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 the coil of wire; B, the magnet; C, the diaphragm; E, the case; F, F, the wires leading from the coil, and connecting at the end of the handle with the ground and line wires.  Fig. 2 shows how a telephone looks on the outside.

[Illustration:  BELL’S TELEPHONE.  Fig. 1 and Fig. 2]

So much for description.  You will understand it better, perhaps, if you experiment a little.  You can easily make a pair for yourself, rude and imperfect, it is true, but good enough for all the tests you may want to apply.