WHAT IS ELECTRICITY?
The question which heads this, our final chapter, is one which must
occur to every one who takes even the most casual interest in matters
scientific, and it would be very satisfactory if we could bring this
volume to a conclusion by providing a full and complete answer.
Unfortunately this is impossible. In years to come the tireless labours
of scientific investigators may lead to a solution of the problem; but,
as Professor Fleming puts it: “The question--What is electricity?--no
more admits of a complete and final answer to-day than does the
question--What is life?”
From the earliest days of electrical science theories of electricity
have been put forward. The gradual extension and development of these
theories, and the constant substitution of one idea for another as
experimental data increased, provide a fascinating subject for study.
To cover this ground however, even in outline, would necessitate many
chapters, and so it will be better to consider only the theory which,
with certain reservations in some cases, is held by the scientific
world of to-day. This is known as the _electron_ theory of electricity.
We have referred already, in Chapter XXIV., to atoms and electrons.
All matter is believed to be constituted of minute particles called
“atoms.” These atoms are so extremely small that they are quite
invisible, being far beyond the range of the most powerful microscope;
and their diameter has been estimated at somewhere about one millionth
of a millimetre. Up to a few years ago the atom was believed to be
quite indivisible, but it has been proved beyond doubt that this is
not the case. An atom may be said to consist of two parts, one much
larger than the other. The smaller part is negatively electrified,
and is the same in all atoms; while the larger part is positively
electrified, and varies according to the nature of the atom. The small
negatively electrified portion of the atom consists of particles called
“electrons,” and these electrons are believed to be indivisible units
or atoms of negative electricity. To quote Professor Fleming: “An atom
of matter in its neutral condition has been assumed to consist of an
outer shell or envelope of negative electrons associated with some
core or matrix which has an opposite electrical quality, such that if
an electron is withdrawn from the atom the latter is left positively
electrified.”
The electrons in an atom are not fixed, but move with great velocity,
in definite orbits. They repel one another, and are constantly
endeavouring to fly away from the atom, but they are held in by the
attraction of the positive core. So long as nothing occurs to upset
the constitution of the atom, a state of equilibrium is maintained
and the atom is electrically neutral; but immediately the atom is
broken up by the action of an external force of some kind, one or more
electrons break their bonds and fly away to join some other atom. An
atom which has lost some of its electrons is no longer neutral, but is
electro-positive; and similarly, an atom which has gained additional
electrons is electro-negative. Electrons, or atoms of negative
electricity, can be isolated from atoms of matter, as in the case of
the stream of electrons proceeding from the cathode of a vacuum tube.
So far, however, it has been found impossible to isolate corresponding
atoms of positive electricity.
From these facts it appears that we must regard a positively charged
body as possessing a deficiency of electrons, and a negatively charged
body as possessing an excess of electrons. In Chapter I. we spoke of
the electrification of sealing-wax or glass rods by friction, and we
saw that according to the nature of the substance used as the rubber,
the rods were either positively or negatively electrified. Apparently,
when we rub a glass rod with a piece of silk, the surface atoms of
each substance are disturbed, and a certain number of electrons leave
the glass atoms, and join the silk atoms. The surface atoms of the
glass, previously neutral, are now electro-positive through the loss
of electrons; and the surface atoms of the silk, also previously
neutral, are now electro-negative through the additional electrons
received from the glass atoms. As the result we find the glass to
be positively, and silk to be negatively electrified. On the other
hand, if we rub the glass with fur, a similar atomic disturbance and
consequent migration of electrons takes place, but this time the glass
receives electrons instead of parting with them. In this case the glass
becomes negatively, and the fur positively electrified. The question
now arises, why is the movement of the electrons away from the glass in
the first instance, and toward it in the second? To understand this we
may make use of a simple analogy. If we place in contact two bodies,
one hot and the other cold, the hot body gives up some of its heat to
the cold body; but if we place in contact with the hot body another
body which is still hotter, then the hot body receives heat instead of
parting with it. In a somewhat similar manner an atom is able to give
some of its electrons to another atom which, in comparison with it,
is deficient in electrons; and at the same time it is able to receive
electrons from another atom which, compared with it, has an excess of
electrons. Thus we may assume that the glass atoms have an excess of
electrons as compared with silk atoms, and a deficiency in electrons
as compared with fur atoms.
