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184 THE BOY ELECTRICIAN

through. This method of telephoning is unsatisfactory
over any appreciable distances. The time utilized in making
a transmitter will be well spent. '

A simple form of transmitter .is shown in Figure 148.
The wooden back, B, is three and one-half inches square
and three-quarters of an inch thick. The front face of the
block is hollowed out in the center as shown in the cross-
section view.

The faceplate, A, is two and one-half inches square and
one-half an inch thick. A hole, seven-eighths of an inch
in diameter, is bored through the center. One side is then
hollowed out to a diameter of one and three-quarter inches,
so as to give space for the diaphragm to vibrate as shown
in the cross-sectional drawing, '

The carbon buttons are one inch—in diameter and three-
sixteenths of an inch thick. A small hole is bored in the
center of each to receive a brass machine screw. The
hole is countersunk, so as to bring the head of the screw
down as close to the surface of the carbon as is possible.
Then, using a sharp knife or a three-comered file, score
the surface of the carbon until it is covered with criss-cross
lines.

The diaphragm is a piece of thin sheet-iron cut in the
form of a circle two and one-half inches in diameter. A
small hole is bored through the center of this. One of the
carbon buttons is fastened to the center of the diaphragm
with a small screw and a nut. '

Cut out a strip of flannel or thin felt, nine-sixteenths of

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