n sound-waves could.

Just when Mr. Bell was thinking how he could make the instrument he
wanted, an important discovery in magnetism was made known to him--a
discovery that helped him wonderfully. You know that if you hold a
piece of iron close to a magnet the magnet will pull it, and the
closer the iron comes to the magnet the harder it is pulled. Now, some
one experimenting with a magnet having a coil of silk-covered wire
around it, found that when a piece of iron was moved in front of the
magnet and close to it without touching, the motion would give rise to
electric waves in the coil of wire, which waves could be transmitted
to considerable distances.

This was just what Mr. Bell wanted. He said to himself, "The sound
of my voice will give motion to a thin plate of iron as well as to a
sheet of goldbeaters'-skin; and if I bring this vibrating plate
of iron close to a magnet, the motion will set up in it waves of
electricity answering exactly to the sound-waves which move the iron
plate."

So far, good. But something more was wanted. The instrument must not
only translate sound-waves into electric impulses, but change these
back again into sound-waves; it must not only hear, but also _speak!_

You remember our first fact in regard to sound: it is caused by
motion. All that is needed to make anything speak is to cause it to
move so as to give rise to just such air-waves as the voice makes. Mr.
Bell's idea was to make the iron plate of his sound-receiver speak.

He reasoned in this way: From the nature of the magnet it follows that
when waves of electricity are passed through the wire coil around the
magnet, the strength of the magnet must vary with the force of the
electric impulses. Its pull on the plate of iron near it must vary in
the same manner. The varying pull on the plate must make it move,
and this movement must set the air against the plate in motion in
sound-waves corresponding exactly with the motion setting up the
electric waves in the first place; in other words, the sound-motion in
one telephone must be exactly reproduced as sound-waves in a similar
instrument joined to it by wire.

Experiment proved the reasoning correct; and thus the
speaking-telephone was invented. But it took a long time to find
the simplest and best way to make it. At last, however, Mr. Bell's
telephone was perfected in the form illustrated below. Fig. 1 shows
the inner structure of the instrument. A is the spool carrying t