g to give you a simple rule. Divide 300,000,000 by the "wave length"
and you'll have the frequency. For example, ships are supposed to use
wave lengths of 300 meters or 600 meters. Dividing three hundred million
by three hundred gives one million and that is one of the frequencies
which I told you were used by ship sets. Dividing by six hundred gives
500,000 or just half the frequency. You can remember that sets
transmitting with long waves have low frequencies, but sets with short
waves have high frequencies. The frequency and the wave length don't
change in the same way. They change in opposite ways or inversely, as we
say. The higher the frequency the shorter the wave length.

I'll tell you about wave lengths later. First let's see how to control
the frequency of an audion oscillator like that of Fig. 38.

[Illustration: Fig 38]

It takes time to get a full-sized stream going through a coil because of
the inductance of the coil. That you have learned. And also it takes
time for such a current to stop completely. Therefore, if we make the
inductance of the coil small, keeping the condenser the same, we shall
make the time required for the current to start and stop smaller. That
will mean a higher frequency for there will be more oscillations each
second. One rule, then, for increasing the frequency of an audion
oscillator is to decrease the inductance.

Later in this letter I shall tell you how to increase or decrease the
inductance of a coil. Before I do so, however, I want to call your
attention to the other way in which we can change the frequency of an
audion oscillator.

Let's see how the frequency will depend upon the capacity of the
condenser. If a condenser has a large capacity it means that it can
accommodate in its waiting-room a large number of electrons before the
e. m. f. of the condenser becomes large enough to stop the stream of
electrons which is charging the condenser. If the condenser in the grid
circuit of Fig. 38 is of large capacity it means that it must receive in
its upper waiting-room a large number of electrons before the grid will
be negative enough to make the plate current zero. Therefore, the
charging current will have to flow a long time to store up the necessary
number of electrons.

You will get the same idea, of course, if you think about the electrons
in the lower room. The current in the plate circuit will not stop
increasing until the voltage of the grid has become positive en