down
by the hand of the telegraphist it breaks contact with the
receiver R, and puts the line-wire L in connection with the earth
E through the battery B, as shown on the left. A current then
flows into the line and traverses the receiver R' at the distant
station, returning or seeming to return to the sending battery by
way of the earth plate E' on the right and the intermediate
ground.
The duration of the current is at the will of the operator who
works the sending-key, and it is plain that signals can be made by
currents of various lengths. In the "Morse code" of signals, which
is now universal, only two lengths of current are employed--
namely, a short, momentary pulse, produced by instant contact of
the key, and a jet given by a contact about three times longer.
These two signals are called "dot" and "dash," and the code is
merely a suitable combination of them to signify the several
letters of the alphabet. Thus e, the commonest letter in English,
is telegraphed by a single "dot," and the letter t by a single
"dash," while the letter a is indicated by a "dot" followed after
a brief interval or "space" by a dash.
Obviously, if two kinds of current are used, that is to say, if
the poles of the battery are reversed by the sending-key, and the
direction of the current is consequently reversed in the circuit,
there is no need to alter the length of the signal currents,
because a momentary current sent in one direction will stand for a
"dot" and in the other direction for a "dash." As a matter of
fact, the code is used in both ways, according to the nature of
the line and receiving instrument. On submarine cables and with
needle and "mirror" instruments, the signals are made by reversing
currents of equal duration, but on land lines worked by "Morse"
instruments and "sounders," they are produced by short and long
currents.
The Morse code is also used in the army for signalling by waving
flags or flashing lights, and may also be serviceable in private
life. Telegraph clerks have been known to "speak" with each other
in company by winking the right and left eye, or tapping with
their teaspoon on a cup and saucer. Any two distinct signs,
however made, can be employed as a telegraph by means of the Morse
code, which runs as shown in figure 46.
The receiving instruments R R' may consist of a magnetic needle
pivotted on its centre and surrounded by a coil of wire, through
which the current passes and deflects the needle to one side or
the other, according to the direction in which it flows. Such was
the pioneer instrument of Cooke and Wheatstone, which is still
employed in England in a simplified form as the "single" and
"double" needle-instrument on some of the local lines and in
railway telegraphs. The signals are made by sending momentary
currents in opposite directions by a "double current" key, which
(unlike the key K in figure 45) reverses the poles
A .- J -.-.
B -... K -.-
C ... L --
D -.. M - -
E . N -.
F .-. O . .
G --. Q ..-.
H . .. R . ..
I ..
S ... 1 .--.
T - 2 ..-..
U ..- 3 ...-.
V ...- 4 ....-
W .- 5 ---
X .-.. 6 ......
Y .. .. 7 --..
Z ... . 8 -.. ..
& . .. 9 -..-
Period ..--.. 0 ----
Comma .-.-
The International (Morse) code used elsewhere is the same as the
above, with the following exceptions:
C -.-. Q --.-
F . -. R .-.
J .--- X -..-
L .-.. Y -.--
O --- Z --..
P .--.
FIG. 46.--Morse Signal Alphabet.
of the battery, in putting the line to one or the other, and thus
making the "dot" signal with the positive and the "dash" signal
with the negative pole. It follows that if the "dot" is indicated
by a throw of the needle to the right side, a "dash" will be given
by a throw to the left.
Most of the telegraph instruments for land lines are based on the
principle of the electro-magnet. We have already seen (page 59)
how Ampere found that a spiral of wire with a current flowing in
it behaved like a magnet and was able to suck a piece of soft iron
into it. If the iron is allowed to remain there as a core, the
combination of coil and core becomes an electro-magnet, that is to
say, a magnet which is only a magnet so long as the current
passes. Figure 47 represents a simple "horse-shoe" electro-magnet
as invented by Sturgeon. A U-shaped core of soft iron is wound
with insulated wire W, and when a current is sent through the
wire, the core is found to become magnetic with a "north" pole in
one end and a "south" pole in the other. These poles are therefore
able to attract a separate piece of soft iron or armature A. When
the current is stopped, however, the core ceases to be a magnet
and the armature drops away. In practice the electromagnet usually
takes the form shown in figure 48, where the poles are two bobbins
or solenoids of wire 61 having straight cores of iron which are
united by an iron bar B, and A is the armature.
Such an electromagnet is a more powerful device than a swinging
needle, and better able to actuate a mechanism. It became the
foundation of the recording instrument of Samuel Morse, the father
of the telegraph in America. The Morse, or, rather, Morse and Vail
instrument, actually marks the signals in "dots" and "dashes" on a
ribbon of moving paper. Figure 49 represents the Morse instrument,
in which an electromagnet M attracts an iron armature A when a
current passes through its bobbins, and by means of a lever L
connected with the armature raises the edge of a small disc out of
an ink-pot I against the surface of a travelling slip of paper P,
and marks a dot or dash upon it as the case may be. The rest of
the apparatus consists of details and accessories for its action
and adjustment, together with the sending-key K, which is used in
asking for repetitions of the words, if necessary.
A permanent record of the message is of course convenient,
nevertheless the operators prefer to "read" the signals by the
ear, rather than the eye, and, to the annoyance of Morse, would
listen to the click of the marking disc rather than decipher the
marks on the paper. Consequently Alfred Vail, the collaborator of
Morse, who really invented the Morse code, produced a modification
of the recording instrument working solely for the ear. The
"sounder," as it is called, has largely driven the "printer" from
the field. This neat little instrument is shown in figure 50,
where M is the electromagnet, and A is the armature which chatters
up and down between two metal stops, as the current is made and
broken by the sending-key, and the operator listening to the
sounds interprets the message letter by letter and word by word.
The motion of the armature in both of these instruments takes a
sensible time, but Alexander Bain, of Thurso, by trade a
watchmaker, and by nature a genius, invented a chemical telegraph
which was capable of a prodigious activity. The instrument of Bain
resembled the Morse in marking the signals on a tape of moving
paper, but this was done by electrolysis or electro-chemical
decomposition. The paper was soaked in a solution of iodide of
potassium in starch and water, and the signal currents were passed
through it by a marking stylus or pencil of iron. The electricity
decomposed the solution in its passage and left a blue stain on
the paper, which corresponded to the dot and dash of the Morse
apparatus. The Bain telegraph can record over 1000 words a minute
as against 40 to 50 by the Morse or sounder, nevertheless it has
fallen into disuse, perhaps because the solution was troublesome.
It is stated that a certain blind operator could read the signals
by the smell of the chemical action; and we can well believe it.
In fact, the telegraph appeals to every sense, for a deaf clerk
can feel the movements of a sounder, and the signals of the
current can be told without any instrument by the mere taste of
the wires inserted in the mouth.
A skilful telegraphist can transmit twenty-five words a minute
with the single-current key, and nearly twice as many by the
double-current key, and if we remember that an average English
word requires fifteen separate signals, the number will seem
remarkable; but by means of Wheatstone's automatic sender 150
words or more can be sent in a minute.
Among telegraphs designed to print the message in Roman type, that
of Professor David Edward Hughes is doubtless the fittest, since
it is now in general use on the Continent, and conveys our
Continental news. In this apparatus the electromagnet, on
attracting its armature, presses the paper against a revolving
type wheel and receives the print of a type, so that the message
can be read by a novice. To this effect the type wheel at the
receiving station has to keep in perfect time as it revolves, so
that the right letter shall be above the paper when the current
passes. Small varieties of the type-printer are employed for the
distribution of news and prices in most of the large towns, being
located in hotels, restaurants, saloons, and other public places,
and reporting prices of stocks and bonds, horse races, and
sporting and general news. The "duplex system," whereby two
messages, one in either direction, can be sent over one wire
simultaneously without interfering, and the quadruplex system,
whereby four messages, two in either direction, are also sent at
once, have come into use where the traffic over the lines is very
great. Both of these systems and their modifications depend on an
ingenious arrangement of the apparatus at each end of the line, by
which the signal currents sent out from one station do not
influence the receivers there, but leave them free to indicate the
currents from the distant station. When the Wheatstone Automatic
Sender is employed with these systems about 500 words per minute
can be sent through the line. Press news is generally sent by
night, and it is on record, that during a great debate in
Parliament, as many as half a million words poured out of the
Central Telegraph Station at St. Martin's-le-Grand in a single
night to all parts of the country.
Errors occur now and then through bad penmanship or the similarity
of certain signals, and amusing telegrams have been sent out, as
when the nomination of Mr. Brand for the Speakership of the
Commons took the form of "Proposed to brand Speaker"; and an
excursion party assured their friends at home of their security by
the message, "Arrived all tight."
Telegraphs, in the literal sense of the word, which actually write
the message as with a pen, and make a copy or facsimile of the
original, have been invented from time to time. Such are the
"telegraphic pen" of Mr. E. A. Cowper, and the "telautographs" of
Mr. J. H. Robertson and Mr. Elisha Gray. The first two are based
on a method of varying the strength of the current in accordance
with the curves of the handwriting, and making the varied current
actuate by means of magnetism a writing pen or stylus at the
distant station. The instrument of Gray, which is the most
successful, works by intermittent currents or electrical impulses,
that excite electro-magnets and move the stylus at the far end of
the line. They are too complicated for description here, and are
not of much practical importance.
Telegraphs for transmitting sketches and drawings have also been
devised by D'Ablincourt and others, but they have not come into
general use. Of late another step forward has been taken by Mr.
Amstutz, who has invented an apparatus for transmitting
photographic pictures to a distance by means of electricity. The
system may be described as a combination of the photograph and
telegraph. An ordinary negative picture is taken, and then
impressed on a gelatine plate sensitised with bichromate of
potash. The parts of the gelatine in light become insoluble, while
the parts in shade can be washed away by water. In this way a
relief or engraving of the picture is obtained on the gelatine,
and a cross section through the plate would, if looked at
edgeways, appear serrated, or up and down, like a section of
country or the trace of the stylus in the record of a phonograph.
The gelatine plate thus carved by the action of light and water is
wrapped round a revolving drum or barrel, and a spring stylus or
point is caused to pass over it as the barrel revolves, after the
manner of a phonographic cylinder. In doing so the stylus rises
and falls over the projections in the plate and works a lever
against a set of telegraph keys, which open electric contacts and
break the connections of an electric battery which is joined
between the keys and the earth. There are four keys, and when they
are untouched the current splits up through four by-paths or
bobbins of wire before it enters the line wire and passes to the
distant station. When any of the keys are touched, however, the
corresponding by-path or bobbin is cut out of circuit. The
suppression of a by-path or channel for the current has the effect
of adding to the "resistance" of the line, and therefore of
diminishing the strength of the current. When all the keys are
untouched the resistance is least and the current strongest. On
the other hand, when all the keys but the last are touched, the
resistance is greatest and the current weakest. By this device it
is easy to see that as the stylus or tracer sinks into a hollow of
the gelatine, or rises over a height, the current in the line
becomes stronger or weaker. At the distant station the current
passes through a solenoid or hollow coil of wire connected to the
earth and magnetises it, so as to pull the soft iron plug or
"core" with greater or less force into its hollow interior. The up
and down movement of the plug actuates a graving stylus or point
through a lever, and engraves a copy of the original gelatine
trace on the surface of a wax or gelatine plate overlying another
barrel or drum, which revolves at a rate corresponding to that of
the barrel at the transmitting station. In this way a facsimile of
the gelatine picture is produced at the distant station, and an
electrotype or cliche of it can be made for printing purposes. The
method is, in fact, a species of electric line graving, and Mr.
Amstutz hopes to apply it to engraving on gold, silver, or any
soft metal, not necessarily at a distance.
We know that an electric current in one wire can induce a
transient current in a neighbouring wire, and the fact has been
utilised in the United States by Phelps and others to send
messages from moving trains. The signal currents are intermittent,
and when they are passed through a conductor on the train they
excite corresponding currents in a wire run along the track, which
can be interpreted by the hum they make in a telephone.
Experiments recently made by Mr. W. H. Preece for the Post Office
show that with currents of sufficient strength and proper
apparatus messages can be sent through the air for five miles or
more by this method of induction.
We come now to the submarine telegraph, which differs in many
respects from the overland telegraph. Obviously, since water and
moist earth is a conductor, a wire to convey an electric current
must be insulated if it is intended to lie at the bottom of the
sea or buried underground. The best materials for the purpose yet
discovered are gutta-percha and india-rubber, which are both
flexible and very good insulators.
The first submarine cable was laid across the Channel from Dover
to Calais in 1851, and consisted of a copper strand, coated with
gutta-percha, and protected from injury by an outer sheath of hemp
and iron wire. It is the general type of all the submarine cables
which have been deposited since then in every part of the world.
As a rule, the armour or sheathing is made heavier for shore water
than it is for the deep sea, but the electrical portion, or
"core," that is to say, the insulated conductor, is the same
throughout.
The first Atlantic cable was laid in 1858 by Cyrus W. Field and a
company of British capitalists, but it broke down, and it was not
until 1866 that a new and successful cable was laid to replace it.
Figure 51 represents various cross-sections of an Atlantic cable
deposited in 1894.
The inner star of twelve copper wires is the conductor, and the
black circle round it is the gutta-percha or insulator which keeps
the electricity from escaping into the water. The core in shallow
water is protected from the bites of teredoes by a brass tape, and
the envelope or armour consists of hemp and iron wire preserved
from corrosion by a covering of tape and a compound of mineral
pitch and sand.
The circuit of a submarine line is essentially the same as that of
a land line, except that the earth connection is usually the iron
sheathing of the cable in lieu of an earth-plate. On a cable,
however, at least a long cable, the instruments for sending and
receiving the messages are different from those employed on a land
line. A cable is virtually a Leyden jar or condenser, and the
signal currents in the wire induce opposite currents in the water
or earth. As these charges hold each other the signals are
retarded in their progress, and altered from sharp sudden jets to
lagging undulations or waves, which tend to run together or
coalesce. The result is that the separate signal currents which
enter a long cable issue from it at the other end in one
continuous current, with pulsations at every signal, that is to
say, in a lapsing stream, like a jet of water flowing from a
constricted spout. The receiving instrument must be sufficiently
delicate to manifest every pulsation of the current. Its
indicator, in fact, must respond to every rise and fall of the
current, as a float rides on the ripples of a stream.
Such an instrument is the beautiful "mirror" galvanometer of Lord
Kelvin, Ex-President of the Royal Society, which we illustrate in
figure 52, where C is a coil of wire with a small magnetic needle
suspended in its heart, and D is a steel magnet supported over it.
The needle (M figure 53) is made of watch spring cemented to the
back of a tiny mirror the size of a half-dime which is hung by a
single fibre of floss silk inside an air cell or chamber with a
glass lens G in front, and the coil C surrounds it. A ray of light
from a lamp L (figure 52) falls on the mirror, and is reflected
back to a scale S, on which it makes a bright spot. Now, when the
coil C is connected between the end of the cable and the earth,
the signal current passing through it causes the tiny magnet to
swing from side to side, and the mirror moving with it throws the
beam up and down the scale. The operator sitting by watches the
spot of light as it flits and flickers like a fire-fly in the
darkness, and spells out the mysterious message.
A condenser joined in the circuit between the cable and the
receiver, or between the receiver and the earth, has the effect of
sharpening the waves of the current, and consequently of the
signals. The double-current key, which reverses the poles of the
battery and allows the signal currents to be of one length, that
is to say, all "dots," is employed to send the message.
Another receiving instrument employed on most of the longer cables
is the siphon recorder of Lord Kelvin, shown in figure 54, which
marks or writes the message on a slip of travelling paper.
Essentially it is the inverse of the mirror instrument, and
consists of a light coil of wire S suspended in the field between
the poles of a strong magnet M. The coil is attached to a fine
siphon (T5) filled with ink, and sometimes kept in vibration by an
induction coil so as to shake the ink in fine drops upon a slip of
moving paper. The coil is connected between the cable and the
earth, and, as the signal current passes through, it swings to one
side or the other, pulling the siphon with it. The ink, therefore,
marks a wavy line on the paper, which is in fact a delineation of
the rise and fall of the signal current and a record of the
message. The dots in this case are represented by the waves above,
and the "dashes" by the waves below the middle line, as may be
seen in the following alphabet, which is a copy of one actually
written by the recorder on a long submarine cable.
Owing to induction, the speed of signalling on long cables is much
slower than on land lines of the same length, and only reaches
from 25 to 45 words a minute on the Atlantic cables, or 30 to 50
words with an automatic sending-key; but this rate is practically
doubled by employing the Muirhead duplex system of sending two
messages, one from each end, at the same time.
The relation of the telegraph to the telephone is analogous to
that of the lower animals and man. In a telegraph circuit, with
its clicking key at one end and its chattering sounder at the
other, we have, in fact, an apish forerunner of the exquisite
telephone, with its mysterious microphone and oracular plate.
Nevertheless, the telephone descended from the telegraph in a very
indirect manner, if at all, and certainly not through the sounder.
The first practical suggestion of an electric telephone was made
by M. Charles Bourseul, a French telegraphist, in 1854, but to all
appearance nothing came of it. In 1860, however, Philipp Reis, a
German schoolmaster, constructed a rudimentary telephone, by which
music and a few spoken words were sent. Finally, in 1876, Mr.
Alexander Graham Bell, a Scotchman, residing in Canada, and
subsequently in the United States, exhibited a capable speaking
telephone of his invention at the Centennial Exhibition,
Philadelphia.
Figure 56 represents an outside view and section of the Bell
telephone as it is now made, where M is a bar magnet having a
small bobbin or coil of fine insulated wire C girdling one pole.
In front of this coil there is a circular plate of soft iron
capable of vibrating like a diaphragm or the drum of the ear. A
cover shaped like a mouthpiece O fixes the diaphragm all round,
and the wires W W serve to connect the coil in the circuit.
The soft iron diaphragm is, of course, magnetised by the induction
of the pole, and would be attracted bodily to the pole were it not
fixed by the rim, so that only its middle is free to move. Now,
when a person speaks into the mouthpiece the sonorous waves
impinge on the diaphragm and make it vibrate in sympathy with
them. Being magnetic, the movement of the diaphragm to and from
the bobbin excites corresponding waves of electricity in the coil,
after the famous experiment of Faraday (page 64). If this
undulatory current is passed through the coil of a similar
telephone at the far end of the line, it will, by a reverse
action, set the diaphragm in vibration and reproduce the original
sonorous waves. The result is, that when another person listens at
the mouthpiece of the receiving telephone, he will hear a faithful
imitation of the original speech.
The Bell telephone is virtually a small magneto-electric generator
of electricity, and when two are joined in circuit we have a
system for the transmission of energy. As the voice is the motive
power, its talk, though distinct, is comparatively feeble, and
further improvements were made before the telephone became as
serviceable as it is now.
