some carbon granules between the mouthpiece disk
and a disk behind it; and there are various other complications,
such as the bell-ringing apparatus and the connections in the central
office. But the principle of the telephone is almost exactly the same
as the principle of the telegraph. Both depend entirely on the fact
that an electric current passing around a piece of iron magnetizes the
iron.

EXPERIMENT 78. By means of your battery, make an electric
bell ring. Examine the bell and trace the current through
it. Notice how the current passes around two iron bars and
magnetizes them, as it did in the telegraph instrument.
Notice that the circuit is completed through a little metal
attachment on the base of the clapper, and that when the
clapper is pulled toward the electromagnet the circuit is
broken. The iron bars are then no longer magnetized. Notice
that a spring pulls the clapper back into place as soon as the
iron stops attracting it. This completes the circuit again and
the clapper is pulled down. That breaks the circuit and
the clapper springs back. See how this constant making and
breaking of the circuit causes the bell clapper to fly back
and forth.

[Illustration: FIG. 143. The bell is rung by electromagnets.]

The electric bell, like the telephone and telegraph, works on the
simple principle that electricity flowing through a wire that is
wrapped around and around a piece of iron will turn that piece of iron
into a magnet as long as the electricity flows.

THE ELECTRIC MOTOR. The motor of a street car is a still more
complicated carrying out of the same principle. In the next experiment
you will see the working of a motor.

EXPERIMENT 79. Connect the wires from the laboratory battery
to the two binding posts of the toy motor, and make the motor
run. Examine the motor and see that it is made of several
electromagnets which keep attracting each other around and
around.

Motors, and therefore all things that are _moved_ by electricity,
including trolley cars and electric railways, submarines while
submerged, electric automobiles, electric sewing machines, electric
vacuum cleaners, and electric player-pianos, are moved by magnetizing
a piece of iron and letting this pull on another piece of iron. And
the iron is magnetized by letting a current of electricity flow around
and around it.

[Illustration: FIG. 144. A toy electric m