We have made mention several times of turning on or off one or more
lights, and now, perhaps, you would like to know how this is done.
Suppose the electricity was traveling through wires to one or several
lamps, it would light up those lamps as long as the wires provided
a path to travel in, but if you were to cut out one of them, which
is called "breaking the circuit," there would be no road for the
electricity to follow, and, consequently, its course would be stopped
short and the lamps would go out. You will remember that _electricity
must have a complete circuit_ or it can do no work, and in electric
lighting it is always a _metallic circuit_ that is used.
Now, the switch is simply a device which is used to break the circuit
so that the current cannot pass on. The simplest form of switch is seen
in the sketch. (Fig. 26.)
[Illustration: Fig. 26]
You will see that there is a wire cut in two, and to one piece is
attached a metallic piece, A, which turns one way or the other, and
when it is turned so as to touch the other part of the wire the circuit
is closed and the electricity goes from the lower part of the wire
through the metallic piece A to the other part of the wire, thus making
a complete circuit or path for the electricity to travel in.
If we turn the piece A away from the upper wire this breaks the circuit
and cuts off the path, and, of course, the lamps would go out.
This is the principle of the switch, and, although they are made in
thousands of ways, switches all have the same object--namely, the
closing and breaking of the circuit, whether it is for one or a hundred
lamps.