Edison, in 1877, was the first to invent a working telephone,
which, instead of generating the current, merely controlled the
strength of it, as the sluice of a mill-dam regulates the flow of
water in the lead. Du Moncel had observed that powder of carbon
altered in electrical resistance under pressure, and Edison found
that lamp-black was so sensitive as to change in resistance under
the impact of the sonorous waves. His transmitter consisted of a
button or wafer of lamp-black behind a diaphragm, and connected in
the circuit. On speaking to the diaphragm the sonorous waves
pressed it against the button, and so varied the strength of the
current in a sympathetic manner. The receiver of Edison was
equally ingenious, and consisted of a cylinder of prepared chalk
kept in rotation and a brass stylus rubbing on it. When the
undulatory current passed from the stylus to the chalk, the stylus
slipped on the surface, and, being connected to a diaphragm, made
it vibrate and repeat the original sounds. This "electro-
motograph" receiver was, however, given up, and a combination of
the Edison transmitter and the Bell receiver came into use.
At the end of 1877 Professor D. E. Hughes, a distinguished
Welshman, inventor of the printing telegraph, discovered that any
loose contact between two conductors had the property of
transmitting sounds by varying the strength of an electric current
passing through it. Two pieces of metal--for instance, two nails
or ends of wire--when brought into a loose or crazy contact under
a slight pressure, and traversed by a current, will transmit
speech. Two pieces of hard carbon are still better than metals,
and if properly adjusted will make the tread of a fly quite
audible in a telephone connected with them. Such is the famous
"microphone," by which a faint sound can be magnified to the ear.
Figure 57 represents what is known as the "pencil" microphone, in
which M is a pointed rod of hard carbon, delicately poised between
two brackets of carbon, which are connected in circuit with a
battery B and a Bell telephone T. The joints of rod and bracket
are so sensitive that the current flowing across them is affected
in strength by the slightest vibration, even the walking of an
insect. If, therefore, we speak near this microphone, the sonorous
waves, causing the pencil to vibrate, will so vary the current in
accordance with them as to reproduce the sounds of the voice in
the telephone.
The true nature of the microphone is not yet known, but it is
evident that the air or ether between the surfaces in contact
plays an important part in varying the resistance, and, therefore,
the current. In fact, a small "voltaic arc," not luminous, but
dark, seems to be formed between the points, and the vibrations
probably alter its length, and, consequently, its resistance. The
fact that a microphone is reversible and can act as a receiver,
though a poor one, tends to confirm this theory. Moreover, it is
not unlikely that the slipping of the stylus in the
electromotograph is due to a similar cause. Be this as it may,
there can be no doubt that carbon powder and the lamp-black of the
Edison button are essentially a cluster of microphones.
Many varieties of the Hughes microphone under different names are
now employed as transmitters in connection with the Bell
telephone. Figure 58 represents a simple micro-telephone circuit,
where M is the Hughes microphone transmitter, T the Bell telephone
receiver, JB the battery, and E E the earth-plates; but sometimes
a return wire is used in place of the "earth." The line wire is
usually of copper and its alloys, which are more suitable than
iron, especially for long distances. Just as the signal currents
in a submarine cable induce corresponding currents in the sea
water which retard them, so the currents in a land wire induce
corresponding currents in the earth, but in aerial lines the earth
is generally so far away that the consequent retardation is
negligible except in fast working on long lines. The Bell
telephone, however, is extremely sensitive, and this induction
affects it so much that a conversation through one wire can be
overheard on a neighbouring wire. Moreover, there is such a thing
as "self-induction" in a wire--that is to say, a current in a wire
tends to induce an opposite current in the same wire, which is
practically equivalent to an increase of resistance in the wire.
It is particularly observed at the starting and stopping of a
current, and gives rise to what is called the "extra-spark" seen
in breaking the circuit of an induction coil. It is also active in
the vibratory currents of the telephone, and, like ordinary
induction, tends to retard their passage. Copper being less
susceptible of self-induction than iron, is preferred for trunk
lines. The disturbing effect of ordinary induction is avoided by
using a return wire or loop circuit, and crossing the going and
coming wires so as to make them exchange places at intervals.
Moreover, it is found that an induction coil in the telephone
circuit, like a condenser in the cable circuit, improves the
working, and hence it is usual to join the battery and transmitter
with the primary wire, and the secondary wire with the line and
the receiver.
The longest telephone line as yet made is that from New York to
Chicago, a distance of 950 miles. It is made of thick copper wire,
erected on cedar poles 35 feet above the ground.
Induction is so strong on submarine cables of 50 or 100 miles in
length that the delicate waves of the telephone current are
smoothed away, and the speech is either muffled or entirely
stifled. Nevertheless, a telephone cable 20 miles long was laid
between Dover and Calais in 1891, and another between Stranraer
and Donaghadee more recently, thus placing Great Britain on
speaking terms with France and other parts of the Continent.
Figure 59 shows a form of telephone apparatus employed in the
United Kingdom. In it the transmitter and receiver, together with
a call-bell, which are required at each end of the line, are
neatly combined. The transmitter is a Blake microphone, in which
the loose joint is a contact of platinum on hard carbon. It is
fitted up inside the box, together with an induction coil, and M
is the mouthpiece for speaking to it. The receiver is a pair of
Bell telephones T T, which are detached from their hooks and held
to the ear. A call-bell B serves to "ring up" the correspondent at
the other end of the line.
Excepting private lines, the telephone is worked on the "exchange
system"--that is to say, the wires running to different persons
converge in a central exchange, where, by means of an apparatus
called a "switch board," they are connected together for the
purpose of conversation
A telephone exchange would make an excellent subject for the
artist. He delights to paint us a row of Venetian bead stringers
or a band of Sevilhan cigarette makers, but why does he shirk a
bevy of industrious girls working a telephone exchange? Let us
peep into one of these retired haunts, where the modern Fates are
cutting and joining the lines of electric speech between man and
man in a great city.
The scene is a long, handsome room or gallery, with a singular
piece of furniture in the shape of an L occupying the middle. This
is the switchboard, in which the wires from the offices and homes
of the subscribers are concentrated like the nerves in a ganglion.
It is known as the "multiple switchboard," an American invention,
and is divided into sections, over which the operators preside.
The lines of all the subscribers are brought to each section, so
that the operator can cross connect any two lines in the whole
system without leaving her chair. Each section of the board is, in
fact, an epitome of the whole, but it is physically impossible for
a single operator to make all the connections of a large exchange,
and the work is distributed amongst them. A multiplicity of wires
is therefore needed to connect, say, two thousand subscribers.
These are all concealed, however, at the back of the board, and in
charge of the electricians. The young lady operators have nothing
to do with these, and so much the better for them, as it would
puzzle their minds a good deal worse than a ravelled skein of
thread. Their duty is to sit in front of the board in comfortable
seats at a long table and make the needful connections. The call
signal of a subscriber is given by the drop of a disc bearing his
number. The operator then asks the subscriber by telephone what he
wants, and on hearing the number of the other subscriber he wishes
to speak with, she takes up a pair of brass plugs coupled by a
flexible conductor and joins the lines of the subscribers on the
switchboard by simply thrusting the plugs into holes corresponding
to the wires. The subscribers are then free to talk with each
other undisturbed, and the end of the conversation is signalled to
the operator. Every instant the call discs are dropping, the
connecting plugs are thrust into the holes, and the girls are
asking "Hullo! hullo!" "Are you there?" "Who are you?" "Have you
finished?" Yet all this constant activity goes on quietly, deftly
--we might say elegantly--and in comparative silence, for the low
tones of the girlish voices are soft and pleasing, and the harsher
sounds of the subscriber are unheard in the room by all save the
operator who attends to him.
CHAPTER VII.
ELECTRIC LIGHT AND HEAT.
The electric spark was, of course, familiar to the early
experimenters with electricity, but the electric light, as we know
it, was first discovered by Sir Humphrey Davy, the Cornish
philosopher, in the year 1811 or thereabout. With the magic of his
genius Davy transformed the spark into a brilliant glow by passing
it between two points of carbon instead of metal. If, as in figure
60, we twist the wires (+ and--) which come from a voltaic
battery, say of 20 cells, about two carbon pencils, and bring
their tips together in order to start the current, then draw them
a little apart, we shall produce an artificial or mimic star. A
sheet of dazzling light, which is called the electric arc, is seen
to bridge the gap. It is not a true flame, for there is little
combustion, but rather a nebulous blaze of silvery lustre in a
bluish veil of heated air. The points of carbon are white-hot, and
the positive is eaten away into a hollow or crater by the current,
which violently tears its particles from their seat and whirls
them into the fierce vortex of the arc. The negative remains
pointed, but it is also worn away about half as fast as the
positive. This wasting of the carbons tends to widen the arc too
much and break the current, hence in arc lamps meant to yield the
light for hours the sticks are made of a good length, and a self-
acting mechanism feeds them forward to the arc as they are slowly
consumed, thus maintaining the splendour of the illumination.
Many ingenious lamps have been devised by Serrin, Dubosq, Siemens,
Brockie, and others, some regulating the arc by clockwork and
electro-magnetism, or by thermal and other effects of the current.
They are chiefly used for lighting halls and railway stations,
streets and open spaces, search-lights and lighthouses. They are
sometimes naked, but as a rule their brightness is tempered by
globes of ground or opal glass. In search-lights a parabolic
mirror projects all the rays in any one direction, and in
lighthouses the arc is placed in the focus of the condensing
lenses, and the beam is visible for at least twenty or thirty
miles on clear nights. Very powerful arc lights, equivalent to
hundreds of thousands of candles, can be seen for 100 or 150
miles.
Figure 61 illustrates the Pilsen lamp, in which the positive
Carbon G runs on rollers rr through the hollow interior of two
solenoids or coils of wire MM' and carries at its middle a
spindle-shaped piece of soft iron C. The current flows through the
solenoid M on its way to the arc, but a branch or shunted portion
of it flows through the solenoid M', and as both of these
solenoids act as electromagnets on the soft iron C, each tending
to suck it into its interior, the iron rests between them when
their powers are balanced. When, however, the arc grows too wide,
and the current therefore becomes too weak, the shunt solenoid M'
gains a purchase over the main solenoid M, and, pulling the iron
core towards it, feeds the positive carbon to the arc. In this way
the balance of the solenoids is readjusted, the current regains
its normal strength, the arc its proper width, and the light its
brilliancy.
Figure 62 is a diagrammatic representation of the Brush arc lamp.
X and Y are the line terminals connecting the lamp in circuit. On
the one hand, the current splits and passes around the hollow
spools H H', thence to the rod N through the carbon K, the arc,
the carbon K', and thence through the lamp frame to Y. On the
other hand, it runs in a resistance fine-wire coil around the
magnet T, thence to Y. The operation of the lamp is as follows: K
and K' being in contact, a strong current starts through the lamp
energising H and H', which suck in their core pieces N and S,
lifting C, and by it the "washer-clutch" W and the rod N and
carbon K, establishing the arc. K is lifted until the increasing
resistance of the lengthening arc weakens the current in H H' and
a balance is established. As the carbons burn away, C gradually
lowers until a stop under W holds it horizontal and allows N to
drop through W, and the lamp starts anew. If for any reason the
resistance of the lamp becomes too great, or the circuit is
broken, the increased current through T draws up its armature,
closing the contacts M, thus short-circuiting the lamp through a
thick, heavy wire coil on T, which then keeps M closed, and
prevents the dead lamp from interfering with the others on its
line. Numerous modifications of this lamp are in very general use.
Davy also found that a continuous wire or stick of carbon could be
made white-hot by sending a sufficient current through it, and
this fact is the basis of the incandescent lamp now so common in
our homes.
Wires of platinum, iridium, and other inoxidisable metals raised
to incandescence by the current are useful in firing mines, but
they are not quite suitable for yielding a light, because at a
very high temperature they begin to melt. Every solid body becomes
red-hot--that is to say, emits rays of red light, at a temperature
of about 1000 degrees Fahrenheit, yellow rays at 1300 degrees,
blue rays at 1500 degrees, and white light at 2000 degrees. It is
found, however, that as the temperature of a wire is pushed beyond
this figure the light emitted becomes far more brilliant than the
increase of temperature would seem to warrant. It therefore pays
to elevate the temperature of the filament as high as possible.
Unfortunately the most refractory metals, such as platinum and
alloys of platinum with iridium, fuse at a temperature of about
3450 degrees Fahrenheit. Electricians have therefore forsaken
metals, and fallen back on carbon for producing a light. In 1845
Mr. Staite devised an incandescent lamp consisting of a fine rod
or stick of carbon rendered white-hot by the current, and to
preserve the carbon from burning in the atmosphere, he enclosed it
in a glass bulb, from which the air was exhausted by an air pump.
Edison and Swan, in 1878, and subsequently, went a step further,
and substituted a filament or fine thread of carbon for the rod.
The new lamp united the advantages of wire in point of form with
those of carbon as a material. The Edison filament was made by
cutting thin slips of bamboo and charring them, the Swan by
carbonising linen fibre with sulphuric acid. It was subsequently
found that a hard skin could be given to the filament by
"flashing" it--that is to say, heating it to incandescence by the
current in an atmosphere of hydrocarbon gas. The filament thus
treated becomes dense and resilient.
Figure 63 represents an ordinary glow lamp of the Edison-Swan
type, where E is the filament, moulded into a loop, and cemented
to two platinum wires or electrodes P penetrating the glass bulb
L, which is exhausted of air.
Platinum is chosen because it expands and contracts with
temperature about the same as glass, and hence there is little
chance of the glass cracking through unequal stress. The vacuum in
the bulb is made by a mercurial air pump of the Sprengel sort, and
the pressure of air in it is only about one-millionth of an
atmosphere. The bulb is fastened with a holder like that shown in
figure 64, where two little hooks H connected to screw terminals T
T are provided to make contact with the platinum terminals of the
lamp (P, figure 63), and the spiral spring, by pressing on the
bulb, ensures a good contact.
Fig. 65 is a cut of the ordinary Edison lamp and socket. One end
of the filament is connected to the metal screw ferule at the
base. The other end is attached to the metal button in the centre
of the extreme bottom of the base. Screwing the lamp into the
socket automatically connects the filament on one end to the
screw, on the other to an insulated plate at the bottom of the
socket.
The resistance of such a filament hot is about 200 ohms, and to
produce a good light from it the battery or dynamo ought to give
an electromotive force of at least 100 volts. Few voltaic cells or
accumulators have an electromotive force of more than 2 volts,
therefore we require a battery of 50 cells joined in series, each
cell giving 2 volts, and the whole set 100 volts. The strength of
current in the circuit must also be taken into account. To yield a
good light such a lamp requires or "takes" about 1/2 an ampere.
Hence the cells must be chosen with regard to their size and
internal resistance as well as to their kind, so that when the
battery, in series, is connected to the lamp, the resistance of
the whole circuit, including the filament or lamp, the battery
itself, and the connecting wires shall give by Ohm's law a current
of 1\2 an ampere. It will be understood that the current has the
same strength in every part of the circuit, no matter how it is
made up. Thus, if 1/2 of an ampere is flowing in the lamp, it is
also flowing in the battery and wires. An Edison-Swan lamp of this
model gives a light of about 15 candles, and is well adapted for
illuminating the interior of houses. The temperature of the carbon
filament is about 3450 degrees Fahr--that is to say, the
temperature at which platinum melts. Similar lamps of various
sizes and shapes are also made, some equivalent to as many as 100
candles, and fitted for large halls or streets, others emitting a
tiny beam like the spark of a glow-worm, and designed for medical
examinations, or lighting flowers, jewels, and dresses in theatres
or ball-rooms.
The electric incandescent lamp is pure and healthy, since it
neither burns nor pollutes the air. It is also cool and safe, for
it produces little heat, and cannot ignite any inflammable stuffs
near it. Hence its peculiar merit as a light for colliers working
in fiery mines. Independent of air, it acts equally well under
water, and is therefore used by divers. Moreover, it can be fixed
wherever a wire can be run, does not tarnish gilding, and lends
itself to the most artistic decoration.
Electric lamps are usually connected in circuit on the series,
parallel, and three wire system.
The series system is shown in figure 66, where the lamps L L
follow each other in a row like beads on a string. It is commonly
reserved for the arc lamp, which has a resistance so low that a
moderate electromotive force can overcome the added resistance of
the lamps, but, of course, if the circuit breaks at any point all
the lamps go out.
The parallel system is illustrated in figure 67, where the lamps
are connected between two main conductors cross-wise, like the
steps of a ladder. The current is thus divided into cross
channels, like water used for irrigating fields, and it is obvious
that, although the circuit is broken at one point, say by the
rupture of a filament, all the lamps do not go out.
Fig. 68 exhibits the Edison three-wire system, in which two
batteries or dynamos are connected together in series, and a third
or central main conductor is run from their middle poles. The plan
saves a return wire, for if two generators had been used
separately, four mains would have been necessary.
The parallel and three-wire systems in various groups, with or
without accumulators as local reservoirs, are chiefly employed for
incandescent lamps.
The main conductors conveying the current from the dynamos are
commonly of stout copper insulated with air like telegraph wires,
or cables coated with india-rubber or gutta-percha, and buried
underground or suspended overhead. The branch and lamp conductors
or "leads" are finer wires of copper, insulated with india-rubber
or silk.
The current of an installation or section of one is made and
broken at will by means of a "switch" or key turned by hand. It is
simply a series of metal contacts insulated from each other and
connected to the conductors, with a sliding contact connected to
the dynamo which travels over them. To guard against an excess of
current on the lamps, "cut-outs," or safety-fuses, are inserted
between the switch and the conductors, or at other leading points
in the circuit. They are usually made of short slips of metal foil
or wire, which melt or deflagrate when the current is too strong,
and thus interrupt the circuit.
There is some prospect of the luminosity excited in a vacuum tube
by the alternating currents from a dynamo or an induction coil
becoming an illuminant. Crookes has obtained exquisitely beautiful
glows by the phosphorescence of gems and other minerals in a
vacuum bulb like that shown in figure 69, where A and B are the
metal electrodes on the outside of the glass. A heap of diamonds
from various countries emit red, orange, yellow, green, and blue
rays. Ruby, sapphire, and emerald give a deep red, crimson, or
lilac phosphorescence, and sulphate of zinc a magnificent green
glow. Tesla has also shown that vacuum bulbs can be lit inside
without any outside connection with the current, by means of an
apparatus like that shown in figure 70, where D is an alternating
dynamo, C a condenser, P S the primary and secondary coils of a
sparking transformer, T T two metal sheets or plates, and SB the
exhausted bulbs. The alternating or see-saw current in this case
charges the condenser and excites the primary coil P, while the
induced current in the secondary coil 5 charges the terminal
plates T T. So long as the bulbs or tubes are kept within the
space between the plates, they are filled with a soft radiance,
and it is easy to see that if these plates covered the opposite
walls of a room, the vacuum lamps would yield a light in any part
of it.
Electric heating bids fair to become almost as important as
electric illumination. When the arc was first discovered it was
noticed that platinum, gold, quartz, ruby, and diamond--in fine,
the most refractory minerals--were melted in it, and ran like wax.
