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MICROPHONES AND TELEPHONES I81

can be purchased for forty-five to seventy-five cents at
almost any electrical supply house. They are very useful
to the amateur experimenter in many ways.

A telephone receiver capable of giving fair results on a
short telephone line can be very easily made, but of course
will not prove as eflicient as one which is purchased ready-
made from a reliable electrical manufacturer.

The first practical telephone receiver was invented by
Alexander Graham Bell and was made somewhat along the
same lines as that shown in Figure I47.

Such a receiver may be made from a piece of curtain-
pole, three and three-quarter inches long and about one
and one-eighth inches in diameter. A hole, three-eighths
of an inch in diameter, is bored along the axis throughout
its entire length, to receive the permanent magnet.

The shell of. the receiver is a cup-shaped piece of hard
wood, two and one-half inches in diameter and one inch
deep. It will have to be turned on a lathe. Its exact shape
and dimensions are best understood from the dimensions
shown in the cross section in Figure 147. The shell is firmly
attached to one end of the piece of curtain-pole by gluing.

The permanent magnet is a piece of hard steel, three-
eighths of an inch in diameter and four and five-eighths of
an inch in length. The steel will have to be tempered or
hardened before it will make a suitable magnet, and the
best way to accomplish this is to have a blacksmith do it
for you by heating the rod and then plunging it into water
when just at the right temperature.

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