he
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.

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 impu