Ores and salts of the metals were also vapourised, and it was
clear that a powerful engine of research had been placed in the
hands of the chemist. As a matter of fact, the temperature of the
carbons in the arc is comparable to that of the Sun. It measures
5000 to 10,000 degrees Fahrenheit, and is the highest artificial
heat known. Sir William Siemens was among the first to make an
electric furnace heated by the arc, which fused and vapourised
metallic ores, so that the metal could be extracted from them.
Aluminium, chromium, and other valuable metals are now smelted by
its means, and rough brilliants such as those found in diamond
mines and meteoric stones have been crystallised from the fumes of
carbon, like hoar frost in a cold mist.
The electric arc is also applied to the welding of wires, boiler
plates, rails, and other metal work, by heating the parts to be
joined and fusing them together.
Cooking and heating by electricity are coming more and more into
favour, owing to their cleanliness and convenience. Kitchen
ranges, including ovens and grills, entirely heated by the
electric current, are finding their way into the best houses and
hotels. Most of these are based on the principle of incandescence,
the current heating a fine wire or other conductor of high
resistance in passing through it. Figure 71 represents an electric
kettle of this sort, which requires no outside fire to boil it,
since the current flows through fine wires of platinum or some
highly resisting metal embedded in fireproof insulating cement in
its bottom. Figures 72 and 73 are a sauce-pan and a flat-iron
heated in the same way. Figure 74 is a cigar-lighter for smoking
rooms, the fusee F consisting of short platinum wires, which
become red-hot when it is unhooked, and at the same time the lamp
Z is automatically lit. Figure 75 is an electric radiator for
heating rooms and passages, after the manner of stoves and hot
water pipes. Quilts for beds, warmed by fine wires inside, have
also been brought out, a constant temperature being maintained by
a simple regulator, and it is not unlikely that personal clothing
of the kind will soon be at the service of invalids and chilly
mortals, more especially to make them comfortable on their
travels.
An ingenious device places an electric heater inside a hot water
bag, thus keeping it at a uniform temperature for sick-room and
hospital use.
CHAPTER VIII.
ELECTRIC POWER.
On the discovery of electromagnetism (Chap. IV.), Faraday, Barlow,
and others devised experimental apparatus for producing rotary
motion from the electric current, and in 1831, Joseph Henry, the
famous American electrician, invented a small electromagnetic
engine or motor. These early machines were actuated by the current
from a voltaic battery, but in the middle of the century Jacobi
found that a dynamo-electric generator can also work as a motor,
and that by coupling two dynamos in circuit--one as a generator,
the other as a motor--it was possible to transmit mechanical power
to any distance by means of electricity. Figure 76 is a diagram of
a simple circuit for the transmission of power, where D is the
technical symbol for a dynamo as a generator, having its poles (+
and -) connected by wire to the poles of M, the distant dynamo, as
a motor. The generator D is driven by mechanical energy from any
convenient source, and transforms it into electric energy, which
flows through the circuit in the direction of the arrows, and, in
traversing the motor M, is re-transformed into mechanical energy.
There is, of course, a certain waste of energy in the process, but
with good machines and conductors, it is not more than 10 to 25
per cent., or the "efficiency" of the installation is from 75 to
90 per cent--that is to say, for every 100 horse-power put into
the generator, from 75 to 90 horse-power are given out again by
the motor.
It was not until 1870, when Gramme had improved the dynamo, that
power was practically transmitted in this way, and applied to
pumping water, and other work. Since then great progress has been
made, and electricity is now recognised, not only as a rival of
steam, but as the best means of distributing steam, wind, water,
or any other power to a distance, and bringing it to bear on the
proper point.
The first electric railway, or, rather, tramway, was built by Dr.
Werner von Siemens at Berlin in 1879, and was soon followed by
many others. The wheels of the car were driven by an electric
motor drawing its electricity from the rails, which were insulated
from the ground, and being connected to the generator, served as
conductors. It was found very difficult to insulate the rails, and
keep the electricity from leaking to the ground, however, and at
the Pans Electrical Exhibition of 1881, von Siemens made a short
tramway in which the current was drawn from a bare copper
conductor running on poles, like a telegraph wire, along the line.
The system will be understood from figure 77, where L is the
overhead conductor joined to the positive pole of the dynamo or
generator in the power house, and C is a rolling contact or
trolley wheel travelling with the car and connected by the wire W
to an electric motor M under the car, and geared to the axles.
After passing through the motor the current escapes to the rail R
by a brush or sliding contact C', and so returns to the negative
pole of the generator. A very general way is to allow the return
current to escape to the rails through the wheels. Many tramways,
covering thousands of miles, are now worked on this plan in the
United States. At Bangor, Maine, a modification of it is in use
whereby the conductor is divided into sections, alternately
connected to the positive and negative poles of two generators,
coupled together as in the "three-wire system" of electric
lighting (page 119), their middle poles being joined to the earth
--that is to say, the rails. It enables two cars to be run on the
same line at once, and with a considerable saving of copper.
To make the car independent of the conductor L for a short time,
as in switching, a battery of accumulators B may be added and
charged from the conductor, so that when the motor is disconnected
from the conductor, the discharge from the accumulator may still
work it and drive the wheels.
Attempts have been made to run tramcars with the electricity
supplied by accumulators alone, but the system is not economical
owing to the dead weight of the cells, and the periodical trouble
of recharging them at the generating station.
On heavy railroads worked by electricity the overhead conductor is
replaced by a third rail along the middle of the track, and
insulated from the ground In another system the middle conductor
is buried underground, and the current is tapped at intervals by
the motor connecting with it for a moment by means of spring
contacts as the car travels In each case, however, the outer rails
serve as the return conductors
Another system puts one or both the conductors in a conduit
underground, the trolley pole entering through a narrow slot
similar to that used on cable roads
The first electric carriages for ordinary roads were constructed
in 1889 by Mr. Magnus Volk of Brighton. Figure 78 represents one
of these made for the Sultan of Turkey, and propelled by a one-
horse-power Immisch electric motor, geared to one of the hind
wheels by means of a chain. The current for the motor was supplied
by thirty "EPS" accumulators stowed in the body of the vehicle,
and of sufficient power to give a speed of ten miles an hour. The
driver steers with a hand lever as shown, and controls the speed
by a switch in front of him.
Vans, bath chairs, and tricycles are also driven by electric
motors, but the weight of the battery is a drawback to their use.
In or about the year 1839, Jacobi sailed an electric boat on the
Neva, with the help of an electromagnetic engine of one horse-
power, fed by the current from a battery of Grove cells, and in
1882 a screw launch, carrying several passengers, and propelled by
an electric motor of three horse-power, worked by forty-five
accumulators, was tried on the Thames. Being silent and smokeless
in its action, the electric boat soon came into favour, and there
is now quite a flotilla on the river, with power stations for
charging the accumulators at various points along the banks.
Figure 79 illustrates the interior of a handsome electric launch,
the Lady Cooper, built for the "E P S," or Electric Power Storage
Company. An electric motor in the after part of the hull is
coupled directly to the shaft of the screw propeller, and fed by
"E P S" accumulators in teak boxes lodged under the deck
amidships. The screw is controlled by a switch, and the rudder by
an ordinary helm. The cabin is seven feet long, and lighted by
electric lamps. Alarm signals are given by an electric gong, and a
search-light can be brought into operation whenever it is
desirable. The speed attained by the Lady Cooper is from ten to
fifteen knots.
M. Goubet, a Frenchman, has constructed a submarine boat for
discharging torpedoes and exploring the sea bottom, which is
propelled by a screw and an electric motor fed by accumulators. It
can travel entirely under water, below the agitation of the waves,
where sea-sickness is impossible, and the inventor hopes that
vessels of the kind will yet carry passengers across the Channel.
The screw propeller of the Edison and Sim's torpedo is also driven
by an electric motor. In this case the current is conveyed from
the ship or fort which discharges the torpedo by an insulated
conductor running off a reel carried by the torpedo, the "earth"
or return half of the circuit being the sea-water.
All sorts of machinery are now worked by the electric motor--for
instance, cranes, elevators, capstans, rivetters, lathes, pumps,
chaff-cutters, and saws. Of domestic appliances, figure 80 shows
an air propeller or ventilation fan, where F is a screw-like fan
attached to the spindle of the motor M, and revolving with its
armature. Figure 81 represents a Trouve motor working a sewing-
machine, where N is the motor which gears with P the driving axle
of the machine. Figure 82 represents a fine drill actuated by a
Griscom motor. The motor M is suspended from a bracket A B C by
the tackle D E, and transmits the rotation of its armature by a
flexible shaft S T to the terminal drill O, which can be applied
at any point, and is useful in boring teeth.
Now that electricity is manufactured and distributed in towns and
villages for the electric light, it is more and more employed for
driving the lighter machinery. Steam, however, is more economical
on a large scale, and still continues to be used in great
factories for the heavier machinery. Nevertheless a day is coming
when coal, instead of being carried by rail to distant works and
cities, will be burned at the pit mouth, and its heat transformed
by means of engines and dynamos into electricity for distribution
to the surrounding country. I have shown elsewhere that peat can
be utilised in a similar manner, and how the great Bog of Allen is
virtually a neglected gold field in the heart of Ireland.
[Footnote: The Nineteenth Century for December 1894.] The sunshine
of deserts, and perhaps the electricity of the atmosphere, but at
all events the power of winds, waves, and waterfalls are also
destined to whirl the dynamo, and yield us light, heat, or motion.
Much has already been done in this direction. In 1891 the power of
turbines driven by the Falls of Neckar at Lauffen was transformed
into electricity, and transmitted by a small wire to the
Electrical Exhibition of Frankfort-on-the-Main, 117 miles away.
The city of Rome is now lighted from the Falls of Tivoli, 16 miles
distant. The finest cataract in Great Britain, the Falls of
Foyers, in the Highlands, which persons of taste and culture
wished to preserve for the nation, is being sacrificed to the
spirit of trade, and deprived of its waters for the purpose of
generating electricity to reduce aluminium from its ores.
The great scheme recently completed for utilizing the power of
Niagara Falls by means of electricity is a triumph of human
enterprise which outrivals some of the bold creations of Jules
Verne.
When in 1678 the French missionaries La Salle and Hennepin
discovered the stupendous cataract on the Niagara River between
Lake Ontario and Lake Erie, the science of electricity was in its
early infancy, and little more was known about the mysterious
force which is performing miracles in our day than its
manifestation on rubbed amber, sealing-wax, glass, and other
bodies. Nearly a hundred years had still to pass ere Franklin
should demonstrate the identity of the electric fire with
lightning, and nearly another hundred before Faraday should reveal
a mode of generating it from mechanical power. Assuredly, neither
La Salle nor his contemporaries ever dreamed of a time when the
water-power of the Falls would be distributed by means of
electricity to produce light or heat and serve all manner of
industries in the surrounding district. The awestruck Iroquois
Indians had named the cataract "Oniagahra," or Thunder of the
Waters, and believed it the dwelling-place of the Spirit of
Thunder. This poetical name is none the less appropriate now that
the modern electrician is preparing to draw his lightnings from
its waters and compel the genius loci to become his willing
bondsman.
The Falls of Niagara are situated about twenty-one miles from Lake
Erie, and fourteen miles from Lake Ontario. At this point the
Niagara River, nearly a mile broad, flowing between level banks,
and parted by several islands, is suddenly shot over a precipice
170 feet high, and making a sharp bend to the north, pursues its
course through a narrow gorge towards Lake Ontario. The Falls are
divided at the brink by Goat Island, whose primeval woods are
still thriving in their spray. The Horseshoe Fall on the Canadian
side is 812 yards, and the American Falls on the south side are
325 yards wide. For a considerable distance both above and below
the Falls the river is turbulent with rapids.
The water-power of the cataract has been employed from olden
times. The French fur-traders placed a mill beside the upper
rapids, and the early British settlers built another to saw the
timher used in their stockades. By-and-by, the Stedman and Porter
mills were established below the Falls; and subsequently, others
which derived their water-supply from the lower rapids by means of
raceways or leads. Eventually, an open hydraulic canal, three-
fourths of a mile long, was cut across the elbow of land on the
American side, through the town of Niagara Falls, between the
rapids above and the verge of the chasm below the Falls, where,
since 1874, a cluster of factories has arisen, which discharge
their spent water over the cliff in a series of cascades almost
rivalling Niagara itself. This canal, which only taps a mere drop
from the ocean of power that is running to waste, has been
utilised to the full; and the decrease of water-privileges in the
New England States, owing to the clearing of the forests and
settlement of the country, together with the growth of the
electrical industries, have led to a further demand on the
resources of Niagara.
With the example of Minneapolis, which draws the power for its
many mills from the Falls of St. Anthony, in the Mississippi
River, before them, a group of far-seeing and enterprising
citizens of Niagara Falls resolved to satisfy this requirement by
the foundation of an industrial city in the neighbourhood of the
Falls. They perceived that a better site could nowhere be found on
the American Continent. Apart from its healthy air and attractive
scenery, Niagara is a kind of half-way house between the East and
West, the consuming and the producing States. By the Erie Canal at
Tonawanda it commands the great waterway of the Lakes and the St.
Lawrence. A system of trunk railways from different parts of the
States and Canada are focussed there, and cross the river by the
Cantilever and Suspension bridges below the Falls. The New York
Central and Hudson River, the Lehigh Valley, the Buffalo,
Rochester, and Pittsburgh, the Michigan Central, and the Grand
Trunk of Canada, are some of these lines. Draining as it does the
great lakes of the interior, which have a total area of 92,000
square miles, with an aggregate basin of 290,000 square miles, the
volume of water in the Niagara River passing over the cataract
every second is something like 300,000 cubic feet; and this, with
a fall of 276 feet from the head of the upper rapids to the
whirlpool rapids below, is equivalent to about nine million, or,
allowing for waste in the turbines, say, seven million horse-
power. Moreover, the great lakes discharging--into each other form
a chain of immense reservoirs, and the level of the river being
little affected by flood or drought, the supply of pure water is
practically constant all the year round. Mr. R. C. Reid has shown
that a rainfall of three inches in twenty-four hours over the
basin of Lake Superior would take ninety days to run off into Lake
Huron, which, with Lake Michigan, would take as long to overflow
into Lake Erie; and, therefore, six months would elapse before the
full effect of the flood was expended at the Falls.
The first outcome of the movement was the Niagara River Hydraulic
Power and Sewer Company, incorporated in 1886, and succeeded by
the Niagara Falls Power Company. The old plan of utilising the
water by means of an open canal was unsuited to the circumstances,
and the company adopted that of the late Mr. Thomas Evershed,
divisional engineer of the New York State Canals. Like the other,
it consists in tapping the river above the Falls, and using the
pressure of the water to drive the number of turbines, then
restoring the water to the river below the Falls; but instead of a
surface canal, the tail-race is a hydraulic tunnel or underground
conduit. To this end some fifteen hundred acres of spare land,
having a frontage just above the upper rapids, was quietly secured
at the low price of three hundred dollars an acre; and we believe
its rise in value owing to the progress of the works is such that
a yearly rental of two hundred dollars an acre can even now be got
for it. This land has been laid out as an industrial city, with a
residential quarter for the operatives, wharves along the river,
and sidings or short lines to connect with the trunk railways. In
carrying out their purpose the company has budded and branched
into other companies--one for the purchase of the land; another
for making the railways; and a third, the Cataract Construction
Company, which is charged with the carrying out of the engineering
works, for the utilisation of the water-power, and is therefore
the most important of all. A subsidiary company has also been
formed to transmit by electricity a portion of the available power
to the city of Buffalo, at the head of the Niagara River, on Lake
Erie, some twenty miles distant. All these affiliated bodies are,
however, under the directorate of the Cataract Construction
Company; and amongst those who have taken the most active part in
the work we may mention the president, Mr. E. D. Adams; Professor
Coleman Sellers, the consulting engineer; and Professor George
Forbes, F. R. S., the consulting electrical engineer, a son of the
late Principal Forbes of Edinburgh.
In securing the necessary right of way for the hydraulic tunnel or
in the acquisitom of land, the Company has shown consummate tact.
A few proprietors declined to accept its terms, and the Company
selected a parallel route. Having obtained the right of way for
the latter, it informed the refractory owners on the first line of
their success, and intimated that the Company could now dispense
with that. On this the sticklers professed their willingness to
accept the original terms, and the bargain was concluded, thus
leaving the Company in possession of the rights of way for two
tunnels, both of which they propose to utilise.
The liberal policy of the directors is deserving of the highest
commendation. They have risen above mere "chauvinism," and instead
of narrowly confining the work to American engineers, they have
availed themselves of the best scientific counsel which the entire
world could afford. The great question as to the best means of
distributing and applying the power at their command had to be
settled; and in 1890, after Mr. Adams and Dr. Sellers had made a
visit of inspection to Europe, an International Commission was
appointed to consider the various methods submitted to them, and
award prizes to the successful competitors. Lord Kelvin (then Sir
William Thomson) was the president, and Professor W. C. Unwin, the
well-known expert in hydraulic engineering, the secretary, while
other members were Professor Mascart of the Institute, a leading
French electrician; Colonel Turretini of Geneva, and Dr. Sellers.
A large number of schemes were sent in, and many distinguished
engineers gave evidence before the Commission. The relative merits
of compressed air and electricity as a means of distributing the
power were discussed, and on the whole the balance of opinion was
in favour of electricity. Prizes of two hundred and two hundred
and fifty pounds were awarded to a number of firms who had
submitted plans, but none of these were taken up by the Company.
The impulse turbines of Messrs. Faesch & Piccard, of Geneva, who
gained a prize of two hundred and fifty pounds, have, however,
been adopted since. It is another proof of the determination of
the Company to procure the best information on the subject,
regardless of cost, that Professor Forbes had carte blanche to go
to any part of the world and make a report on any system of
electrical distribution which he might think fit.
With the selection of electricity another question arose as to the
expediency of employing continuous or alternating currents. At
that time continuous currents were chiefly in vogue, and it speaks
well for the sagacity and prescience of Professor Forbes that he
boldly advocated the adoption of alternating currents, more
especially for the transmission of power to Buffalo. His proposals
encountered strong opposition, even in the highest quarters; but
since then, partly owing to the striking success of the Lauffen to
Frankfort experiment in transmitting power by alternating currents
over a bare wire on poles a distance of more than a hundred miles,
the directors and engineers have come round to his view of the
matter, and alternating currents have been employed, at all events
for the Buffalo line, and also for the chief supply of the
industrial city. Continuous currents, flowing always in the same
direction, like the current of a battery, can, it is true, be
stored in accumulators, but they cannot be converted to higher or
lower pressure in a transformer. Alternating currents, on the
other hand, which see-saw in direction many times a second, cannot
be stored in accumulators, but they can be sent at high pressure
along a very fine wire, and then converted to higher or lower
pressures where they are wanted, and even to continuous currents.
