opposes them will be measured in volts. Therefore we say that a coil has
one henry of inductance when an electron stream which is increasing one
ampere's worth each second stirs up in the coil a conscientious
objection of one volt. Don't try to remember this now; you can come back
to it later.

There is one more effect of inductance which we must know before we can
get very far with our radio. Suppose an electron stream is flowing
through a coil because a battery is driving the electrons along. Now let
the battery be removed or disconnected. You'd expect the electron stream
to stop at once but it doesn't. It keeps on for a moment because the
electrons have got the habit.

[Illustration: Fig 28]

If you look again at Fig. 28 you will see what I mean. Suppose the
switch is closed and a steady stream of electrons is flowing through the
coil from _a_ to _b_. There will be no current in the other
part of the coil. Now open the switch. There will be a motion of the
needle of the current-measuring instrument, showing a momentary current.
The direction of this motion, however, shows that the momentary stream
of electrons goes through the coil from _c_ to _d_.

Do you see what this means? The moment the battery is disconnected there
is nothing driving the electrons in the part _ab_ and they slow
down. Immediately, and just for an instant, a stream of electrons starts
off in the part _cd_ in the same direction as if the battery was
driving them along.

Now look again at Fig. 29. If the battery is suddenly disconnected there
is a momentary rush of electrons in the same direction as the battery
was driving them. Just as the self-inductance of a coil opposes the
starting of a stream of electrons, so it opposes the stopping of a
stream which is already going.

[Illustration: Fig 29]

So far we haven't said much about making an audion produce alternating
e. m. f.'s and thus making it useful for radio-telephony. Before radio
was possible all these things that I have just told you, and some more
too, had to be known. It took hundreds of good scientists years of
patient study and experiment to find out those ideas about electricity
which have made possible radio-telephony.

Two of these ideas are absolutely necessary for the student of
radio-communication. First: A condenser is a gap in a circuit where
there are waiting-rooms for the electrons. Second: Electrons form
habits. It's hard to get them going through a coil of wire,