A current of electricity is believed to be nothing more or less
than a stream of electrons, set in motion by the application of
an electro-motive force. We have seen that some substances are
good conductors of electricity, while others are bad conductors or
non-conductors. In order to produce an electric current, that is a
current of electrons, it is evidently necessary that the electrons
should be free to move. In good conductors, which are mostly metals,
it is believed that the electrons are able to move from atom to atom
without much hindrance, while in a non-conductor their movements are
hampered to such an extent that inter-atomic exchange of electrons is
almost impossible. Speaking on this point, Professor Fleming says:
“There may be (in a good conductor) a constant decomposition and
recomposition of atoms taking place, and any given electron so to speak
flits about, now forming part of one atom and now of another, and anon
enjoying a free existence. It resembles a person visiting from house
to house, forming a unit in different households, and, in between,
being a solitary person in the street. In non-conductors, on the other
hand, the electrons are much restricted in their movements, and can be
displaced a little way but are pulled back again when released.”
Let us try to see now how an electric current is set up in a simple
voltaic cell, consisting of a zinc plate and a copper plate immersed in
dilute acid. First we must understand the meaning of the word _ion_.
If we place a small quantity of salt in a vessel containing water, the
salt dissolves, and the water becomes salt, not only at the bottom
where the salt was placed, but throughout the whole vessel. This means
that the particles of salt must be able to move through the water.
Salt is a chemical compound of sodium and chlorine, and its molecules
consist of atoms of both these substances. It is supposed that each
salt molecule breaks up into two parts, one part being a sodium atom,
and the other a chlorine atom; and further, that the sodium atom loses
an electron, while the chlorine atom gains one. These atoms have the
power of travelling about through the solution, and they are called
_ions_, which means “wanderers.” An ordinary atom is unable to wander
about in this way, but it gains travelling power as soon as it is
converted into an ion, by losing electrons if it be an atom of a metal,
and by gaining electrons if it be an atom of a non-metal.
Returning to the voltaic cell, we may imagine that the atoms of the
zinc which are immersed in the acid are trying to turn themselves into
ions, so that they can travel through the solution. In order to do
this each atom parts with two electrons, and these electrons try to
attach themselves to the next atom. This atom however already has two
electrons, and so in order to accept the newcomers it must pass on
its own two. In this way electrons are passed on from atom to atom of
the zinc, then along the connecting wire, and so to the copper plate.
The atoms of zinc which have lost their electrons thus become ions,
with power of movement. They leave the zinc plate immediately, and so
the plate wastes away or dissolves. So we get a constant stream of
electrons travelling along the wire connecting the two plates, and this
constitutes an electric current.
The electron theory gives us also a clear conception of magnetism. An
electric current flowing along a wire produces magnetic effects; that
is, it sets up a field of magnetic force. Such a current is a stream
of electrons, and therefore we conclude that a magnetic field is
produced by electrons in motion. This being so, we are led to suppose
that there must be a stream of electrons in a steel magnet, and this
stream must be constant because the magnetic field of such a magnet is
permanent. The electron stream in a permanent magnet however is not
quite the same as the electron stream in a wire conveying a current. We
have stated that the electrons constituting an atom move in definite
orbits, so that we may picture them travelling round the core of the
atom as the planets travel round the Sun. This movement is continuous
in every atom of every substance. Apparently we have here the necessary
conditions for the production of a magnetic field, that is, a constant
stream of electrons; but one important thing is still lacking. In an
unmagnetized piece of steel the atoms are not arranged symmetrically,
so that the orbits of their electrons lie some in one plane and some
in another. Consequently, although the electron stream of each atom
undoubtedly produces an infinitesimally small magnetic field, no
magnetic effect that we can detect is produced, because the different
streams are not working in unison and adding together their forces.