Each kind, therefore, has its peculiar advantages, and both will
be employed to some extent.
With regard to the engineering works, the hydraulic tunnel starts
from the bank of the river where it is navigable, at a point a
mile and a half above the Falls, and after keeping by the shore,
it cuts across the bend beneath the city of Niagara Falls, and
terminates below the Suspension Bridge under the Falls at the
level of the water. It is 6700 yards long, and of a horseshoe
section, 19 feet wide by 21 feet high. It has been cut 160 feet
below the surface through the limestone and shale, but is arched
with brick, having rubble above, and at the outfall is lined on
the invert or under side with iron. The gradient is 36 feet in the
mile, and the total fall is 205 feet, of which 140 feet are
available for use. The capacity of the tunnel is 100,000 horse-
power. In the lands of the company it is 400 feet from the margin
of the river, to which it is connected by a canal, which is over
1500 feet long, 500 feet wide at the mouth, and 12 feet deep.
Out of this canal, head-races fitted with sluices conduct the
water to a number of wheel-pits 160 feet deep, which have been dug
near the edge of the canal, and communicate below with the tunnel.
At the bottom of each wheel-pit a 5000 horse-power Girard double
turbine is mounted on a vertical shaft, which drives a propeller
shaft rising to the surface of the ground; a dynamo of 5000 horse-
power is fixed on the top of this shaft, and so driven by it. The
upward pressure of the water is ingeniously contrived to relieve
the foundation of the weight of the turbine shaft and dynamo.
Twenty of these turbines, which are made by the I. P. Morris
Company of Philadelphia, from the designs of Messrs. Faesch and
Piccard, will be required to utilize the full capacity of the
tunnel.
The company possesses a strip of land extending two miles along
the shore; and in excavating the tunnel a coffer-dam was made with
the extracted rock, to keep the river from flooding the works.
This dam now forms part of a system by which a tract of land has
been reclaimed from the river. Part of it has already been
acquired by the Niagara Paper Pulp Company, which is building
gigantic factories, and will employ the tailrace or tunnel of the
Cataract Construction Company. Wharfs for the use of ships and
canal boats will also be constructed on this frontage. By land and
water the raw materials of the West will be conveyed to the
industrial town which is now coming into existence; grain from the
prairies of Illinois and Dakota; timber from the forests of
Michigan and Wisconsin; coal and copper from the mines of Lake
Superior; and what not. It is expected that one industry having a
seat there will attract others. Thus, the pulp mills will bring
the makers of paper wheels and barrels; the smelting of iron will
draw foundries and engine works; the electrical refining of copper
will lead to the establishment of wire-works, cable factories,
dynamo shops, and so on. Aluminum, too, promises to create an
important industry in the future. In the meantime, the Cataract
Construction Company is about to start an electrical factory of
its own, which will give employment to a large number of men. It
has also undertaken the water supply of the adjacent city of
Niagara Falls. The Cataract Electric Company of Buffalo has
obtained the exclusive right to use the electricity transmitted to
that city, and the line will be run in a subway. This underground
line will be more expensive to make than an overhead line, but it
will not require to be renewed every eight to fifteen years, and
it will not be liable to interruption from the heavy gales that
sweep across the lakes, or the weight of frozen sleet: moreover,
it will be more easily inspected, and quite safe for the public.
We should also add that, in addition to the contemplated duplicate
tunnel of 100,000 horse-power, the Cataract Construction Company
owns a concession for utilising 250,000 horse-power from the
Horseshoe Falls on the Canadian side in the same manner. It has
thus a virtual monopoly of the available water-power of Niagara,
and the promoters have not the least doubt that the enterprise
will be a great financial success. Already the Pittsburg Reduction
Company have begun to use the electricity in reducing aluminum
from the mineral known as bauxite, an oxide of the metal, by means
of the electric furnace.
Another portion of the power is to be used to produce carbide of
calcium for the manufacture of acetylene gas. At a recent
electrical exhibition held in New York city a model of the Niagara
plant was operated by an electric current brought from Niagara,
450 miles distant; and a collection of telephones were so
connected that the spectator could hear the roar of the real
cataract.
Thanks to the foresight of New York State and Canada, the scenery
of the Falls has been preserved by the institution of public
parks, and the works in question will do nothing to spoil it,
especially as they will be free from smoke. Mr. Bogarts, State
Engineer of New York, estimates that the water drawn from the
river will only lower the mean depth of the Falls about two
inches, and will therefore make no appreciable difference in the
view. Altogether, the enterprise is something new in the history
of the world. It is not only the grandest application of
electrical power, but one of the most remarkable feats in an age
when romance has become science, and science has become romance.
CHAPTER IX.
MINOR USES OF ELECTRICITY.
The electric "trembling bell," now in common use, was first
invented by John Mirand in 1850. Figure 83 shows the scheme of the
circuit, where
B is a small battery, say two or three "dry" or Leclanche cells,
joined by insulated wire to P, a press-button or contact key, and
G an electromagnetic gong or bell. On pressing the button P, a
spring contact is made, and the current flowing through the
circuit strikes the bell. The action of the contact key will be
understood from figure 84, where P is the press-button removed to
show the underlying mechanism, which is merely a metal spring A
over a metal plate B. The spring is connected by wire to a pole of
the battery, and the plate to a terminal or binding screw of the
bell, or vice versa. When the button P is pressed by the finger
the spring is forced against the plate, the circuit is made, and
the bell rings. On releasing the button it springs back, the
circuit is broken, and the bell stops.
Figure 85 shows the inner mechanism of the bell, which consists of
a double-poled electromagnet M, having a soft iron armature A
hinged on a straight spring or tongue S, with one end fixed, and
the other resting against a screw contact T. The hammer H projects
from the armature beside the edge of the gong E.
In passing through the instrument the current proceeds from one
terminal, say that on the right, by the wire W to the screw
contact T, and thence by the spring S through the bobbins of the
electromagnet to the other terminal. The electromagnet attracts
the armature A, and the hammer H strikes the gong; but in the act
the spring S is drawn from the contact T, and the circuit is
broken. Consequently the electromagnet, no longer excited, lets
the armature go, and the spring leaps back against the contact T,
withdrawing the hammer from the gong. But the instrument is now as
it was at first, the current again flows, and the hammer strikes
the gong, only to fly back a second time. In this way, as long as
the button is pressed by the operator, the hammer will continue to
tap the bell and give a ringing sound. Press-buttons are of
various patterns, and either affixed to the wall or inserted in
the handle of an ordinary bell-pull, as shown in figure 86.
The ordinary electric bell actuated by a battery is liable to get
out of order owing to the battery spending its force, or to the
contacts becoming dirty. Magnetoelectric bells have, therefore,
been introduced of late years. With these no battery or
interrupting contacts are required, since the bell-pull or press-
button is made in the form of a small dynamo which generates the
current when it is pulled or pushed. Figure 87 illustrates a form
of this apparatus, where M P is the bell-pull and B the bell,
these being connected by a double wire W, to convey the current.
The bell-pull consists of a horseshoe magnet M, having a bobbin of
insulated wire between its poles, and mounted on a spindle. When
the key P is turned round by the hand, the bobbin moves in the
magnetic field between the poles of the magnet, and the current
thus generated circulates in the wires W, and passing through an
electromagnet under the bell, attracts its armature, and strikes
the hammer on the bell. Of course the bell may be placed at any
distance from the generator. In other types the current is
generated and the bell rung by the act of pulling, as in a common
house-bell.
Electric bells in large houses and hotels are usually fitted up
with indicators, as shown in figure 88, which tell the room from
which the call proceeds. They are serviceable as instantaneous
signals, annunciators, and alarms in many different ways. An
outbreak of fire can be announced by causing the undue rise of
temperature to melt a piece of tallow or fusible metal, and thus
release a weight, which tails on a press-button, and closes the
circuit of an electric bell. Or, the rising temperature may expand
the mercury in a tube like that of a thermometer until it connects
two platinum wires fused through the glass and in circuit with a
bell. Some employ a curving bi-metallic spring to make the
necessary contact. The spring is made by soldering strips of brass
and iron back to back, and as these metals expand unequally when
heated, the spring is deformed, and touches the contact which is
connected in the circuit, thus permitting the current to ring the
bell. A still better device, however, is a small box containing a
thin metallic diaphragm, which expands with the heat, and sagging
in the centre, touches a contact screw, thus completing the
circuit, and allowing the current to pass.
These automatic or self-acting fire-alarms can, of course, be
connected in the circuit of the ordinary street fire-alarms, which
are usually worked by pulling a handle to make the necessary
contact.
From what has been said, it will be easy to understand how the
stealthy entrance of burglars into a house can be announced by an
electric bell or warning lamp. If press-buttons or contact-keys
are placed on the sashes of the windows, the posts of the door, or
the treads of the stair, so that when the window or door is
opened, or the tread bends under the footstep, an electric circuit
is closed, the alarm will be given. Of course, the connections
need only be arranged when the device is wanted. Shops and offices
can be guarded by making the current show a red light from a lamp
hung in front of the premises, so that the night watchman can see
it on his beat. This can readily be done by adjusting an
electromagnet to drop a screen of red glass before the flame of
the lamp. Safes and showcases forcibly opened can be made to
signal the fact, and recently in the United States a thief was
photographed by a flashlight kindled in this way, and afterwards
captured through the likeness.
The level of water in cisterns and reservoirs, can be told in a
similar manner by causing a float to rise with the water and make
the required contact. The degree of frost in a conservatory can
also be announced by means of the mercury "thermostat," already
described, or some equivalent device. There are, indeed, many
actual or possible applications of a similar kind.
The Massey log is an instrument for telling the speed of a ship by
the revolutions of a "fly" as it is towed through the water, and
by making the fly complete a circuit as it revolves the number of
turns a second can be struck by a bell on board. In one form of
the "electric log," the current is generated by the chemical
action of zinc and copper plates attached to the log, and immersed
in the sea water, and in others provided by a battery on the ship.
Captain M'Evoy has invented an alarm for torpedoes and torpedo
boats, which is a veritable watchdog of the sea. It consists of an
iron bell-jar inverted in the water, and moored at a depth below
the agitation of the waves. In the upper part of the jar, where
the pressure of the air keeps back the water, there is a delicate
needle contact in circuit with a battery and an electric bell or
lamp, as the case may be, on the shore. Waves of sound passing
through the water from the screw propeller of the torpedo, or,
indeed, any ship, make and break the sensitive contact, and ring
the bell or light the lamp. The apparatus is intended to alarm a
fleet lying at anchor or a port in time of war.
Electricity has also been employed to register the movements of
weathercocks and anemometers. A few years ago it was applied
successfully to telegraph the course marked by a steering compass
to the navigating officer on the bridge. This was done without
impeding the motion of the compass card by causing an electric
spark to jump from a light pointer on the card to a series of
metal plates round the bowl of the compass, and actuate an
electric alarm.
The "Domestic Telegraph," an American device, is a little dial
apparatus by which a citizen can signal for a policeman, doctor,
messenger, or carriage, as well as a fire engine, by the simple
act of setting a hand on the dial.
Alexander Bain was the first to drive a clock with electricity
instead of weights, by employing a pendulum having an iron bob,
which was attracted to one side and the other by an electromagnet,
but as its rate depends on the constancy of the current, which is
not easy to maintain, the invention has not come into general use.
The "butterfly clock" of Lemoine, which we illustrate in figure
89, is an improved type, in which the bob of soft iron P swings to
and fro over the poles of a double electro magnet M in circuit
with a battery and contact key. When the rate is too slow the key
is closed, and a current passing through the electromagnet pulls
on the pendulum, thus correcting the clock. This is done by the
ingenious device of Hipp, shown in figure 90, where M is the
electromagnet, P the iron bob, from which projects a wire bearing
a light vane B of mica in the shape of a butterfly. As the bob
swings the wire drags over the hump of the metal spring S, and
when the bob is going too slowly the wire thrusts the spring into
contact with another spring T below, thus closing the circuit, and
sending a current through the magnet M, which attracts the bob and
gives a fillip to the pendulum.
Local clocks controlled from a standard clock by electricity have
been more successful in practice, and are employed in several
towns--for example, Glasgow. Behind local dials are electromagnets
which, by means of an armature working a frame and ratchet wheel,
move the hands forward every minute or half-minute as the current
is sent from the standard clock.
The electrical chronograph is an instrument for measuring minute
intervals of time by means of a stylus tracing a line on a band of
travelling paper or a revolving barrel of smoked glass. The
current, by exciting an electromagnet, jerks the stylus, and the
interval between two jerks is found from the length of the trace
between them and the speed of the paper or smoked surface.
Retarded clocks are sometimes employed as electric meters for
registering the consumption of electricity. In these the current
to be measured flows through a coil beneath the bob of the
pendulum, which is a magnet, and thus affects the rate. In other
meters the current passes through a species of galvanometer called
an ampere meter, and controls a clockwork counter. In a third kind
of meter the chemical effect of the current is brought into play--
that of Edison, for example, decomposing sulphate of copper, or
more commonly of zinc.
The electric light is now used for signalling and advertising by
night in a variety of ways. Incandescent lamps inside a
translucent balloon, and their light controlled by a current key,
as in a telegraph circuit, so as to give long and short flashes,
according to the Morse code, are employed in the army. Signals at
sea are also made by a set of red and white glow-lamps, which are
combined according to the code in use. The powerful arc lamp is
extremely useful as a "search light," especially on men of war and
fortifications, and it has also been tried in signalling by
projecting the beam on the clouds by way of a screen, and
eclipsing it according to a given code.
In 1879, Professor Graham Bell, the inventor of the speaking
telephone, and Mr Summer Tamter, brought out an ingenious
apparatus called the photophone, by which music and speech were
sent along a beam of light for several hundred yards. The action
of the photophone is based on the peculiar fact observed in 1873
by Mr J E Mayhew, that the electrical resistance of crystalline
selenium diminishes when a ray of light falls upon it. Figure 91
shows how Bell and Tamter utilised this property in the telephone.
A beam of sun or electric light, concentrated by a lens L, is
reflected by a thin mirror M, and after traversing another lens L,
travels to the parabolic reflector R, in the focus of which there
is a selenium resistance in circuit with a battery S and two
telephones T T'. Now, when a person speaks into the tube at the
back of the mirror M, the light is caused to vibrate with the
sounds, and a wavering beam falls on the selenium, changing its
resistance to the current. The strength of the current is thus
varied with the sonorous waves, and the words spoken by the
transmitter are heard in the telephones by the receiver. The
photophone is, however, more of a scientific toy than a practical
instrument.
Becquerel, the French chemist, found that two plates of silver
freshly coated with silver from a solution of chloride of silver
and plunged into water, form a voltaic cell which is sensitive to
light. This can be seen by connecting the plates through a
galvanometer, and allowing a ray of light to fall upon them. Other
combinations of the kind have been discovered, and Professor
Minchin, the Irish physicist, has used one of these cells to
measure the intensity of starlight.
The "induction balance" of Professor Hughes is founded on the
well-known fact that a current passing in one wire can induce a
sympathetic current in a neighbouring wire. The arrangement will
be understood from figure 92, where P and P1 are two similar coils
or bobbins of thick wire in circuit with a battery B and a
microphone M, while S and S1 are two similar coils or bobbins of
fine wire in circuit with a telephone T. It need hardly be said
that when the microphone M is disturbed by a sound, the current in
the primary coils P P1 will induce a corresponding current in the
secondary coils S S1; but the coils S S1 are so wound that the
induction of P on S neutralises the induction of P1 on S1; and no
current passes in the secondary circuit, hence no sound is heard
in the telephone. When, however, this balance of induction is
upset by bringing a piece of metal--say, a coin--near one or other
of the coils S S1, a sound will be heard in the telephone.
The induction balance has been used as a "Sonometer" for measuring
the sense of hearing, and also for telling base coins. The writer
devised a form of it for "divining" the presence of gold and
metallic ores which has been applied by Captain M'Evoy in his
"submarine detector" for exploring the sea bottom for lost anchors
and sunken treasure. When President Garfield was shot, the
position of the bullet was ascertained by a similar arrangement.
The microphone as a means of magnifying feeble sounds has been
employed for localising the leaks in water pipes and in medical
examinations. Some years ago it saved a Russian lady from
premature burial by rendering the faint beating of her heart
audible.
Edison's electric pen is useful in copying letters. It works by
puncturing a row of minute holes along the lines of the writing,
and thus producing a stencil plate, which, when placed over a
clean sheet of paper and brushed with ink, gives a duplicate of
the writing by the ink penetrating the holes to the paper below.
It is illustrated in figure 93, where P is the pen, consisting of
a hollow stem in which a fine needle actuated by the armature of a
small electromagnet plies rapidly up and down and pierces the
paper. The current is derived from a small battery B, and an
inking roller like that used in printing serves to apply the ink.
In 1878 Mr. Edison announced his invention of a machine for the
storage and reproduction of speech, and the announcement was
received with a good deal of incredulity, notwithstanding the
partial success of Faber and others in devising mechanical
articulators. The simplicity of Edison's invention when it was
seen and heard elicited much admiration, and although his first
instrument was obviously imperfect, it was nevertheless regarded
as the germ of something better. If the words spoken into the
instrument were heard in the first place, the likeness of the
reproduction was found to be unmistakable. Indeed, so faithful was
the replica, that a member of the Academy of Sciences, Paris,
stoutly maintained that it was due to ventriloquism or some other
trickery. It was evident, however, that before the phonograph
could become a practical instrument, further improvements in the
nicety of its articulation were required. The introduction of the
electric light diverted Mr. Edison from the task of improving it,
although he does not seem to have lost faith in his pet invention.
During the next ten years he accumulated a large fortune, and was
the principal means of introducing both electric light and power
to the world at large. This done, however, he returned to his
earlier love, and has at length succeeded in perfecting it so as
to redeem his past promises and fulfill his hopes regarding it.
The old instrument consisted, as is well known, of a vibrating
tympan or drum, from the centre of which projected a steel point
or stylus, in such a manner that on speaking to the tympan its
vibrations would urge the stylus to dig into a sheet of tinfoil
moving past its point. The foil was supported on a grooved barrel,
so that the hollow of the groove behind it permitted the foil to
give under the point of the stylus, and take a corrugated or wavy
surface corresponding to the vibrations of the speech. Thus
recorded on a yielding but somewhat stiff material, these
undulations could be preserved, and at a future time made to
deflect the point of a similar stylus, and set a corresponding
diaphragm or tympan into vibration, so as to give out the original
sounds, or an imitation of them.
Tinfoil, however, is not a very satisfactory material on which to
receive the vibrations in the first place. It does not precisely
respond to the movements of the marking stylus in taking the
impression, and does not guide the receiving stylus sufficiently
well in reproducing sounds. Mr. Edison has therefore adopted wax
in preference to it; and instead of tinfoil spread on a grooved
support, he now employs a cylinder of wax to take the print of the
vibrations. Moreover, he no longer uses the same kind of diaphragm
to print and receive the sounds, but employs a more delicate one
for receiving them. The marking cylinder is now kept in motion by
an electric motor, instead of by hand-turning, as in the earlier
instrument.
