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.