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The telephone receiver is an extremely sensitive instru-
ment, and it only requires a very weak current to operate
it and produce a sound. The currents or oscillations gen-
erated in the aerial, however, are alternating currents of
high frequency, that is, they flow in one direction and then
reverse and flow in the other several thousand times a
second. Such a current cannot be made to pass through a


nu. PHON ’










FIG. 195. —A Simple Receptor.

telephone receiver, and in order to do so the nature of the
current must be changed by converting it into direct cur-
rent flowing in one direction only.

Certain minerals and crystals possess the remarkable
ability to do this, silicon, galena, and iron pyrites are among
the best.

The diagram in Figure 195 shows the arrangement of a
simple receiving outfit. The detector consists of a sensitive

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