The new phonograph, which we illustrate in figure 94, is about the
size of an ordinary sewing machine, and is of exquisite
workmanship, the performance depending to a great extent on the
perfection and fitness of the mechanism. It consists of a
horizontal spindle S, carrying at one end the wax cylinder C, on
which the sonorous vibrations are to be imprinted. Over the
cylinder is supported a diaphragm or tympan T, provided with a
conical mouthpiece M for speaking into. Under the tympan there is
a delicate needle or stylus, with its point projecting from the
centre of the tympan downwards to the surface of the wax cylinder,
so that when a person speaks into the mouthpiece, the voice
vibrates the tympan and drives the point of the stylus down into
the wax, making an imprint more or less deep in accordance with
the vibrations of the voice. The cylinder is kept revolving in a
spiral path, at a uniform speed, by means of an electric motor E,
fitted with a sensitive regulator and situated at the base of the
machine. The result is that a delicate and ridgy trace is cut in
the surface of wax along a spiral line. This is the sound record,
and by substituting a finer tympan for the one used in producing
it, the ridges and inequalities of the trace can be made to
agitate a light stylus resting on them, and cause it to set the
delicate tympan into vibrations corresponding very accurately to
those of the original sounds. The tympan employed for receiving is
made of gold-beater's skin, having a stud at its centre and a
springy stylus of steel wire. The sounds emitted by this device
are almost a whisper as compared to the original ones, but they
are faithful in articulation, which is the main object, and they
are conveyed to the ear by means of flexible hearing-tubes.
These tympans are interchangeable at will, and the arm which
carries them is also provided with a turning tool for smoothing
the wax cylinder prior to its receiving the print. The cylinders
are made of different sizes, from 1 to 8 inches long and 4 inches
in diameter. The former has a storage capacity of 200 words. The
next in size has twice that, or 400 words, and so on. Mr. Edison
states that four of the large 8-inch cylinders can record all
"Nicholas Nickleby," which could therefore be automatically read
to a private invalid or to a number of patients in a hospital
simultaneously, by means of a bunch of hearing-tubes. The
cylinders can be readily posted like letters, and made to deliver
their contents viva voce in a duplicate phonograph, every tone and
expression of the writer being rendered with more or less
fidelity. The phonograph has proved serviceable in recording the
languages and dialects of vanishing races, as well as in teaching
pronunciation.
The dimensions, form, and consequent appearance of the present
commercial American phonograph are quite different from that above
described, but the underlying principles and operations are
identical.
A device for lighting gas by the electric spark is shown in figure
95, where A is a flat vulcanite box, containing the apparatus
which generates the electricity, and a stem or pointer L, which
applies the spark to the gas jet. The generator consists of a
small "influence" machine, which is started by pressing the thumb-
key C on the side of the box. The rotation of a disc inside the
box produces a supply of static electricity, which passes in a
stream of sparks between two contact-points in the open end of the
stem D. The latter is tubular, and contains a wire insulated from
the metal of the tube, and forming with the tube the circuit for
the electric discharge. The handle enables the contrivance to be
readily applied. The apparatus is one of the few successful
practical applications of static electricity.
Other electric gas-lighters consist of metal points placed on the
burner, so that the electric spark from a small induction coil or
dynamo kindles the jet.
A platinum wire made white-hot by the passage of a current is
sometimes used to light lamps, as shown in figure 96, where W is a
small spiral of platinum connected in circuit with a generator by
the terminals T T. When the lamp L is pressed against the button B
the wire glows and lights it.
Explosives, such as gunpowder and guncotton, are also ignited by
the electric spark from an induction coil or the incandescence of
a wire. Figure 97 shows the interior of an ordinary electric fuse
for blasting or exploding underground mines. It consists of a box
of wood or metal primed with gunpowder or other explosive, and a
platinum wire P soldered to a pair of stout copper wires W,
insulated with gutta-percha. When the current is sent along these
wires, the platinum glows and ignites the explosive. Detonating
fuses are primed with fulminate of mercury.
Springs for watches and other purposes are tempered by heating
them with the current and quenching them in a bath of oil.
Electrical cautery is performed with an incandescent platinum wire
in lieu of the knife, especially for such operations as the
removal of the tongue or a tumour.
It was known to the ancients that a fish called a torpedo existed
in the Mediterranean which was capable of administering a shock to
persons and benumbing them. The torpedo, or "electric ray," is
found in the Atlantic as well as the Mediterranean, and is allied
to the skate. It has an electric organ composed of 800 or 1000
polygonal cells in its head, and the discharge, which appears to
be a vibratory current, passes from the back or positive pole to
the belly or negative pole through the water. The gymotus, or
Surinam eel, which attains a length of five or six feet, has an
electric organ from head to tail, and can give a shock sufficient
to kill a man. Humboldt has left a vivid picture of the frantic
struggles of wild horses driven by the Indians of Venezuela into
the ponds of the savannahs infested by these eels, in order to
make them discharge their thunderbolts and be readily caught.
Other fishes--the silurus, malapterurus, and so on--are likewise
endowed with electric batteries for stunning and capturing their
prey. The action of the organs is still a mystery, as, indeed, is
the whole subject of animal electricity. Nobili and Matteucci
discovered that feeble currents are generated by the excitation of
the nerves and the contraction of the muscles in the human
subject.
Electricity promises to become a valuable remedy, and currents--
continuous, intermittent, or alternating--are applied to the body
in nervous and muscular affections with good effect; but this
should only be done under medical advice, and with proper
apparatus.
In many cases of severe electric shock or lightning stroke, death
is merely apparent, and the person may be brought back to life by
the method of artificial respiration and rhythmic traction of the
tongue, as applied to the victims of drowning or dead faint.
A good lightning conductor should not have a higher electrical
resistance than 10 ohms from the point to the ground, including
the "earth" contact. Exceptionally good conductors have only about
5 ohms. A high resistance in the rod is due either to a flaw in
the conductor or a bad earth connection, and in such a case the
rod may be a source of danger instead of security, since the
discharge is apt to find its way through some part of the building
to the ground, rather than entirely by the rod. It is, therefore,
important to test lightning conductors from time to time, and the
magneto-electric tester of Siemens, which we illustrate in figures
98 and 99, is very serviceable for the purpose, and requires no
battery. The apparatus consists of a magneto-electric machine AT,
which generates the testing current by turning a handle, and a
Wheatstone bridge. The latter comprises a ring of German silver
wire, forming two branches. A contact lever P moves over the ring,
and is used as a battery key. A small galvanometer G shows the
indications of the testing current. A brass sliding piece S puts
the galvanometer needle in and out of action. There are also
several connecting terminals, b b', l, &c., and a comparison
resistance R (figure 98). A small key K is fixed to the terminal l
(figure 99), and used to put the current on the lightning-rod, or
take it off at will. A leather bag A at one side of the wooden
case (figure 99) holds a double conductor leading wire, which is
used for connecting the magneto-electric machine to the bridge. On
turning the handle of M the current is generated, and on closing
the key K it circulates from the terminals of the machine through
the bridge and the lightning-rod joined with the latter. The
needle of the galvanometer is deflected by it, until the
resistance in the box R is adjusted to balance that in the rod.
When this is so, the galvanometer needle remains at rest. In this
way the resistance of the rod is told, and any change in it noted.
In order to effect the test, it is necessary to have two earth
plates, E1 and E2, one (El) that of the rod, and the other (E2)
that for connecting to the testing apparatus by the terminal b1
(figure 99). The whole instrument only weighs about 9 lbs. In
order to test the "earth" alone, a copper wire should be soldered
to the rod at a convenient height above the ground, and terminal
screws fitted to it, as shown at T (figure 99), so that instead of
joining the whole rod in circuit with the apparatus, only that
part from T downwards is connected. The Hon. R. Abercrombie has
recently drawn attention to the fact that there are three types of
thunderstorm in Great Britain. The first, or squall thunderstorms,
are squalls associated with thunder and lightning. They form on
the sides of primary cyclones. The second, or commonest
thunderstorms, are associated with secondary cyclones, and are
rarely accompanied by squalls The third, or line thunderstorms,
take the form of narrow bands of rain and thunder--for example,
100 miles long by 5 to 10 miles broad. They cross the country
rapidly, and nearly broadside on. These are usually preceded by a
violent squall, like that which capsized the Eurydice.
The gloom of January, 1896, with its war and rumours of war, was,
at all events, relieved by a single bright spot. Electricity has
surprised the world with a new marvel, which confirms her title to
be regarded as the most miraculous of all the sciences. Within the
past twenty years she has given us the telephone of Bell, enabling
London to speak with Paris, and Chicago with New York; the
microphone of Hughes, which makes the tread of a fly sound like
the "tramp of an elephant," as Lord Kelvin has said; the
phonograph of Edison, in which we can hear again the voices of the
dead; the electric light which glows without air and underwater,
electric heat without fire, electric power without fuel, and a
great deal more beside. To these triumphs we must now add a means
of photographing unseen objects, such as the bony skeletons in the
living body, and so revealing the invisible.
Whether it be that the press and general public are growing more
enlightened in matters of science, or that Professor Rontgen's
discovery appeals in a peculiar way to the popular imagination, it
has certainly evoked a livelier and more sudden interest than
either the telephone, microphone, or phonograph. I was present
when Lord Kelvin first announced the invention of the telephone to
a British audience, and showed the instrument itself, but the
intelligence was received so apathetically that I suspect its
importance was hardly realised. It fell to my own lot, a few years
afterwards, to publish the first account of the phonograph in this
country, and I remember that, between incredulity on the one hand,
and perhaps lack of scientific interest on the other, a
considerable time elapsed before the public at large were really
impressed by the invention. Perhaps the uncanny and mysterious
results of Rontgen's discovery, which seem to link it with the
"black arts," have something to do with the quickness of its
reception by all manner of people.
Like most, if not all, discoveries and inventions, it is the
outcome of work already done by other men. In the early days of
electricity it was found that when an electric spark from a
frictional machine was sent through a glass bulb from which the
air had been sucked by an air pump, a cloudy light filled the
bulb, which was therefore called an "electric egg". Hittorf and
others improved on this effect by employing the spark from an
induction coil and large tubes, highly exhausted of air, or
containing a rare infusion of other gases, such as hydrogen. By
this means beautiful glows of various colours, resembling the
tender hues of the tropical sky, or the fleeting tints of the
aurora borealis, were produced, and have become familiar to us in
the well-known Geissler tubes.
Crookes, the celebrated English chemist, went still further, and
by exhausting the bulbs with an improved Sprengel air-pump,
obtained an extremely high vacuum, which gave remarkable effects
(page 120). The diffused glow or cloudy light of the tube now
shrank into a single stream, which joined the sparking points
inserted through the ends of the tube as with a luminous thread A
magnet held near the tube bent the streamer from its course; and
there was a dark space or gap in it near the negative point or
cathode, from which proceeded invisible rays, having the property
of impressing a photographic plate, and of rendering matter in
general on which they impinged phosphorescent, and, in course of
time, red-hot. Where they strike on the glass of the tube it is
seen to glow with a green or bluish phosphorescence, and it will
ultimately soften with heat.
These are the famous "cathode rays" of which we have recently
heard so much. Apparently they cannot be produced except in a very
high vacuum, where the pressure of the air is about 1-100th
millionth of an atmosphere, or that which it is some 90 or 100
miles above the earth. Mr Crookes regards them as a stream of airy
particles electrified by contact with the cathode or negative
discharging point, and repelled from it in straight lines. The
rarity of the air in the tube enables these particles to keep
their line without being jostled by the other particles of air in
the tube. A molecular bombardment from the cathode is, in his
opinion, going on, and when the shots, that is to say, the
molecules of air, strike the wall of the tube, or any other body
within the tube, the shock gives rise to phosphorescence or
fluorescence and to heat. This, in brief, is the celebrated
hypothesis of "radiant matter," which has been supported in the
United Kingdom by champions such as Lord Kelvin, Sir Gabriel
Stokes, and Professor Fitzgerald, but questioned abroad by
Goldstem, Jaumann, Wiedemann, Ebert, and others.
Lenard, a young Hungarian, pupil of the illustrious Heinrich
Hertz, was the first to inflict a serious blow on the hypothesis,
by showing that the cathode rays could exist outside the tube in
air at ordinary pressure. Hertz had found that a thin foil of
aluminium was penetrated by the rays, and Lenard made a tube
having a "window" of aluminium, through which the rays darted into
the open air. Their path could be traced by the bluish
phosphorescence which they excited in the air, and he succeeded in
getting them to penetrate a thin metal box and take a photograph
inside it. But if the rays are a stream of radiant matter which
can only exist in a high vacuum, how can they survive in air at
ordinary pressure? Lenard's experiments certainly favour the
hypothesis of their being waves in the luminiferous ether.
Professor Rontgen, of Wirzburg, profiting by Lenard's results,
accidentally discovered that the rays coming from a Crookes tube,
through the glass itself, could photograph the bones in the living
hand, coins inside a purse, and other objects covered up or hid in
the dark. Some bodies, such as flesh, paper, wood, ebonite, or
vulcanised fibre, thin sheets of metal, and so on, are more or
less transparent, and others, such as bones, carbon, quartz, thick
plates of metal, are more or less opaque to the rays. The human
hand, for example, consisting of flesh and bones, allows the rays
to pass easily through the flesh, but not through the bones.
Consequently, when it is interposed between the rays and a
photographic plate, the skeleton inside is photographed on the
plate. A lead pencil photographed in this way shows only the black
lead, and a razor with a horn handle only the blade.
Thanks to the courtesy of Mr. A. A. Campbell Swinton, of the firm
of Swinton & Stanton, the well-known electrical engineers, of
Victoria Street, Westminster, a skilful experimentalist, who was
the first to turn to the subject in England, I have witnessed the
taking of these "shadow photographs," as they are called, somewhat
erroneously, for "radiographs" or "cryptographs" would be a better
word, and shall briefly describe his method. Rontgen employs an
induction coil insulated in oil to excite the Crookes tube and
yield the rays, but Mr. Swinton uses a "high frequency current,"
obtained from apparatus similar to that of Tesla, and shown in
figure 100, namely, a high frequency induction coil insulated by
means of oil and excited by the continuous discharge of twelve
half-gallon Leyden jars charged by an alternating current at a
pressure of 20,000 volts produced by an ordinary large induction
coil sparking across its high pressure terminals.
A vacuum bulb connected between the discharge terminals of the
high frequency coil, as shown in figure 101, was illuminated with
a pink glow, which streamed from the negative to the positive
pole--that is to say, the cathode to the anode, and the glass
became luminous with bluish phosphorescence and greenish
fluorescence. Immediately under the bulb was placed my naked hand
resting on a photographic slide containing a sensitive bromide
plate covered with a plate of vulcanised fibre. An exposure of
five or ten minutes is sufficient to give a good picture of the
bones, as will be seen from the frontispiece.
The term "shadow" photograph requires a word of explanation. The
bones do not appear as flat shadows, but rounded like solid
bodies, as though the active rays passed through their substance.
According to Rontgen, these "x" rays, as he calls them, are not
true cathode rays, partly because they are not deflected by a
magnet, but cathode rays transformed by the glass of the tube; and
they are probably not ultra-violet rays, because they are not
refracted by water or reflected from surfaces. He thinks they are
the missing "longitudinal" rays of light whose existence has been
conjectured by Lord Kelvin and others--that is to say, waves in
which the ether sways to and fro along the direction of the ray,
as in the case of sound vibrations, and not from side to side
across it as in ordinary light.
Be this as it may, his discovery has opened up a new field of
research and invention. It has been found that the immediate
source of the rays is the fluorescence and phosphorescence of the
glass, and they are more effective when the fluorescence is
greenish-yellow or canary colour. Certain salts--for example, the
sulphates of zinc and of calcium, barium platino-cyanide,
tungstate of calcium, and the double sulphate of uranyle and
potassium--are more active than glass, and even emit the rays
after exposure to ordinary light, if not also in the dark.
Salvioni of Perugia has invented a "cryptoscope," which enables us
to see the hidden object without the aid of photography by
allowing the rays to fall on a plate coated with one of these
phosphorescent substances. Already the new method has been applied
by doctors in examining malformations and diseases of the bones or
internal organs, and in localising and extracting bullets,
needles, or other foreign matters in the body. There is little
doubt that it will be very useful as an adjunct to hospitals,
especially in warfare, and, if the apparatus can be reduced in
size, it will be employed by ordinary practitioners. It has also
been used to photograph the skeleton of a mummy, and to detect
true from artificial gems. However, one cannot now easily predict
its future value, and applications will be found out one after
another as time goes on.
CHAPTER X.
THE WIRELESS TELEGRAPH.
Magnetic waves generated in the ether (see pp. 53-95) by an
electric current flowing in a conductor are not the only waves
which can be set up in it by aid of electricity. A merely
stationary or "static" charge of electricity on a body, say a
brass ball, can also disturb the ether; and if the strength of the
charge is varied, ether oscillations or waves are excited. A
simple way of producing these "electric waves" in the ether is to
vary the strength of charge by drawing sparks from the charged
body. Of course this can be done according to the Morse code; and
as the waves after travelling through the ether with the speed of
light are capable of influencing conductors at a distance, it is
easy to see that signals can be sent in this way. The first to do
so in a practical manner was Signer Marconi, a young Italian
hitherto unknown to fame. In carrying out his invention, Marconi
made use of facts well known to theoretical electricians, one of
whom, Dr, Oliver J. Lodge, had even sent signals with them in
1894; but it often happens in science as in literature that the
recognised professors, the men who seem to have everything in
their favour--knowledge, even talent--the men whom most people
would expect to give us an original discovery or invention, are
beaten by an outsider whom nobody heard of, who had neither
learning, leisure, nor apparatus, but what he could pick up for
himself.
Marconi produces his waves in the ether by electric sparks passing
between four brass balls, a device of Professor Righi, following
the classical experiments of Heinrich Hertz. The balls are
electrified by connecting them to the well-known instrument called
an induction coil, sometimes used by physicians to administer
gentle shocks to invalids; and as the working of the coil is
started and stopped by an ordinary telegraph key for interrupting
the electric current, the sparking can be controlled according to
the Morse code. In our diagram, which explains the apparatus, the
four balls are seen at D, the inner and larger pair being partly
immersed in vaseline oil, the outer and smaller pair being
connected to the secondary or induced circuit of the induction
coil C, which is represented by a wavy line. The primary or
inducing circuit of the coil is connected to a battery B through a
telegraph signalling key K, so that when this key is opened and
closed by the telegraphist according to the Morse code, the
induction coil is excited for a longer or shorter time by the
current from the battery, in agreement with the longer and shorter
signals of the message. At the same time longer or shorter series
of sparks corresponding to these signals pass across the gaps
between the four balls, and give rise to longer or shorter series
of etheric waves represented by the dotted line. So much for the
"Transmitter." But how does Marconi transform these invisible
waves into visible or audible signals at the distant place? He
does this by virtue of a property discovered by Mr. S. A. Varley
as far back as 1866, and investigated by Mr. E. Branly in 1889.
