age of the grid changes. Both
curves start from the instant when the switch is closed; and the two
taken together show just what happens in the tube from instant to
instant.

Fig. 37 shows the two curves. You will notice how I have drawn them
beside and below the audion characteristic. The grid voltage and the
plate current are related, as I have told you, and the audion
characteristic is just a convenient way of showing the relationship. If
we know the current in the plate circuit we can find the voltage of the
grid and vice versa.

As time goes on, the plate current grows to its maximum and decreases to
zero and then goes on climbing up and down between these two extremes.
The grid voltage meanwhile is varying alternately, having its maximum
positive value when the plate current is a maximum and its maximum
negative value when the plate current is zero. Look at the two curves
and see this for yourself.

[Illustration: Fig 37]

Now I want to tell you something about how fast these oscillations
occur. We start by learning two words. One is "cycle" with which you are
already partly familiar and the other is "frequency." Take cycle first.
Starting from zero the current increases to a maximum, decreases to
zero, and is ready again for the same series of changes. We say the
current has passed through "a cycle of values." It doesn't make any
difference where we start from. If we follow the current through all its
different values until we are back at the same value as we started with
and ready to start all over, then we have followed through a cycle of
values.

Once you get the idea of a cycle, and the markings on the curves in Fig.
31 will help you to understand, then the other idea is easy. By
"frequency" we mean the number of cycles each second. The electric
current which we use in lighting our house goes through sixty cycles a
second. That means the current reverses its direction 120 times a
second.

In radio we use alternating currents which have very high frequencies.
In ship sets the frequency is either 500,000 or 1,000,000 cycles per
second. Amateur transmitting sets usually have oscillators which run at
well over a million cycles per second. The longer range stations use
lower frequencies.

You'll find, however, that the newspaper announcements of the various
broadcast stations do not tell the frequency but instead tell the "wave
length." I am not going to stop now to explain what that means but I am
goin