e iron core and upon the strength of the current
which is sent through the coils.

[Illustration: FIG. 213.--An electromagnet.]

[Illustration: FIG. 214.--A horseshoe electromagnet is powerful enough
to support heavy weights.]

To increase the strength of the electromagnet still further, the
so-called horseshoe shape is used (Fig. 214). In such an arrangement
there is practically the strength of two separate electromagnets.

297. The Electric Bell. The ringing of the electric bell is due to
the attractive power of an electromagnet. By the pushing of a button
(Fig. 215) connection is made with a battery, and current flows
through the wire wound on the iron spools, and further to the screw
_P_ which presses against the soft iron strip or armature _S_; and
from _S_ the current flows back to the battery. As soon as the
current flows, the coils become magnetic and attract the soft iron
armature, drawing it forward and causing the clapper to strike the
bell. In this position, _S_ no longer touches the screw _P_, and hence
there is no complete path for the electricity, and the current ceases.
But the attractive, magnetic power of the coils stops as soon as the
current ceases; hence there is nothing to hold the armature down, and
it flies back to its former position. In doing this, however, the
armature makes contact at _P_ through the spring, and the current
flows once more; as a result the coils again become magnets, the
armature is again drawn forward, and the clapper again strikes the
bell. But immediately afterwards the armature springs backward and
makes contact at _P_ and the entire operation is repeated. So long as
we press the button this process continues producing what sounds like
a continuous jingle; in reality the clapper strikes the bell every
time a current passes through the electromagnet.

[Illustration: FIG. 215.--The electric bell.]

298. The Push Button. The push button is an essential part of every
electric bell, because without it the bell either would not ring at
all, or would ring incessantly until the cell was exhausted. When the
push button is free, as in Figure 216, the cell terminals are not
connected in an unbroken path, and hence the current does not flow.
When, however, the button is pressed, the current has a complete path,
provided there is the proper connection at _S_. That is, the pressure
on the push button permits current to flow to the bell. The flow of
this current then depends sole