They found that powder of metals, carbon, and other conductors,
while offering a great resistance to the passage of an electric
current when in a loose state, coheres together when electric
waves act upon it, and opposes much less resistance to the
electric current. It follows that if a Morse telegraph instrument
at the distant place be connected in circuit with a battery and
some loose metal dust, it can be adjusted to work when the etheric
waves pass through the dust, and only then. In the diagram R is
this Morse "Receiver" joined in circuit with a battery B1; and a
thin layer of nickel and silver dust, mixed with a trace of
mercury, is placed between two cylindrical knobs or "electrodes"
of silver fused into the glass tube d, which is exhausted of air
like an electric glow lamp. Now, when the etheric waves proceeding
from the transmitting station traverse the glass of the tube and
act upon the metal dust, the current of the battery B1 works the
Morse receiver, and marks the signals in ink on a strip of
travelling paper. Inasmuch as the dust tends to stick together
after a wave passes through it, however, it requires to be shaken
loose after each signal, and this is done by a small round hammer
head seen on the right, which gives a slight tap to the tube. The
hammer is worked by a small electromagnet E, connected to the
Morse instrument, and another battery b in what is called a
"relay" circuit; so that after the Morse instrument marks a
signal, the hammer makes a tap on the tube. As this tap has a
bell-like sound, the telegraphist can also read the signals of the
message by his ear.
Two "self-induction bobbins," L Ll, a well-known device of
electricians for opposing resistance to electric waves, are
included in the circuit of the Morse instrument the better to
confine the action of the waves to the powder in the tube.
Further, the tube d is connected to two metal conductors V Vl,
which may be compared to resonators in music. They can be adjusted
or attuned to the electric waves as a string or pipe is to
sonorous waves. In this way the receiver can be made to work only
when electric waves of a certain rate are passing through the
tube, just as a tuning-fork resounds to a certain note; it being
understood that the length of the waves can be regulated by
adjusting the balls of the transmitter. As the etheric waves
produced by the sparks, like ripples of water caused by dropping a
stone into a pool, travel in all directions from the balls, a
single transmitter can work a number of receivers at different
stations, provided these are "tuned" by adjusting the conductors V
Vl to the length of the waves.
This indeed was the condition of affairs at the time when the
young Italian transmitted messages from France to England in
March, 1899, and it is a method that since has been found useful
over limited distances. But to the inventor there seemed no reason
why wireless telegraphy should be limited by any such distances.
Accordingly he immediately developed his method and his apparatus,
having in mind the transmission of signals over considerable
intervals. The first question that arose was the effect of the
curvature of the Earth and whether the waves follow the surface of
the Earth or were propagated in straight lines, which would
require the erection of aerial towers and wires of considerable
height. Then there was the question of the amount of power
involved and whether generators or other devices could be used to
furnish waves of sufficient intensity to traverse considerable
distances.
Little by little progress was made and in January, 1901, wireless
communication was established between the Isle of Wight and Lizard
in Cornwall, a distance of 186 miles with towers less than 300
feet in height, so that it was demonstrated that the curvature of
the Earth did not seriously affect the transmission of the waves,
as towers at least a mile high would have been required in case
the waves were so cut off. This was a source of considerable
encouragement to Marconi, and his apparatus was further improved
so that the resonance of the circuit and the variation of the
capacity of the primary circuit of the oscillation transformer
made for increased efficiency. The coherer was still retained and
by the end of 1900 enough had been accomplished to warrant Marconi
in arranging for trans-Atlantic experiments between Poldhu,
Cornwall and the United States, stations being located on Cape Cod
and in Newfoundland. The trans-Atlantic transmission of signals
was quite a different matter from working over 100 miles or so in
Great Britain. The single aerial wire was supplanted by a set of
fifty almost vertical wires, supported at the top by a horizontal
wire stretched between two masts 157 1/2 feet high and 52 1/2 feet
apart, converging together at the lower end in the shape of a
large fan. The capacity of the condenser was increased and instead
of the battery a small generator was employed so that a spark 1
1/2 inches in length would be discharged between spheres 3 inches
in diameter. At the end of the year 1901 temporary stations at
Newfoundland were established and experiments were carried on with
aerial wires raised in the air by means of kites. It was here
realized that various refinements in the receiving apparatus were
necessary, and instead of the coherer a telephone was inserted in
the secondary circuit of the oscillation transformer, and with
this device on February 12th the first signals to be transmitted
across the Atlantic were heard. These early experiments were
seriously affected by the fact that the antennae or aerial wires
were constantly varying in height with the movement of the kites,
and it was found that a permanent arrangement of receiving wires,
independent of kites or balloons, was essential. Yet it was
demonstrated at this time that the transmission of electric waves
and their detection over distances of 2000 miles was distinctly
possible.
A more systematic and thorough test occurred in February, 1902,
when a receiving station was installed on the steamship
Philadelphia, proceeding from Southampton to New York. The
receiving aerial was rigged to the mainmast, the top of which was
197 feet above the level of the sea, and a syntonic receiver was
employed, enabling the signals to be recorded on the tape of an
ordinary Morse recorder. On this voyage readable messages were
received from Poldhu up to a distance of 1551 miles, and test
letters were received as far as 2099 miles. It was on this voyage
that Marconi made the interesting discovery of the effect of
sunlight on the propagation of electric waves over great
distances. He found that the waves were absorbed during the
daytime much more than at night and he eventually reached the
conclusion that the ultraviolet light from the sun ionized the
gaseous molecules of the air, and ionized air absorbs the energy
of the electric waves, so that the fact was established that clear
sunlight and blue skies, though transparent to light, serve as a
fog to the powerful Hertzian waves of wireless telegraphy. For
that reason the transmission of messages is carried on with
greater facility on the shores of England and Newfoundland across
the North Atlantic than in the clearer atmosphere of lower
latitudes. But atmospheric conditions do not affect all forms of
waves the same, and long waves with small amplitudes are far less
subject to the effect of daylight than those of large amplitude
and short wave length, and generators and circuits were arranged
to produce the former. But the difficulty did not prove
insuperable, as Marconi found that increasing the energy of the
transmitting station during the daytime would more than make up
for the loss of range.
The experiments begun at Newfoundland were transferred to Nova
Scotia, and at Glace Bay in 1902 was established a station from
which messages were transmitted and experimental work carried on
until its work was temporarily interrupted by fire in 1909. Here
four wooden lattice towers, each 210 feet in height, were built at
the corner of a square 200 feet on a side, and a conical
arrangement of 400 copper wires supported on stays between the
tops of the towers and connected in the middle at the generating
station was built. Additional machinery was installed and at the
same time a station at Cape Cod for commercial work was built. In
December, 1902, regular communication was established between
Glace Bay and Poldhu, but it was only satisfactory from Canada to
England as the apparatus at the Poldhu station was less powerful
and efficient than that installed in Canada. The transmission of a
message from President Roosevelt to King Edward marked the
practical beginning of trans-Atlantic wireless telegraphy. By this
time a new device for the detection of messages was employed, as
the coherer we have described even in its improved forms was found
to possess its limitations of sensitiveness and did not respond
satisfactorily to long distance signals. A magnetic detector was
devised by Marconi while other inventors had contrived
electrolytic, mercurial, thermal, and other forms of detector,
used for the most part with a telephone receiver in order to
detect minute variations in the current caused by the reception of
the electro-magnetic waves. With one of Marconi's magnetic
detectors signals from Cape Cod were read at Poldhu.
In 1903 wireless telegraphy had reached such a development that
the transmission of news messages was attempted in March and April
of that year. But the service was suspended, owing to defects
which manifested themselves in the apparatus, and in the meantime
a new station in Ireland was erected. But there was no cessation
of the practical experiments carried on, and in 1903 the Cunard
steamship Lucania received, during her entire voyage across from
New York to Liverpool, news transmitted direct from shore to
shore. In the meantime intercommunication between ships had been
developed and the use of wireless in naval operations was
recognized as a necessity.
Various improvements from time to time were made in the aerial
wires, and in 1905 a number of horizontal wires were connected to
an aerial of the inverted cone type previously used. The
directional aerial with the horizontal wires was tried at Glace
Bay, and adopted for all the long distance stations, affording
considerable strengthening of the received signals at Poldhu
stations. Likewise improvements in the apparatus were effected at
both trans-Atlantic stations, consisting of the adoption of air
condensers composed of insulated metallic plate suspended in the
air, which were found much better than the condensers where glass
was previously used to separate the plates. For producing the
energy employed for transmitting the signals a high tension
continuous current dynamo is used. An oscillatory current of high
potential is produced in a circuit which consists of rapidly
rotating disks in connection with the dynamo and suitable
condensers.
The production of electric oscillations can be accomplished in
several ways and waves of the desired frequency and amplitude
produced. Thus in 1903 it was found by Poulsen, elaborating on a
principle first discovered by Duddell, that an oscillatory current
may be derived from an electric arc maintained under certain
conditions and that undamped high frequency waves so produced were
suitable for wireless telegraphy. This discovery was of
importance, as it was found that the waves so generated were
undamped, that is, capable of proceeding to their destination
without loss of amplitude. On this account they were especially
suitable for wireless telephony where they were early applied, as
it was found possible so to arrange a circuit with an ordinary
microphone transmitter that the amplitude of the waves would be
varied in harmony with the vibrations of the human voice. These
waves so modulated could be received by some form of sensitive
wave detector at a distant station and reproduced in the form of
sound with an ordinary telephone receiver. With undamped waves
from the arc and from special forms of generators wireless
telephony over distances as great as 200 miles has been
accomplished and over shorter distances, especially at sea and for
sea to shore, communication has found considerable application. It
is, however, an art that is just at the beginning of its
usefulness, standing in much the same relation to wireless
telegraphy that the ordinary telephone does to the familiar system
employing metallic conductors.
On the spark and arc systems various methods of wireless
telegraphy have been developed and improved so that Marconi no
longer has any monopoly of methods or instruments. Various
companies and government officials have devised or modified
systems so that to-day wireless is practically universal and is
governed by an international convention to which leading nations
of the world subscribe.
One of the recent features of wireless telegraphy of interest is
the success of various directional devices. As we have seen,
various schemes were tried by Marconi ranging from metallic
reflectors used by Hertz in his early experiments with the
electric waves to the more successful arrangement of aerial
conductors. In Europe Bellini and Tosi have developed a method for
obtaining directed aerial waves which promises to be of
considerable utility, enabling them to be projected in a single
direction just as a searchlight beam and thus restrict the number
of points at which the signals could be intercepted and read.
Likewise an arrangement was perfected which enabled a station to
determine the direction in which the waves were being projected
and consequently the bearing of another vessel or lighthouse or
other station. The fundamental principle was the arrangement of
the antennae, two triangular systems being provided on the same
mast, but in one the current is brought down in a perpendicular
direction. The action depends upon the difference of the current
in the two triangles.
Wireless telegraph apparatus is found installed in almost every
seagoing passenger vessel of large size engaged in regular
traffic, and as a means of safety as well as a convenience its
usefulness has been demonstrated. Thus on the North Atlantic the
largest liners are never out of touch with land on one side of the
ocean or the other, and news is supplied for daily papers which
are published on shipboard. Every ship in this part of the ocean
equipped with the Marconi system, for example, is in communication
on an average with four vessels supplied with instruments of the
same system every twenty-four hours. In case of danger or disaster
signals going out over the sea speedily can bring succour, as
clearly was demonstrated in the case of the collision between the
White Star steamship Republic and the steamship Florida on January
26, 1909. Here wireless danger messages were sent out as long as
the Republic was afloat and its wireless apparatus working. These
brought aid from various steamers in the vicinity and the
passengers were speedily transferred from the sinking Republic. On
April 15, 1912, the White Star liner Titanic, the largest ship
afloat, sank off Newfoundland, after colliding with an iceberg.
Wireless SOS calls for help brought several steamships to the
scene, and 703 persons from a total of 2,206, were rescued. On
October 9, 1913, the Uranium liner Volturno caught fire in mid-
ocean, and her wireless calls brought ten steamships to her aid,
which, despite a heavy sea, rescued 532 persons from a total of
657. Again, on November 14, 1913, the Spanish steamship Balmes
caught fire off Bermuda, and at her wireless call the Cunard liner
Pannonia saved all of her passengers--103. The Titanic horror led
the principal maritime nations to take immediate steps to perfect
their wireless systems, and the installation of apparatus and
operators soon became a prime requisite of the equipment of the
world's shipping. Wireless telegraphy has been developed to great
efficiency in all the leading navies, and powerful plants are
installed on all warships. The United States, Great Britain, and
Germany, most noticeably, have established shore stations, by
which they can "talk all around the world" from any ship or
station. In operation secrecy is most important. For in the navy
practically all important messages are sent in code or cipher
under all conditions while in commercial work the tapping of land
wires or the stealing of messages while illegal is physically
possible for the evil disposed yet has never proved in practice a
serious evil. The problem of interference, however, seems to have
been fairly solved by the large systems though the activity of
amateurs is often a serious disturbance for government and other
stations.
Despite the progress of wireless telegraphy it has not yet
supplanted the submarine cable and the land wire, and in
conservative opinion it will be many years before it will do so.
In fact, since Marconi's work there has been no diminution in the
number or amount of cables laid and the business handled, nor is
there prospect of such for years to come. While the cable has
answered admirably for telegraphic purposes yet for telephony over
considerable distances it has failed entirely so that wireless
telephony over oceans starts with a more than favorable outlook.
But wireless telegraphy to a large extent has made its own field
and here its work has been greatly successful. Thus when Peary's
message announcing his discovery of the North Pole came out of the
Frozen North, it was by way of the wireless station on the distant
Labrador coast that it reached an anxious and interested
civilization. It is this same wireless that watches the progress
of the fishing fleets at stations where commercial considerations
would render impossible the maintenance of a submarine cable. It
is the wireless telegraph that maintains communication in the
interior of Alaska and between islands in the Pacific and
elsewhere where conditions of development do not permit of the
more expensive installation of submarine cable or climatic or
other conditions render impossible overland lines. At sea its
advantages are obvious. Everywhere the ether responds to the
impulses of the crackling sparks, and even from the airship we
soon may expect wireless messages as the few untrodden regions of
our globe are explored.
CHAPTER XL
ELECTRO-CHEMISTRY AND ELECTRO-METALLURGY.
In no department of the application of electricity to practical
work has there been a greater development than in electro-
metallurgy and electro-chemistry. To-day there are vast industries
depending upon electrical processes and the developments of a
quarter of a century have been truly remarkable. Already more than
one-half of the copper used in the arts is derived by electrolytic
refining. The production of aluminum depends entirely on
electricity, the electric furnace as a possible rival to the blast
furnace for the production of iron and steel is being seriously
considered, and many other metallurgical processes are being
undertaken on a large scale. We have seen in our chapter on
Electrolysis how a metal may be deposited from a solution of its
salt and how this process could be used for deriving a pure metal
or for plating or coating with the desired metal the surface of
another metal or one covered with graphite. In the following pages
it is intended to take up some of the more notable accomplishments
in this field achieved by electricity, which have been developed
to a state of commercial importance.
The electric arc not only supplies light, but heat of great
intensity which the electrical engineer as well as the pure
scientist has found so valuable for many practical operations. It
is of course obvious that for most chemical operations, and
especially in the field of metallurgy, heat is required for the
separation of combinations of various elements, for their
purification, as well as for the combination with other elements
into alloys or compounds of direct utility. The usual method of
generating heat is by the combustion of some fuel, such as coal,
coke, gas or oil, and this has been utilized for hundreds of years
in smelting metals and ores and in refining the material from a
crude state. Now it may happen that a nation or region may be rich
in metalliferous ores, but possess few, if any, coal deposits.
Accordingly the ore must be mined and transported considerable
distances for treatment and the advantages of manufacturing
industries are lost to the neighborhood of its original
production. But if water power is available, as it is in many
mountainous countries where various ores are found, then this
power can be transformed into electricity which is available as
power not only in various manufacturing operations, but for
primary metallurgical work in smelting the ores and obtaining the
metal therefrom. A striking instance of this is the kingdom of
Sweden, which contains but little coal, yet is rich in minerals
and in water power, so that its waterfalls have been picturesquely
alluded to as the country's "white coal." Likewise, at Niagara
Falls a portion of the vast water power developed there has been
used in the manufacture of aluminum, calcium carbide, carborundum,
and other materials, while at other points in the United States
and Canada, not to mention Europe, large industries where
electricity is used for metallurgical or chemical work are carried
on and the erection of new plants is contemplated.
The application of electricity to metallurgical and chemical work
has been, in nearly all cases, the result of scientific research,
and elaborate experimental laboratories are maintained by the
various corporations interested in the present or future use of
electrical processes. It is recognized by many of the older
workers in this field that electrical developments are bound to
come in the near future, and while they have not installed such
appliances in their works yet they are keeping close watch of
present developments, and in many cases experimental investigation
and research is being carried on where electrical methods have not
yet been introduced generally into the plant.
Prior to 1886 the refining of copper was the only electro-
metallurgical industry and at that time it was carried on on a
very limited scale. To-day the production of electrolytic copper
as an industry is second in importance only to the actual
production of that metal. From the small refinery started by James
Elkington at Pembury in South Wales, a vast industry has developed
in which there has been a change in the size of operations and in
the details of methods rather than in the fundamental process. For
a solution of copper sulphate is employed as the electrolyte,
blocks of raw copper as the anodes, and thin sheets of pure copper
as the cathodes. The passage of the electric current, as we have
seen on page 79, in the chapter on Electrolysis, is able to
decompose the copper in the electrolyte and to precipitate
chemically pure copper on the cathode, the copper of the solution
being replenished from the raw material used as the anode by which
the current is passed into the bath. At this Welsh factory 250
tons yearly were produced, and small earthenware pots sufficed for
the electrolyte. Thirty years later one American factory alone was
able to produce at least 350 tons of electrolytic copper in
twenty-four hours, and over 400,000 tons is the aggregate output
of the refineries of the world, which is about 53 per cent, of the
total raw copper production. Of this amount 85 per cent, comes
from American refineries, whose output has more than doubled since
1900.
