BY M.F.
What is a telephone?
Up go a hundred hands of the brightest and sharpest of the readers of ST. NICHOLAS, and a hundred confident voices reply:
"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.