“An instrument to convey sounds by means of electricity.”

Good.  That shows you have some definite idea of it; but, after all, that answer is not the right one.  The telephone does not convey sound.

“What does its name mean, then?” do you ask?

Simply, that it is a far-sounder; but that does not necessarily imply that it carries sounds afar.  Strictly speaking, the telephone only changes sound-waves into waves of electricity and back again.  When two telephones are connected by means of a wire, they act in this way,—­the first telephone changes the sound-waves it receives into electric impulses which travel along the wire until they reach the second telephone, here they are changed back to sound-waves exactly like those received by the first telephone.  Accordingly, the listener in New York seems to hear the very tones of his friend who is speaking at the other end of the line, say, in Boston.

Still you don’t see how.

It is not surprising, for in this description several scientific facts and principles are involved; and all boys and girls cannot be expected to know much about the laws of sound and electricity.  Perhaps a little explanation may make it clearer.

The most of you probably know that sound is produced by rapid motion.  Put your finger on a piano wire that is sounding, and you will feel the motion, or touch your front tooth with a tuning-fork that is singing; in the last case you will feel very distinctly the raps made by the vibrating fork.  Now, a sounding body will not only jar another body which touches it, but it will also give its motion to the air that touches it; and when the air-motions or air-waves strike the sensitive drums of our ears, these vibrate, and we hear the sound.

You all have heard the windows rattle when it thunders loudly, or when cannons have been fired near-by.  The sound waves in the air fairly shake the windows; and, sometimes, when the windows are closed, so that the air-waves cannot pass readily, the windows are shattered by the shock.  Fainter sounds act less violently, yet similarly.  Every time you speak, your voice sets everything around you vibrating in unison, though ever so faintly.

Thus, from your every-day experience you have proof of two important facts,—­first, sound is caused by rapid motion; second, sound-waves give rise to corresponding motion.  Both these facts are involved in the speaking telephone, which performs a twofold office,—­that of the ear on the one hand, that of our vocal organs on the other.

To serve as an ear, the telephone must be able to take up quickly and nicely the sound-waves of the air.  A tightened drum-head will do that; or better, a strip of goldbeaters’-skin drawn tightly over a ring or the end of a tube.  But these would not help Professor Bell, the inventor of the telephone we shall describe, since he wanted an ear that would translate the waves of sound into waves of electricity, which would travel farther and faster than sound-waves could.