The chief reason for this increased output of electrolytic copper
has been the great demand for its use in the electrical industries
where not only a vast amount is consumed, but where copper of high
purity, to give the maximum conductivity required by the
electrical engineer, is demanded. When it is realized that every
dynamo is wound with copper wire and that the same material is
used for the trolley wire and for the distribution wires in
electric lighting, it will be apparent how the demand for copper
has increased in the last quarter of a century. Electrolytic
methods not only supply a purer article and are economical to
operate, especially if there is water power in the vicinity, but
the copper ores contain varying amounts of silver and gold which
can be recovered from the slimes obtained in the electrolytic
process. Wherever possible machinery has been substituted for hand
labor, the raw copper anodes have been cast, and the charging and
discharging of the vats is carried on by the most modern
mechanical methods in which efficiency and economy are secured. On
the chemical side of the process attempts have been made to
improve the electrolyte, notably by the addition of a small amount
of hydrochloric acid to prevent the loss of silver in the slimes,
and this part of the work is watched with quite as much care as
the other stages. Electric furnaces have also been constructed for
smelting copper ores, but these have not found wide application,
and the problem is one of the future. For the most part the copper
electrically refined is produced in an ordinary smelter. The mints
of the United States are now all equipped with electrolytic
refining plants to produce the pure metal needed for coinage and
they have proved most satisfactory and economical.
As the electrolytic production of copper is an industry of great
present importance, so the production of iron and steel by
electricity promises to be of the greatest future importance.
Electric furnaces for making steel are now maintained, and the
industry has passed beyond an experimental condition. But it has
not reached the point where it is competing with the Bessemer or
the open hearth process of the manufacture of steel, while for the
smelting of iron ores the electric furnace has not yet been found
practical from an economic standpoint. Before 1880 Sir William
Siemens showed that an electric arc could be used to melt iron or
steel in a crucible, and he patented an electric crucible furnace
which was the first attempt to use electricity in iron and steel
manufacture. He stated that the process would not be too costly
and that it had a great future before it. This was an application
of the intense heat of the arc, which supplies a higher
temperature than any source known except that of the sun. This
heat is used to melt the metal, in which condition various
impurities can be removed and necessary ingredients added.
Siemens' furnace did not find extensive application, largely on
account of the great metallurgical developments then taking place
in the iron industry and the thorough knowledge of metallurgical
processes as carried on, possessed by metallurgical engineers. But
the idea by no means languished, and in 1899 Paul Heroult and
other electro-metallurgists were active in developing a practical
electric furnace for iron and steel work. The Swedish engineer, F.
A. Kjellin, was also active and as the result of the efforts of
these and other workers, by 1909 electric furnaces were employed,
not only in the manufacture of special steels whose composition
and making were attended with special care, but for rails and
structural material. There were reported to be between thirty and
forty electric steel plants in various countries, and the outlook
for the future was distinctly bright. The application of electro-
metallurgy at this time was confined to the manufacture of steel,
as the smelting of iron had not emerged from the experimental
stage of its development, though extensive trials on a large scale
of various furnaces have been undertaken in Europe and by the
Canadian government at Sault Ste. Marie, where the Heroult
furnace, soon to be described, was employed. Electro-metallurgy of
steel, as in all utilization of electrical power, depends upon
obtaining electricity at a reasonable cost, and then utilizing the
heat of the arc or of the current in the most practical and
economical form. One of the pioneer furnaces for this purpose
which has seen considerable development and practical application
is the Heroult furnace, which is a tilting furnace of the crucible
type, whose operation depends upon both the heat of the arc and on
the heat produced by the resistance of the molten material. In the
Heroult process the impurities of the molten iron are washed out
by treatment with suitable slags. The furnace consists of a
crucible in the form of a closed shallow iron tank, thickly lined
with dolomite and magnazite brick, with a hearth of crushed
dolomite. The electric current enters the crucible through two
massive electrodes of solid carbon, 70 inches in length and 14
inches in diameter, so mounted that they can be moved either
vertically or horizontally by the electrician in charge. These
electrodes are water-jacketed to reduce the rate of consumption.
The furnace contains an inlet for an air blast and openings in its
covering for charging the material and for the escape of the
gases. The actual process of steel-making consists of charging the
crucible with steel scrap, pig iron, iron ore, and lime of the
proper quality and in the right proportions, placing this material
on the hearth of the furnace. Combined arc and resistance heating
is applied to raise the charge to the melting point. The current
is of 120 volts or the same as that used in an ordinary
incandescent lighting circuit, but is alternating and of 4,000
amperes. This is for a three-ton furnace. As the material melts
the lime and silicates form a slag which fuses rapidly and covers
the iron and steel in the crucible, so that the molten bath is
protected from the action of the gases which are liberated and the
oxygen in the atmosphere. The next step in the process is to lower
the electrodes until they just touch beneath the surface of the
molten slag so that subsequent heating is due not to the effect of
the arc but to the resistance which the bath offers to the passage
of the current.
Air from an air blast is introduced into the crucible to oxidize
the impurities of the metal, particularly the sulphur and the
phosphorus which are carried into the slag and this is removed by
the tilting of the furnace. Fresh quantities of lime, etc., are
added, and the operation is repeated until a comparatively pure
metal remains, when an alloy high in carbon is added and whatever
other constituents are desired for the finished steel. The charge
is then tipped into the casting ladle and the part of the electric
furnace is finished. For three tons of steel eight to ten hours
are required in the Heroult crucible furnace.
Furnaces of an altogether different type are those employing an
alternating current, such as the Kjellin and Rochling furnaces,
where the metal to be heated really forms the secondary circuit of
a large and novel form of transformer which in principle is
analogous to the familiar transformer seen to step down the
potential of alternating current as for house lighting. For such a
transformer the primary coil is formed of heavy wire and the
secondary circuit is the molten metal which is contained in an
annular channel. The current obtained in the metal is of
considerable intensity, but at lower potential than that in the
primary coil, and roughly is equal to that of the primary
multiplied by the number of turns in the coil. The condition is
similar to that in the ordinary induction coil where the current
from a battery at low potential flows around a coil of a few turns
and is surrounded by a second coil with a large number of turns of
fine wire in which current of small intensity but of high
potential is generated. In the induction furnace the reverse takes
place and the current flowing in the metal derived from that of
the heavy coil in the primary is of great intensity. For this type
of furnace molten metal is required and the furnace is never
entirely emptied, so that its process is continuous. The
temperature attained is not as high as in the arc furnace, so that
the raw materials used have to be of a high degree of purity, and
this has proved a restriction of the field of usefulness of this
type of furnace in many cases. It, however, has been improved
recently and two rings of molten metal employed instead of one so
that a wide centre trough is obtained in which the metal is
subjected to ordinary resistance heat by direct or alternating
currents. This furnace permits of various metallurgical operations
and the elimination of impurities as in the Heroult type.
A third type of furnace that is meeting with some extensive use is
the Giroud, which, like the Heroult furnace, is based on the arc
and resistance in principle, but in its construction has a number
of different features. As the current passes horizontally from the
upper electrodes through the slag and molten metal in the furnace
chamber to the base electrodes of the furnace, it permits of the
easy regulation of the arcs and the use of lower electromotive
force, while there is only one arc in the path of the current
instead of two as in the Heroult type.
Sufficient quantities of steel have been made in electric furnaces
to permit of the determination of the quality of the product as
well as the economy of the process. It has been found in Germany
that rail steel made in the induction furnace has a much higher
bending and breaking limit than ordinary Bessemer or Thomas rail
steel, and in Germany in 1908 rails so made commanded a
considerably higher price per ton than those of ordinary rail
steel. After trial orders had proved satisfactory, in 1908 5,000
tons of rails were ordered for the Italian and Swiss governments
at a German works, where furnaces of eight tons capacity had been
installed. In the United States only a few electric steel furnaces
are in operation, and these, for the most part, for purposes of
demonstration and experiment. But in Europe the industry is well
established, and while at present small, is constantly growing and
possesses an assured future.
In addition to the manufacture of steel, the application of the
electric furnace for producing what are known as ferro-alloys, or
alloys of iron, silicon, chromium, manganese, tungsten and
vanadium, is now a large and important industry. Special steels
have their uses in different mechanical applications and the
advantage of alloying them with the rarer metals has been
demonstrated for several important purposes, as for example, the
use of chrome steel for armor plate, and steel containing vanadium
for parts of motor cars. These industries for the most part
contain electric arc furnaces and have, as their object, the
manufacture of ferro-alloys, which are introduced into the steel,
it having been found advantageous to use the rare metals in this
form rather than in their crude state.
There is one electro-metallurgical process that has made possible
the production in commercial form and for ordinary use of a metal
that once was little more than a chemical curiosity. In 1885 there
were produced 3.12 tons of aluminum, and its value was roughly
estimated at about $12 a pound. By 1908 America alone produced
over 9,000 tons valued at over $500,000,000, while European
manufacturers were also large producers. In 1888 the electrolytic
manufacture of aluminum was commenced in America and in the
following year it was begun in Switzerland. Aluminum is formed by
the electrolysis of the aluminum oxide in a fused bath of cryolite
and fluorspar. The aluminum may be obtained in the form of
bauxite, and is produced in large rectangular iron pots with a
thick carbon lining. The pot itself is the cathode, while large
graphite rods suspended in the bath serve as the anodes. After the
arc is formed and the heat of the bath rises to a sufficient
degree the material is decomposed and the metal is separated out
so that it can be removed by ladling or with a siphon. The
application of heat to obtain this metal previous to the invention
of the electric furnace could only be considered a laboratory
problem and the expense involved did not permit of commercial
application. Now, however, aluminum is universally available and
with the expiration of certain patents, the material has sold as
low as 25 cents a pound.
Electrolytic methods serve also for the refining of nickel and for
the production of lead, and as in other fields of metallurgy,
these processes are attracting the attention of chemists and of
engineers. While tin as yet has not yielded to electrolytic or
electro-thermal methods with any success, the removal of tin from
tin scraps and cuttings has been carried on with considerable
success. With zinc the electrolytic and electro-thermal processes
have not been able yet to compete with the older metallurgical
method of distillation, but an important industry is electro-
galvanizing, where a solution of zinc sulphate is deposited on
iron and gives a protective coating. Experimental methods with the
use of electricity in extracting zinc from its ores are being
tested at various European plants, but the matter has not yet
reached a commercial scale.
One of the earliest notable uses of the electric furnace in a
large electro-chemical industry was for the production of
carborundum, a carbide of silicon, which is remarkably useful as
an abrasive, being available in the manufacture of grinding stones
and other like purposes to replace emery and corundum. It is
produced by the use of a simple electric furnace of the resistance
type, where coke, sand, and sawdust are heated to a temperature of
between 2000 degrees and 3000 degrees C. The chemical reaction
involves the production of carbon monoxide, and gives a carbide of
silicon, a crystalline solid which has the excellent abrasive
properties mentioned. The manufacture was first started by its
inventor, E. G. Acheson, about 1891 on a small scale, and in the
following year 1,000 pounds of the material were produced at the
Niagara Falls works. Within fifteen years its output had increased
to well over six million pounds.
The electric furnaces at Niagara Falls have supplied many
interesting electro-chemical processes. After making a carbide in
the electric furnace it was found possible to decompose it by
further increasing the heat to a point where the second element is
volatilized and the pure carbon in the form of artificial graphite
remains. In more recent work the carbide containing the silicon
has been done away with and ordinary anthracite coal used as a
charge from which the pure graphite is obtained. This graphite has
been found especially useful in electrical work as for electrodes,
while a more recent process enables a soft variety of graphite to
be obtained which becomes a competitor of the natural material.
One of the most interesting of the many electro-chemical processes
is the heating of lime and coke in the electric furnace so as to
obtain a product in the form of calcium carbide, which, on
solution in water, forms acetylene gas, a useful and valuable
illuminant. This process dates from 1893 when T. L. Willson in the
United States first started its manufacture on a large scale, and
the great electrochemist, Henri Moissin, about the same time
independently invented a similar process as a result of his
notable work with the electric furnace. The process involves
merely a transformation at a high temperature, a portion of the
carbon in the form of coke, uniting with pulverized lime to give
the calcium carbide or CaC2. Now this material, when water is
added to it, decomposes, and acetylene or C2H2 is formed, which is
a gas of high illuminating value as the carbon separates and glows
brightly after being heated to incandescence in the flame.
The electric furnace at Niagara Falls has been able to produce
still another combination in the form of siloxicon by heating
carbon and silicon to a temperature slightly below that required
to produce carborundum. This product is a highly refractory
material and is valuable for the manufacture of crucibles,
muffles, bricks, etc., for work where extreme temperatures are
employed. The electric furnace enables various elements to be
isolated, such. as silicon, sodium, and phosphorus, and when
obtained in their pure state they find wide application.
The most important electro-chemical work of the future is to
devise some means of obtaining nitrogen from the air. It is stated
by scientists that the nitrogen of the soil is being exhausted and
that at some future time the Earth may not be able to bear crops
sufficient for the sustenance of man, unless some artificial means
be found to replenish the nitrogen. Unlimited supplies of nitrogen
exist in the air, but to fix it with other materials in such form
that it will be useful as a fertilizer has been one of the
problems to which the electro-chemists have recently devoted much
attention. By the use of the electric arc and passing air through
a furnace, various substances have been tried to take up the
nitrogen of the air. Thus when calcium carbide is heated and
brought into contact with nitrogen one atom of carbon is given up
and two atoms of nitrogen take its place, resulting in the
production of cyanamide.
Other important electro-chemical processes are involved in the
electrolysis of the various alkaline salts to obtain metallic
sodium and such products as chlorates. Thus by the electrolysis of
sodium chloride metallic sodium and chlorine is obtained. From the
metallic sodium solid caustic soda is then derived by a secondary
reaction, while the chlorine is combined with lime to form
chloride of lime or bleaching powder. In some processes the
electrolysis affords directly an alkaline hypochlorite or a
chlorate, the former being of wide commercial use as a bleaching
agent in textile works and in the paper industry. The same process
employed in the electrolysis of sodium salts is used in the case
of magnesium and calcium.
Electrolysis is also made use of in the manufacture of chloroform
and iodoform, as the chlorine or iodine which is produced in the
electrolytic cell is allowed to act upon the alcohol or acetone
under such conditions that chloroform or iodoform is produced.
Electro-chemistry plays an important part in many other industries
whose omission from our description must not be considered as
indicating any lack of their importance. New processes constantly
are being discovered which may range all the way from the
production of artificial gems to the wholesale production of the
most common chemicals used in the arts. In many branches of
chemical industry manufacturing processes have been completely
changed, and from the research laboratories, which all large
progressive manufacturers now maintain, as well as from workers in
universities and scientific schools, new methods and discoveries
are constantly forthcoming.
CHAPTER XII.
ELECTRIC RAILWAYS.
The electric railway of Dr. Werner von Siemens constructed at
Berlin in 1879 was the forerunner of a number of systems which
have had the effect of changing materially the problems of
transportation in all parts of the world. The electric railway not
only was found suitable as a substitute for the tramway with its
horse-drawn car, but far more economical than the cable cars,
which were installed to meet the transportation problems of large
cities with heavy traffic, or, as in the case of certain cities on
the Pacific slope, where heavy grades made transportation a
serious problem. Furthermore, the electric railway was found
serviceable for rural lines where small steam engines or "dummies"
were operated with limited success, and then only under
exceptional conditions. As a result, practically every country of
the world where the density of population and the state of
civilization has warranted, is traversed by a network of electric
railways, securing the most complete intercommunication between
the various localities and handling local transportation in a
manner impossible for a railway line employing steam locomotives.
The great advance in electric transportation, aside from its
meeting an economic need, has been due to the development of
systems of generating and transmitting power economically over
long distances. If water power is available, turbines and electric
generators can be installed and power produced and transmitted
over long distances, as, for example, from Niagara Falls to
Buffalo, or even to much greater distances as in the case of power
plants on the Pacific coast where mountain streams and lakes are
employed for this purpose with considerable efficiency. A high
tension alternating current thus can be transmitted over
considerable distances and then transformed into direct current
which flows along the trolley wires and is utilized in the motors.
This transformation is usually accomplished by means of a rotary
converter, that is, an alternating current motor which carries
with it the essential elements of a direct current dynamo and
receiving the alternating current of high potential turns it out
in the form of direct current at a, lower and standard potential.
The alternating current at high potential can be transmitted over
long distances with a minimum of loss, while the direct current at
lower potential is more suitable for the motor and can be used
with greater advantage, yet its potential or pressure decreases
rapidly over long lengths of line, so that it is more economical
to use sub-stations to convert the alternating current from the
power plant. It must not be inferred, however, that all electric
railways employ direct current machinery. In Europe alternating
current has been used with great success and also in the United
States where a number of lines have been equipped with this form
of power. But the greater number of installations employ the
direct current at about 500-600 volts and this is now the usual
practice. Whether it will continue so in the future or not is
perhaps an open question.
The electric car, as we have seen, employs a motor which is geared
to the axle of the driving trucks, and the current is derived from
the trolley wire by the familiar pole and wheel and after flowing
through the controller to the motor returns by the rail. The speed
of the car is regulated by the amount of current which the
motorman allows to pass through the motor and the circuits through
which it flows in order to produce different effects in the
magnetic attraction of the magnet and the armature. In the
ordinary electric car for urban or suburban uses there has been a
constant increase in the power of the motor and size of the cars,
as it has been found that even large cars can be handled with the
required facility necessary in crowded streets and that they are
correspondingly more economical to maintain and operate.
The success of electric traction in large cities had been
demonstrated but a few years when it was appreciated that the
overhead wires of the trolley were unsightly and dangerous,
especially in the case of fire or the breaking of the wires or
supports. Accordingly a system was developed where the current was
obtained from conductors laid in a conduit on insulated supports
through a slot in the centre of the track between the rails. A
plow suspended from the bottom of the car was in contact with the
conductors which were steel rails mounted on insulated supports,
and through them the current passed by suitable conductors to the
controller and motors. This system found an immediate vogue in
American cities, and though more costly to install than the
overhead trolley, was far more satisfactory in its results and
appearance. In certain cities, Washington, D. C., for example, the
conduit is used in the built-up portion of the town and when the
suburbs are reached the plow is removed and the motors are
connected with the trolley wire by the usual pole and wheel.
Perhaps the most important feature of the electric railway in the
United States has been the development and increase of its
efficiency. Wherever possible traffic conditions warranted, it was
comparatively easy to secure the right of way along country
highways with little, if any, expense, and the construction of
track and poles for such work was not a particularly heavy outlay.
It was found, as we have seen, that the current could be
transmitted over considerable distances so that the opportunity
was afforded to supply transportation between two towns at some
small distance where the local business at the time of the
construction of the road would not warrant the outlay. This led to
the systems of interurban lines, small at first, but as their
success was demonstrated, gradually extending and uniting so that
not only two important towns were connected, but eventually a
large territory was supplied with adequate transportation
facilities and even mail, express, and light freight could be
handled.
Again the success of such enterprises made it feasible for the
electric railways to forsake the public highway and to secure a
right of way of their own, and gradually to develop express and
through service, often in direct competition with the local
service of the steam railways in the same territory. Here larger
cars were required and power stations of the most modern and
efficient type in order to secure proper economy of operation. The
general character of machinery, both generators and motors, was
preserved even for these long distance lines, and their operation
became simply an engineering problem to secure the maximum
efficiency with a minimum expenditure.