In fact they are upsetting and neutralizing each other’s efforts. By
stroking the piece of steel with a magnet, or by surrounding it by a
coil of wire conveying a current, the atoms are turned so that their
electron orbits all lie in the same plane. The electron streams now all
work in unison, their magnetic effects are added together, and we get
a strong magnetic field as the result of their combined efforts. Any
piece of steel or iron may be regarded as a potential magnet, requiring
only a rearrangement of its atoms in order to become an active magnet.
In Chapter VI. it was stated that other substances besides iron and
steel show magnetic effects, and this is what we should expect, as the
electron movement is common to all atoms. None of these substances is
equal to iron and steel in magnetic power, but why this is so is not
understood.
This brings us to the production of an electric current by the dynamo.
Here we have a coil of wire moving across a magnetic field, alternately
passing into this field and out of it. A magnetic field is produced,
as we have just seen, by the steady movement of electrons, and we may
picture it as being a region of the ether disturbed or strained by the
effect of the moving electrons. When the coil of wire passes into the
magnetic field, the electrons of its atoms are influenced powerfully
and set in motion in one direction, so producing a current in the
coil. As the coil passes away from the field, its electrons receive a
second impetus, which checks their movement and starts them travelling
in the opposite direction, and another current is produced. The coil
moves continuously and regularly, passing into and out of the magnetic
field without interruption; and so we get a current which reverses its
direction at regular intervals, that is, an alternating current. This
current may be made continuous if desired, as explained in Chapter IX.
Such, stated briefly and in outline, is the electron theory of
electricity. It opens up possibilities of the most fascinating nature;
it gives us a wonderfully clear conception of what might be called the
inner mechanism of electricity; and it even introduces us to the very
atoms of electricity. Beyond this, at present, it cannot take us, and
the actual nature of electricity itself remains an enigma.
INDEX
Accumulators, 38, 90.
Alarms, electric, 120.
Alternating currents, 71, 75.
Amber, discovery of, 2.
Ampère, 33.
Arc lamp, 93.
Armature, 68.
Atlantic cable, 145.
Atom, 287.
Aurora borealis, 25.
Automatic telephone exchange, 165.
Aviation and “wireless,” 280.
Bachelet “flying” train, 271.
Bastian heater, the, 110.
Battery, voltaic, 33.
Bell telephone, the, 156.
Bells and alarms, electric, 116.
Blasting, 256.
Bunsen cell, 223.
Cable-laying, 150.
Cables, telegraph, 144.
Cell, voltaic, 29.
Clocks, electric, 124.
Coherer, the, 183.
Commutator, 70.
Compass, magnetic, 52.
Condenser, 63.
Conductors, 6.
Conduit system, 83.
Convectors, 109.
Cookers, electric, 110.
Creed telegraph, 137.
Crookes, Sir W., 230.
Current, electric, 30.
Daniell cell, 31, 223.
Davy, Sir Humphry, 93.
Detector, in wireless telegraphy, 188, 198.
Diamond-making, 113.
Duplex telegraphy, 139.
Dussaud cold light, 106.
Dynamo, 66.
Edison, Thomas A., 103.
Electric cookers, 110.
Electric heating, 109.
Electric motor, 66.
Electric lighting, 70, 75, 93.
Electricity, early discoveries, 1;
nature of, 287.
Electro-culture, 258.
Electrolysis, 224.
Electro-magnets, 58.
Electron, 287.
Electroplating, 213.
Electrophorus, the, 11.
Electrotyping, 213.
Faraday, 66.
Finsen light treatment, 243.
Franklin, Benjamin, 19.
Frictional electricity, 2.
Furnace, electric, 111.
Galvani, 27.
Galvanometer, 59.
