;
we get millionths of an ampere or "microamperes," as we say. We can
plot the pairs of values which we measure and make a curve like that of
Fig. 71.

[Illustration: Fig 71]

When the voltage across the contact is reversed, of course, the current
reverses. Part of the curve looks something like the lower part of an
audion characteristic.

[Illustration: Fig 72]

Now connect this crystal in a receiving circuit as in Fig. 72. We use an
antenna just as we did for the audion and we tune the antenna circuit to
the frequency of the incoming signal. The receiving circuit is coupled
to the antenna circuit and is tuned to the same frequency. Whatever
voltage there may be across the condenser of this circuit is applied to
the crystal detector. We haven't put the telephone receiver in the
circuit yet. I want to wait until you have seen what the crystal does
when an alternating voltage is applied to it.

[Illustration: Fig 73]

We can draw a familiar form of sketch as in Fig. 73 to show how the
current in the crystal varies. You see that there flows through the
crystal a current very much like that of Fig. 62a. And you know that
such a current is really equivalent to two electron streams, one steady
and the other alternating. The crystal detector gives us much the same
sort of a current as does the vacuum tube detector of Fig. 54. The
current isn't anywhere near as large, however, for it is microamperes
instead of mil-amperes.

Our crystal detector produces the same results so far as giving us a
steady component of current to send through a telephone receiver. So we
can connect a receiver in series with the crystal as shown in Fig. 74.
Because the receiver would offer a large impedance to the high-frequency
current, that is, seriously impede and so reduce the high-frequency
current, we connect a condenser around the receiver.

[Illustration: Fig 74]

There is a simple crystal detector circuit. If the signal intensity
varies then the current which passes through the receiver will vary. If
these variations are caused by a human voice at the sending station the
crystal will permit one to hear from the telephone receiver what the
speaker is saying. That is just what the audion detector does very many
times better.

In the letter on how to experiment you'll find details as to the
construction of a crystal-detector set. Excellent instructions for an
inexpensive set are contained in Bull. No. 120 of the Bureau of
Standards