each day and to pay up the next, as shown in Fig. 61a. Then we
would represent the story of this 60 cents by the graph of Fig. 61b.

[Illustration: Fig 61b]

Now suppose that instead of going in debt each day he uses part of this
60 cents. Each day after the first his father gives him a dollar, just
as before. He starts then with 60 cents as shown in Fig. 61c, increases
in wealth to $1.10, then spends $1.00, bringing his funds down to 10
cents. Then he receives $1.00 from his father and the process is
repeated cyclically.

[Illustration: Fig 61c]

If you saw the graph of Fig. 61c you would be able to say that, whatever
he actually did, the effect was the same as if he had two pockets, in
one of which he kept 60 cents all the time as shown in Fig. 61b. In his
other pocket he either had money or he was in debt as shown in Fig. 61a.
If you did that you would be resolving the money changes of Fig. 61c
into the two components of Figs. 61a and b.

That is what I want you to do with the curve of Fig. 57 which I am
reproducing here, redrawn as Fig. 62a. You see it is really the result
of adding together the two curves of Figs. 62b and c, which are shown on
the following page.

[Illustration: Fig 62a]

We can think, therefore, of the current in the plate circuit as if it
were two currents added together, that is, two electron streams passing
through the same wire. One stream is steady and the other alternates.

[Illustration: Fig 62b]

Now look again at the diagram of our receiving set which I am
reproducing as Fig. 63. When the signal is incoming there flow in the
plate circuit two streams of electrons, one steady and of a value in
mil-amperes corresponding to that of the graph in Fig. 62b, and the
other alternating as shown in Fig. 62c.

The steady stream of electrons will have no more difficulty in getting
through the coiled wire of the receiver than it would through the same
amount of straight wire. On the other hand it cannot pass the gap of the
condenser.

The alternating-current component can't get along in the coil because
its frequency is so high that the coil impedes the motion of the
electrons so much as practically to stop them. On the other hand these
electrons can easily run into the waiting-room offered by the condenser
and then run out again as soon as it is time.

[Illustration: Fig 62c]

When the current in the plate circuit is large all the electrons which
aren't needed for the steady stream