e divide 300
million meters by the frequency we get the wave length, and that's the
same rule as I gave you in the last letter.

In further letters I'll tell you how the audion works as a detector and
how we connect a telephone transmitter to the oscillator to make it send
out energy with a speech significance instead of a mere dot and dash
significance, or signal significance. We shall have to learn quite a
little about the telephone itself and about the human voice.

LETTER 14

WHY AND HOW TO USE A DETECTOR

DEAR SON:

In the last letter we got far enough to sketch, in Fig. 54, a radio
transmitting station and a receiving station. We should never, however,
use just this combination because the transmitting station is intended
to send telegraph signals and the receiving station is best suited to
receiving telephonic transmission. But let us see what happens.

[Illustration: Fig 54]

When the key in the plate circuit of the audion at the sending station
is depressed an alternating current is started. This induces an
alternating current in the neighboring antenna circuit. If this antenna
circuit, which is formed by a coil and a condenser, is tuned to the
frequency of oscillations which are being produced in the audion circuit
then there is a maximum current induced in the antenna.

As soon as this starts the antenna starts to send out energy in all
directions, or "radiate" energy as we say. How this energy, or ability
to do work, gets across space we don't know. However it may be, it does
get to the receiving station. It only takes a small fraction of a second
before the antenna at the receiving station starts to receive energy,
because energy travels at the rate of 186,000 miles a second.

The energy which is received does its work in making the electrons in
that antenna oscillate back and forth. If the receiving antenna is tuned
to the frequency which the sending station is producing, then the
electrons in the receiving antenna oscillate back and forth most widely
and there is a maximum current in this circuit.

The oscillations of the electrons in the receiving antenna induce
similar oscillations in the tuned circuit which is coupled to it. This
circuit also is tuned to the frequency which the distant oscillator is
producing and so in it we have the maximum oscillation of the electrons.
The condenser in that circuit charges and discharges alternately.

The grid of the receiving audion always has t