For each you will want:  (1) a straight magnet; (2) a coil of silk-covered copper wire; (3) a thin plate of soft iron; (4) a box to hold the first three articles.  You will also want as much wire as you can afford, to connect the instruments, and two short pieces of wire to connect your telephones with the ground. (Two wires between the instruments would make the ground-wires unnecessary, but this would use up too much wire.) The magnet and the coil you will have to buy from some dealer in electrical apparatus.  They need not cost much.  A small cigar-box will answer for the case.

[Illustration:  Fig. 3.  A “CIGAR-BOX” TELEPHONE.]

In one end of the box cut a round hole, say, three inches across.  Against this hole fasten a disk of thin sheet-iron for the vibrator or “diaphragm.”  For a mouth-piece use a small can, such as ground spices come in, or even a round paper box.

Now, on the inside of the box, place the magnet, the end carrying the coil almost touching the middle of the diaphragm, and fix it firmly.  Then, to the ends of the copper wire of the main coil fasten two wires,—­one for the line, the other for the “ground-wire.”

This done, you will have an instrument (or rather two of them) very much like Fig. 3.  A is the mouth-piece; B, the diaphragm; C, the coil; D, the magnet; E, E, the wires.

The receiving and sending instruments are precisely alike, each answers for both purposes; but there must be two, since one must always be hearing while the other is speaking.

When you speak into the mouth-piece of one telephone, the sound of your voice causes the “diaphragm” to vibrate in front of the magnet.  The vibrations cause the magnet’s pull upon the diaphragm to vary in force, which variation is answered by electrical waves in the coil and over the wires connected with it.  At the other end of the wire the pull of the magnet of the speaking telephone is varied exactly in proportion to the strength of the electric impulses that come over the wire; the varying pull of the magnet sets the diaphragm in motion, and that sets the air in motion in waves precisely like those of the distant voice.  When those waves strike the listener’s ear, he seems to hear the speaker’s exact tones, and so, substantially, he does hear them.  The circumstance that electric waves, and not sound-waves, travel over the wires, does not change the quality of the resulting sound in the least.

I think you now understand Bell’s telephone.

The telephones of Edison, Gray, and others, involve different principles and are differently constructed.

One invention very often leads to another, and the telephone already has an offspring not less wonderful than itself.  It is called the speaking-phonograph.  It was invented by Mr. Edison, one of the gentlemen, just mentioned.