[Illustration: FIG. 1.]
It will first be necessary to explain how electric currents are produced
by means of chemicals. In a jar A, Fig. 1, are placed two plates B and
C, one zinc, and the other copper, each having connected to it at the
top a copper wire of any convenient length. The plates are kept in
position by means of pieces of wood, and the jar is about half filled
with a solution of salt and water, or sulphuric acid and water; if then
the two wires are joined, a current of electricity at once flows through
them, however long they may be. The current produced in this manner is
very weak, and does not even keep what strength it has for any length of
time, but rapidly gets weaker until quite imperceptible. The current
is, however, continuous; that is, it flows steadily in the one direction
through the wire, and may be used for ringing bells, or for other
purposes where a feeble current only is required to do intermittent
work. The wire E in connection with the copper plate is called the
positive lead, and the other the negative, and the current is said to
flow from the copper plate, through the wire E through the circuit to D,
and thence to the zinc plate, and through the liquid to the copper
plate. The current has often been compared to water flowing through a
pipe, but I think it can be better compared to the blood in the human
body, which through the action of the heart is continually forced
through the arteries and veins in one steady stream. There is, however,
this difference, that there is no actual progression of matter in the
electric current, it being like a ripple on water, which moves from end
to end of a lake without the water itself being moved across. Now that I
have given you an idea of how the current acts, I must try and explain
how different degrees of strength and volume are obtained. In the first
place, let us consider what constitute strength and volume in an
electric current, or at least try and get a general notion about them.
For this purpose I shall compare the electric current to water being
forced through a pipe; and the strength of the electric current, or
electromotive force, written for short E.M.F., will be like the pressure
of water at any part of the pipe. Two pipes may carry different
quantities of water, and yet the pressure may be the same in each; in
one a gallon of water may pass a given point in the same time that a
pint passes the same point in the other, and yet in each case the
different quantities may pass that point at the same speed. Thus in
electricity, two currents may be of different volume or quantity,
measured in amperes, and yet be of the same E.M.F. measured in volts; or
they may be of different E.M.F., or pressure, or intensity, and yet be
of the same volume. If any work is to be done by the water forced
through a pipe, such as turning a turbine, it is evident that pressure
of itself is not sufficient, seeing that a stream an inch in diameter
may be at the same pressure as another a foot in diameter. So with the
electric current, if work is to be done, such as driving a motor or
lighting a lamp, it is not sufficient to have a certain E.M.F.; there
must be quantity or volume in proportion to the amount of work, so that
if it takes a given quantity to work one lamp, it will take twice that
quantity to work two lamps of the same kind. It must not be inferred
from this, that if one lamp requires a certain E.M.F., that two lamps
will require it to be doubled, as such is not the case, except under
certain conditions which I will explain later on.
The action of electricity is practically instantaneous in any length of
wire, so that if the current is used to ring two bells a mile apart,
but connected by wires, they will commence to ring simultaneously. I
have so far not said anything about resistance to the passage of the
current through the wires. I shall therefore refer again to our
comparison of the current to water forced through a pipe, and you will
agree that a certain sized pipe will only convey a certain amount of
water in a given time. If a larger quantity is to be conveyed in the
same time, a greater pressure must be applied, or a larger pipe must be
used.
It is evident that increasing the size of the pipe will get over the
difficulty more readily than increasing the pressure of the water. The
pipes themselves offer a certain resistance to the passage of the water
through them, in the shape of friction; so that if an effect is to be
produced at a distance, rather more pressure is required than if it is
done close at hand, so as to make up for the loss sustained by friction.
Much the same may be said of the electric current; a certain sized wire
will only carry a certain current, and if more current is required, a
thicker wire must be used to convey it, or it must be of a greater
E.M.F. It is usually more convenient to increase the thickness of the
wire than to increase the E.M.F. of the current. The wire offers a
certain resistance to the passage of the current through it, which may
be compared to friction, and this resistance varies according to the
metal of which it is composed. Copper is the metal in ordinary use for
wires for electric lighting purposes, and the purer it is the better
will it convey the current. Iron is used for telegraph wires on account
of cheapness, the current used being so small that this metal conveys it
readily enough; if copper were used, the wires will only require to be
about one-third the diameter of the iron ones. The following are the
respective values for electrical conductivity of various metals when
pure, taking silver as a standard:--Silver 100, copper 99.9, gold 80,
zinc 29, brass 22, iron 16.8, tin 13.1, lead 8.3, mercury 1.6.
If a wire is made to convey a current which is too large for its
electrical capacity, it will get heated, which decreases its
conductivity, with the result that the heat increases until finally the
wire fuses. I shall have more to say about this when speaking of
electric lighting.