With the success of electric railways in cities and for suburban
and interurban service naturally arose the question, why electric
power whose availability and economy had been shown in so many
circumstances could not be used for the great trunk lines where
steam locomotives have been developed and employed for so many
years? The question is not entirely one of engineering unless as
part of the engineering problem we consider the various economic
elements that enter into the question, and their investigation is
the important task of the twentieth century engineer. For he must
answer the question not only is a method possible mechanically,
but is it profitable from a practical and economic standpoint? And
it is here that the question of the electrification of trunk lines
now rests. The steam locomotive has been developed to a point
perhaps of almost maximum efficiency where the greatest speed and
power have been secured that are possible on machines limited by
the standard gauge of the track, 4 feet 8 1/2 inches, and the
curves which present railway lines and conditions of construction
demand. Now, withal, the steam locomotive mechanically considered
is inefficient, as it must take with it a large weight of fuel and
water which must be transformed into steam under fixed conditions.
If for example, we have one train a day working over a certain
line, there would be no question of the economy of a steam
locomotive, but with a number, we are simply maintaining isolated
units for the production of power which could be developed to far
greater advantage in a central plant. Just as the factory is more
economical than a number of workers engaged at their homes, and
the large establishment of the trust still more economical in
production than a number of factories, so the central power
station producing electricity which can be transmitted along a
line and used as required is obviously more advantageous than
separate units producing power on the spot with various losses
inherent in small machines.
But even if the central station is theoretically superior and more
economical it does not imply that it is either good policy or
economy to electrify at once all the trunk lines of a country such
as the United States and to send to the scrap heap thousands of
good locomotives at the sacrifice of millions of dollars and the
outlay of millions more for electrical equipment. In other words,
unless the financial returns will warrant it, there is no good and
positive reason for the electrification of our great trans-
continental lines and even shorter railroads. That is the
situation to-day, but to-morrow is another question, and the far-
seeing railroad man must be ready with his answer and with his
preparations. To-day terminal services in large cities can better
be performed by electricity, and not only is there economy in
their operation, but the absence of dirt, smoke and noise is in
accord with public sentiment if not positively demanded by statute
or ordinance. Suburban service can be worked much more
economically and effectively by trains of motor cars, and time
table and schedule are not limited by the number of available
locomotives on a line so equipped. On mountain grades, where
auxiliary power or engines of extreme capacity are required,
electricity generated by water power from melting snow or mountain
lakes or streams in the vicinity may be availed of. Under such
conditions powerful motors can be used on mountain divisions, not
only with economy, but with increased comfort to passengers,
especially where there are long tunnels. All this and more the
railway man of to-day realizes, and electrification to this extent
has been accomplished or is in course of construction. For each
one of the services mentioned typical installations can be given
as examples, and to accomplish the various ends, there is not only
one system but several systems of electrical working, which have
been devised by electrical engineers to meet the difficulties.
To summarize then, electric working of a trunk line results in
increased economy over steam locomotives by concentration of the
power and especially by the use of water power where possible.
Thus economy is secured to the greatest extent by a complete
electrical service and not by a mixed service of electric and
steam locomotives. Electrification gives an increase in capacity
both in the haulage by a locomotive, an electric locomotive being
capable of more work than a steam locomotive, and in schedule and
rate of speed, as motor car trains and electric terminal
facilities make possible augmented traffic, and an increased use
of dead parts of the system such as track and roadbed. There is a
great gain in time of acceleration and for stopping, and for the
Boston terminal it was estimated that with electricity 50 per
cent, more traffic could be handled, as the headway could be
reduced from three to two minutes. The modern tendency of
electrification deals either with special conditions or where the
traffic is comparatively dense. From such a beginning it is
inevitable that electric working should be extended and that is
the tendency in all modern installations, as for example, at the
New York terminal of the New York Central and Hudson River
Railroad where the electric zone, first installed within little
more than station limits, is gradually being extended. As examples
of density of traffic suitable for electrification, yet at the
same time possessing problems of their own, are the great
terminals such as the Grand Central Station of the New York
Central and Hudson River Railroad in New York City, the new
Pennsylvania Station in the same city, and that of the Illinois
Central Station in the city of Chicago. Not only is there density
here but the varied character of the service rendered, such as
express, local, suburban, and freight, involves the prompt and
efficient handling of trains and cars. Now, with suburban trains
made up of motor cars, a certain number of locomotives otherwise
employed are released; for these cars can be operated or shifted
by their own power. Such terminal stations are often combined with
tunnel sections, as in the case of the great Pennsylvania
terminal, where the tunnel begins at Bergen, New Jersey, and
extends under the Hudson River, beneath Manhattan Island and under
the East River to Long Island City. It is here that electric
working is essential for the comfort of passengers as well as for
efficient operation. But there are tunnel sections not connected
with such vast terminals, as in the case of the St. Clair tunnel
under the Detroit River.
While the field and future direction of electrification is fairly
well outlined and its future is assured, yet this future will be
one of steady progress rather than one of sudden upheaval for the
economic reasons before stated. To-day there are no final
standards either of systems or of motors and the field is open for
the final evolution of the most efficient methods. Notwithstanding
the extraordinary progress that has been made many further
developments are not only possible now but will be demanded with
the progress of the art.
The great problem of the electric railway is the transmission of
energy, and while power may be economically generated at the
central station, yet, as Mr. Frank J. Sprague, one of the pioneers
and foremost workers in the electrical engineering of railways has
so aptly said, it is still at that central station and it will
suffer a certain diminution in being carried to the point of
utilization as well as in being transformed into power to move
locomotives, so that these two considerations lie at the bottom of
the electric railway and on them depend the choice of the system
and the design and construction of the motor. The two fundamental
systems for electric railways, as in other power problems, are the
direct current and the alternating current. In the former we have
the familiar trolley wire, fed perhaps by auxiliary conductors
carried on the supporting poles or the underground trolley in the
conduit, or the third rail laid at the side of the track. All of
these have become standard practice and are operated at the usual
voltage of from 500 to 600 volts. The current on lines of any
considerable length is alternating current, supplied from large
central generating stations and transformed to direct as occasion
may demand at suitable sub-stations. Recently there has been a
tendency to employ high voltage direct current systems where the
advantages of the use of direct current motors are combined with
the economies of high voltage transmission, chief of which are the
avoiding of power losses in transmission and the economy in the
first cost of copper. These high voltage direct current lines were
first used in Europe, and during the year 1907 experimental lines
on the Vienna railway were tested. IN Germany and Switzerland
tests were made of direct current system of 2,000 and 3,000 volts
and in 1908 there was completed the first section of a 1,200-volt
direct current line between Indianapolis and Louisville, which
marked the first use of high tension direct current in the United
States, and this was followed by other successful installations.
With alternating current there can be used the various forms of
single phase or polyphase current familiar in power work, but the
latter is now preferred, and in Europe and in the United States in
the latter part of 1908 the number of single phase lines was
estimated at 27 and 28 respectively, with a total mileage of 782
and 967 miles. A trolley wire or suspended conductor is used. To
employ a single phase current, motors of either the repulsion type
or of the series type are used and are of heavier weight than the
direct current motors, as they must combine the functions of a
transformer and a motor. It is for this reason that we often see
two electric locomotives at the head of a single train on lines
where the single phase system is employed, while on neighboring
lines using direct current, one locomotive of hardly larger size
suffices. With the polyphase current a motor with a rotating field
is used, and they have considerable efficiency as regards weight
when compared with the single phase and with the direct current
motor. The polyphase motor, however, is open to the objection that
it does not lend itself to regulations as well as the direct
current form, and with ingenious devices involving the arrangement
of the magnetic field and the combination of motors, various
running speeds can be had. The usual voltage for these motors is
3,000 volts, but in the polyphase plant designed for the Cascade
Tunnel 6,000 volts are to be used. They possess many advantages,
especially their ability to run at overload, and consequently a
locomotive with polyphase motor will run up grade without serious
loss of speed. The single phase system has been carried on on
Swiss and Italian railroads, notably on the Simplon Tunnel and the
Baltelina lines with great success, and the distribution problems
are reduced to a minimum. In the United States a notable
installation has been on the New York, New Haven & Hartford
Railroad, where the section between Stamford and New York has been
worked by electricity exclusively since July 1, 1908. Here the
single phase motors use direct current while running over the
tracks of the New York Central from Woodlawn to the Grand Central
Terminal. On both the New York, New Haven & Hartford and the New
York Central locomotives the armature is formed directly on the
axle of the driving wheels, so consequently much interest attaches
to the new design adopted for the Pennsylvania tunnels, where the
armatures of the direct current motors are connected with the
driving wheels by connecting rods somewhat after the fashion of
the steam locomotive, and following in this respect some
successful European practice.
APPENDIX.
UNITS OF MEASUREMENT.
(From Munro and Jamieson's Pocket-book of Electrical Rules and
Tables).
I. FUNDAMENTAL UNITS.--The electrical units are derived from the
following mechanical units:--
The Centimetre as a unit of length;
The Gramme as a unit of mass;
The Second as a unit of time.
The Centimetre is equal to 0.3937 inch in length, and nominally
represents one thousand-millionth part, or 1/1,000,000,000 of a
quadrant of the earth.
The Gramme is equal to 15.432 grains, and represents the mass of a
cubic centimetre of water at 4 degrees C. Mass is the quantity of
matter in a body.
The Second is the time of one swing of a pendulum making 86,164.09
swings in a sidereal day, or 1/86,400 part of a mean solar day.
II. DERIVED MECHANICAL UNITS.-
Area.-The unit of area is the square centimetre.
Volume.--The unit of volume is the CUBIC CENTIMETRE.
VELOCITY is rate of change of position. It involves the idea of
direction as well as that of magnitude. VELOCITY is UNIFORM when
equal spaces are traversed in equal intervals of time The unit of
velocity is the velocity of a body which moves through unit
distance in unit time, or the VELOCITY OF ONE CENTIMETRE PER
SECOND.
MOMENTUM is the quantity of motion in a body, and is measured by
mass x velocity.
ACCELERATION is the rate of change of velocity, whether that
change take place in the direction of motion or not. The unit of
acceleration is the acceleration of a body which undergoes unit
change of velocity in unit time, or an acceleration of one
centimetre-per-second per second The acceleration due to gravity
is considerably greater than this, for the velocity imparted by
gravity to falling bodies in one second is about 981 centimetres
per second (or about 32.2 feet per second). The value differs
slightly in different latitudes. At Greenwich the value of the
acceleration due to gravity is g=981.17; at the Equator g=978.1;
at the North Pole g=983.1.
FORCE is that which tends to alter a body's natural state of rest
or of uniform motion in a straight line.
FORCE is measured by the acceleration which it imparts to mass--i.
e., mass x acceleration.
THE UNIT OF FORCE, or DYNE, is that force which, acting for one
second on a mass of one gramme, gives to it a velocity of one
centimetre per second. The force with which the earth attracts any
mass is usually called the "weight" of that mass, and its value
obviously differs at different points of the earth's surface The
force with which a body gravitates--i e, its weight (in dynes), is
found by multiplying its mass (in grammes) by the value of g at
the particular place where the force is exerted.
Work is the product of a force and a distance through which it
acts. The unit of work is the work done in overcoming unit force
through unit distance--i e, in pushing a body through a distance
of one centimetre against a forch of one dyne. It is called the
Erg. Since the "weight" of one gramme is 1 X 981 or 981 dynes, the
work of raising one gramme through the height of one centimetre
against the force of gravity is 981 ergs or g ergs. One
kilogramme-metre = 100,000 (g) ergs = 9 8 1 X 10^7 ergs. One foot-
pound = 13,825 (g) ergs, = 1 356 X 10^7 ergs.
Energy is that property which, possessed by a body, gives it the
capability of doing work. Kinetic energy is the work a body can do
in virtue of its motion. Potential energy is the work a body can
do in virtue of its position. The unit of energy is the Erg.
Power or Activity is the rate of work; the practical unit is
called the Watt--10^7 ergs per second.
A Horse-power = 33,000 ft--Ibs per minute = 550 ft--Ibs per
second, but as seen above under Work, 1 ft--Ib = 1 356 X 10^7
ergs, and under Power, 1 Watt = 10^7 ergs per sec a Horsepower =
550 X 1 356 X 10^7 ergs = 746 Watts; or, =EC/746=C^2R/746=E^2/(746
R) =HP where E = volts, C = amperes, and R = ohms.
The French "force de cheval" = 75 kilogramme metres per sec = 736
Watts = 542 48 ft--lbs. per sec. = .9863 H.P.; or one H.P. =
1.01385 "force de cheval."
DERIVED ELECTRICAL UNITS.--There are two systems of electrical
units derived from the fundamental "C.G.S." units, one set being
based upon the force exerted between two quantities of
electricity, and the other upon the force exerted between two
magnetic poles. The former set are termed electro-static units,
the latter electro-magnetic units.
III. ELECTROSTATIC UNITS.--
UNIT QUANTITY of electricity is that which repels an equal and
similar quantity at unit distance with unit force in air.
UNIT CURRENT is that which conveys unit quantity of electricity
along a conductor in a second.
UNIT ELECTROMOTIVE FORCE, or unit DIFFERENCE OF POTENTIAL exists
between two points when the unit quantity of electricity in
passing from one to the other will do the unit amount of work.
UNIT RESISTANCE is that of a conductor through which unit
electromotive force between its ends can send a unit current.
UNIT CAPACITY is that of a condenser which contains unit quantity
when charged to unit difference of potential.
IV. MAGNETIC UNITS.--
UNIT MAGNETIC POLE is that which repels an equal and similar pole
at unit distance with unit force in air.
STRENGTH OF MAGNETIC FIELD at any point is measured by the force
which would act on a unit magnetic pole placed at that point.
UNIT INTENSITY OF FIELD is that intensity of field which acts on a
unit pole with unit force.
MOMENT OF A MAGNET is the strength of either pole multiplied by
the distance between the poles.
INTENSITY OF MAGNETISATION is the magnetic moment of a magnet
divided by its volume.
MAGNETIC POTENTIAL.--The potential at a point due to a magnet is
the work that must be done in removing a unit pole from that point
to an infinite distance against the magnetic attraction, or in
bringing up a unit pole from an infinite distance to that point
against the magnetic repulsion.
UNIT DIFFERENCE OF MAGNETIC POTENTIAL.--Unit difference of
magnetic potential exists between two points when it requires the
expenditure of one erg of work to bring an (N. or S.) unit
magnetic pole from one point to the other against the magnetic
forces.
V. ELECTRO-MAGNETIC UNITS.--
UNIT CURRENT is that which in a wire of unit length, bent so as to
form an arc of a circle of unit radius, would act upon a unit pole
at the centre of the circle with unit force.
UNIT QUANTITY of electricity is that which a unit current conveys
in unit time.
UNIT ELECTRO-MOTIVE FORCE or DIFFERENCE OF POTENTIAL is that which
is produced in a conductor moving through a magnetic field at such
a rate as to cut one unit line per second.
UNIT RESISTANCE is that of a conductor in which unit current is
produced by unit electro-motive force between its ends.
UNIT CAPACITY is that of a condenser which will be at unit
difference of potential when charged with unit quantity.
Electric and magnetic force varies inversely as the square of the
distance.
PRACTICAL UNITS OF ELECTRICITY.
RESISTANCE-R.--The Ohm is the resistance of a column of mercury
106.3 centimetres long, 1 square millimetre in cross-section,
weighing 14.4521 grammes, and at a temperature of 0 degrees
centigrade. Standards of wire are used for practical purposes. The
ohm is equal to a thousand million, 10^9, electromagnetic or
Centimetre-Gramme-Second ("C. G. S.") units of resistance.
The megohm is one million ohms.
The microhm is one millionth of an ohm.
ELECTROMOTIVE FORCE--E.--The Volt is that electromotive force
which maintains a current of one ampere in a conductor having a
resistance of one ohm. The electromotive force of a Clark standard
cell at a temperature of 15 degrees centigrade is 1.434 volts. The
volt is equal to a hundred million, 10^8, C. G. S. units of
electromotive force.
CURRENT--C.--The Ampere is that current which will decompose
0.09324 milligramme of water (H2O) per second or deposit 1.118
milligrammes of silver per second. It is equal to one-tenth of a
C. G. S. unit of current.
The milliampere is one thousandth of an ampere.
QUANTITY--Q.--The Coulomb is the quantity of electricity conveyed
by an ampere in a second. It is equal to one-tenth of a C. G. S.
unit of quantity.
The micro-coulomb is one millionth of a coulomb.
CAPACITY--K.--The farad is that capacity of a body, say a Leyden
jar or condenser, which a coulomb of electricity will charge to
the potential of a volt. It is equal to one thousand-millionth of
a C. G. S. unit of capacity.
The micro-farad is one millionth of a Farad.
By Ohm's Law, Current = Electromotive Force/ Resistance,
or C = E/R
Ampere = Volt/Ohm
Hence when we know any two of these quantities, we can find the
third. For example, if we know the electromotive force or
difference of potential in volts and the resistance in ohms of an
electric circuit, we can easily find the current in amperes.
POWER--P.--The Watt is the power conveyed by a current of one
ampere through a conductor whose ends differ in potential by one
volt, or, in other words, the rate of doing work when an ampere
passes through an ohm. It is equal to ten million, 10^7, C. G. S.
units of power or ergs per second, that is to say, to a Joule per
second, or 1/746 of a horse-power.
A Watt = volt X ampere, and a Horse-power = Watts/746.
HEAT OR WORK--W.--The Joule is the work done or heat generated by
a Watt in a second, that is, the work done or heat generated in a
second by an ampere flowing through the resistance of an ohm. It
is equal to ten million, 10^7, C. G. S. units of work or ergs.
Assuming "Joule's equivalent" of heat and mechanical energy to be
41,600,000, it is the heat required to raise .24 gramme of water 1
degrees centigrade. A Joule = Volt x ampere x second. Since 1
horse-power = 550 foot pounds of work per second,
W = 550/746 E. Q. = .7373 E. Q. foot pounds.
HEAT UNITS.
The British Unit is the amount of heat required to raise one pound
of water from 60 degrees to 61 degrees Fahrenheit. It is 251.9
times greater than the metric unit, therm or calorie, which is the
amount of heat required to raise one gramme of water from 4
degrees to 5 degrees centigrade.
Joule's Equivalent--J.--is the amount of energy equivalent to a
therm or calorie, the metric unit of heat. It is equal to
41,600,000 ergs.
The heat in therms generated in a wire by a current = Volt X
ampere X time in seconds X 0.24.
LIGHT UNITS
The British Unit is the light of a spermaceti candle 7/8-inch in
diameter, burning 120 grains per hour (six candles to the pound).
They sometimes vary as much as 10 per cent, from the standard. Mr.
Vernon Harcourt's standard flame is equal to an average standard
candle.
The French Unit is the light of a Carcel lamp, and is equivalent
to 9 T/Z British units.
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