disturbed in unison.

HOW TO MAKE A TELEPHONE.--For experimental and testing purposes two of
these telephones should be made at the same time. The case or holder
(A) may be made either of hard wood or hard rubber, so that it is of
insulating material. The core (B) is of soft iron, 3/8 inch in diameter
and 5 inches long, bored and threaded at one end to receive a screw (C)
which passes through the end of the case (A).

The enlarged end of the case should be, exteriorly, 2-1/4 inches in
diameter, and the body of the case 1 inch in diameter.

[Illustration: _Fig. 81._ SECTION OF TELEPHONE RECEIVER]

Interiorly, the large end of the case is provided with a circular recess
1-3/4 inches in diameter and adapted to receive therein a spool which
is, diametrically, a little smaller than the recess. The spool fits
fairly tight upon the end of the core, and when in position rests
against an annular shoulder in the recess. A hollow space (F) is thus
provided behind the spool (D), so the two wires from the magnet may
have room where they emerge from the spool.

The spool is a little shorter than the distance between the shoulder (E)
and the end of the casing, at G, and the core projects only a short
distance beyond the end of the spool, so that when the diaphragm (H) is
put upon the end of the case, and held there by screws (I) it will not
touch the end of the core. A wooden or rubber mouthpiece (J) is then
turned up to fit over the end of the case.

[Illustration: _Fig. 82._ THE MAGNET AND RECEIVER HEAD]

The spool (D) is made of hard rubber, and is wound with No. 24
silk-covered wire, the windings to be well insulated from each other.
The two ends of the wire are brought out, and threaded through holes (K)
drilled longitudinally through the walls of the case, and affixed to the
end by means of screws (L), so that the two wires may be brought
together and connected with a duplex wire (M).

As the screw (C), which holds the core in place, has its head hidden
within a recess, which can be closed up by wax, the two terminals of the
wires are well separated so that short-circuiting cannot take place.

TELEPHONE CONNECTIONS.--The simplest form of telephone connection is
shown in Fig. 83. This has merely the two telephones (A and B), with a
single battery (C) to supply electricity for both. One line wire (D)
connects the two telephones directly, while the other line (E) has the
battery in its circuit.

[Illustration: _Fig. 83._