Glass, 4.
Goldschmidt system, 197.
_Great Eastern_, the, 148.
Half-watt lamp, 105.
Heating by electricity, 109.
Hughes printing telegraph, 136.
Iceberg detector, 267.
Ignition, electric, 253.
Incandescent lamps, 103.
Induction, 9.
Induction coil, 61.
Ion, 291.
Kelvin, Lord, 152.
Korn’s photo-telegraph, 174.
Lamps, electric, 93.
Leclanché cell, 32, 116.
Lemström’s experiments in electro-culture, 258.
Lepel system, 196.
Leyden jar, 15, 181.
Light, 23.
Lighting, electric, 75, 93.
Lightning, 1, 19, 23.
Lightning conductors, 25.
Lindsay, wireless experiments, 180.
Lodge, Sir Oliver, 260.
Machines for producing static electricity, 9.
Magnetic poles, 50.
Magnetism, 44, 56, 291.
Marconi, 186, 195.
Medicine, electricity in, 241.
Mercury-vapour lamp, 99.
Microphone, 159.
Mines, submarine, 283.
Mines, telephones in, 169.
Mono-railway, 89.
Morse, telegraph, 130;
experiments in wireless telegraphy, 180.
Motor, electric, 66.
Motor-car, electric, 91.
Navy, use of wireless, 274;
of electricity, 282.
Negative electricity, 5.
Neon lamps, 102.
Non-conductors, 6.
Ohm, 33.
Oil radiator, 110.
Ozone, 23, 247.
Ozone ventilation, 249.
Petrol, motor, ignition in, 253.
Photographophone, the, 173.
Pile, voltaic, 28.
Pipe locator, 266.
Plant culture, electric, 258.
Polarization, 31.
Pollak-Virag telegraph, 137.
Positive electricity, 5.
Poulsen, Waldemar, 171, 197.
Poultry, electro-culture of, 264.
Power stations, 75.
Preece, wireless experiments, 180.
Primary and secondary coils, 62.
Radiator, 109.
Railways, electric, 87;
use of wireless, 211.
Resistance, 33.
Röntgen rays, 228, 242.
Searchlights, 98.
Ships, use of wireless, 206.
Siphon recorder, the, 252.
Sparking plug, 154.
Static electricity, 7.
Stations, wireless, 204.
Steinheil telegraph, 130.
Submarine telegraphy, 144.
Submarine telephony, 169.
Surface contact system, 83.
Telefunken system, 196.
Telegraph, the, 128, 144, 171, 179, 203.
Telegraphone, 171.
Telephone, the, 154, 171, 179, 201.
Telephone exchange, 160.
Thermopile, 36.
Thermostat, 121.
Thunderstorms, 22, 194.
Trains, electric, 87;
the Bachelet, 271.
Tramways, electric, 78, 83.
Trolley system, 83.
Tubes for X-rays, 233.
Tuning in wireless telegraphy, 191.
Tungsten lamps, 104.
Volt, 33.
Voltaic electricity, 28, 129, 290.
War, electricity in, 274;
telegraph in, 277.
Water, electrolysis of, 38.
Water-power, 81.
Waves, electric, 181, 191, 199.
Welding, electric, 114.
Welsbach lamp, 103.
Wheatstone and Cooke telegraphs, 130.
Wimshurst machine, 12.
Wireless telegraphy and telephony, 179, 203, 270, 280.
Wires, telegraph, 141.
X-rays, 231, 242.
MORRISON & GIBB LIMITED, EDINBURGH
5/15 2½
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.
Illustrations in this eBook have been positioned between paragraphs and
outside quotations. In versions of this eBook that support hyperlinks,
the page references in the List of Plates lead to the corresponding
illustrations. (There is no list of the other illustrations.)
Plate VIII., “Typical Electric Locomotives,” listed as being on page
90, was not in the original book and therefore not in this ebook.
The index was not checked for proper alphabetization or correct page
references.