f a charged condenser, as in Fig. 27, then
something of great interest happens. To understand you must know
something more about electron streams.

Suppose we should wind a few turns of wire on a cylindrical core, say on
a stiff cardboard tube. We shall use insulated wire. Now start from one
end of the coil, say _a_, and follow along the coiled wire for a
few turns and then scratch off the insulation and solder onto the coil
two wires, _b_, and _c_, as shown in Fig. 28. The further end
of the coil we shall call _d_. Now let's arrange a battery and
switch so that we can send a current through the part of the coil
between _a_ and _b_. Arrange also a current-measuring instrument so as
to show if any current is flowing in the part of the coil between _c_
and _d_. For this purpose we shall use a kind of current-measuring
instrument which I have not yet explained. It is different from the
hot-wire type described in Letter 7 for it will show in which direction
electrons are streaming through it.

The diagram of Fig. 28 indicates the apparatus of our experiment. When
we close the switch, _S_, the battery starts a stream of electrons
from _a_ towards _b_. Just at that instant the needle, or
pointer, of the current instrument moves. The needle moves, and thus
shows a current in the coil _cd_; but it comes right back again,
showing that the current is only momentary. Let's say this again in
different words. The battery keeps steadily forcing electrons through
the circuit _ab_ but the instrument in the circuit _cd_ shows
no current in that circuit except just at the instant when current
starts to flow in the neighboring circuit _ab_.

[Illustration: Fig 28]

One thing this current-measuring instrument tells us is the direction of
the electron stream through itself. It shows that the momentary stream
of electrons goes through the coil from _d_ to _c_, that is in
the opposite direction to the stream in the part _ab_.

Now prepare to do a little close thinking. Read over carefully all I
have told you about this experiment. You see that the moment the battery
starts a stream of electrons from _a_ towards _b_, something causes
a momentary, that is a temporary, movement of electrons from _d_ to
_c_. We say that starting a stream of electrons from _a_ to _b_ sets
up or "induces" a stream of electrons from _d_ to _c_.

What will happen then if we connect the battery between _a_ and _d_
as in Fig. 29? Electrons will start streaming a