ig 32]

Now suppose you are an electron in coil _cd_ of Fig. 33 and
"Brownie" is one in coil _ab_. Your motions are induced by his.
What's true of you two is true of all the other electrons. I have
separated the coils a little in this sketch so that you can think of a
hedge between. I don't know how one electron can affect another on the
opposite side of this hedge but it can. And I don't know anything really
about the hedge, which is generally called "the ether." The hedge isn't
air. The effect would be the same if the coils were in a vacuum. The
"ether" is just a name for whatever is left in the space about us when
we have taken out everything which we can see or feel--every molecule,
every proton and every electron.

[Illustration: Fig 33]

Why and how electrons can affect one another when they are widely
separated is one of the great mysteries of science. We don't know any
more about it than about why there are electrons. Let's accept it as a
fundamental fact which we can't as yet explain.

[Illustration: Fig 34]

And now we can see how to make an audion produce an alternating current
or as we sometimes say "make an audion oscillator." We shall set up an
audion with its A-battery as in Fig. 34. Between the grid and the
filament we put a coil and a condenser. Notice that they are in
parallel, as we say. In the plate-filament circuit we connect the
B-battery and a switch, _S_, and another coil. This coil in the
plate circuit of the audion we place close to the other coil so that the
two coils are just like the coils _ab_ and _cd_ of which I
have been telling you. The moment any current flows in coil _ab_
there will be a current flow in the coil _cd_. (An induced electron
stream.) Of course, as long as the switch in the B-battery is open no
current can flow.

The moment the switch _S_ is closed the B-battery makes the plate
positive with respect to the filament and there is a sudden surge of
electrons round the plate circuit and through the coil from _a_ to
_b_. You know what that does to the coil _cd_. It induces an
electron stream from _d_ towards _c_. Where do these electrons
come from? Why, from the grid and the plate 1 of the condenser. Where do
they go? Most of them go to the waiting-room offered by plate 2 of the
condenser and some, of course, to the filament. What is the result? The
grid becomes positive and the filament negative.

[Illustration: Fig 35]

This is the crucial moment in our study. Can