voice is high pitched there are
more rapid variations in the strength of the radio-frequency current.
That is why we say the radio-current is "modulated" by the human voice.

[Illustration: Fig 68]

The signal which radiates out from the transmitting antenna carries all
the little variations in pitch and loudness of the human voice. When
this signal reaches the distant antenna it establishes in that antenna
circuit a current of high frequency which has just the same variations
as did the current in the antenna at the sending station. The human
voice isn't there. It is not transmitted. It did its work at the sending
station by modulating the radio-signal, "modulating the carrier
current," as we sometimes say. But there is speech significance hidden
in the variations in strength of the received signal.

If a telephone-receiver diaphragm can be made to vibrate in accordance
with the variations in signal intensity then the air adjacent to that
diaphragm will be set into vibration and these vibrations will be just
like those which the human voice set up in the air molecules near the
mouth of the speaker. All the different systems of receiving
radio-telephone signals are merely different methods of getting a
current which will affect the telephone receiver in conformity with the
variations in signal strength. Getting such a current is called
"detecting." There are many different kinds of detectors but the vacuum
tube is much to be preferred.

The cheapest detector, but not the most sensitive, is the crystal. If
you understand how the audion works as a detector you will have no
difficulty in understanding the crystal detector.

The crystal detector consists of some mineral crystal and a fine-wire
point, usually platinum. Crystals are peculiar things. Like everything
else they are made of molecules and these molecules of atoms. The atoms
are made of electrons grouped around nuclei which, in turn, are formed
by close groupings of protons and electrons. The great difference
between crystals and substances which are not crystalline, that is,
substances which don't have a special natural shape, is this: In
crystals the molecules and atoms are all arranged in some orderly
manner. In other substances, substances without special form, amorphous
substances, as we call them, the molecules are just grouped together in
a haphazard way.

[Illustration: Fig 69]

For some crystals we know very closely indeed how their molecules or
r