h reached at the
same time--then the battery has to stop driving electrons around the
circuit. The battery has not enough e. m. f. to drive any more
electrons. Why? Because the condenser has now just enough e. m. f. with
which to oppose the battery.

It would be well to learn at once the right words to use in describing
this action. We say that the battery sends a "charging current" around
its circuit and "charges the condenser" until it has the same e. m. f.
When the battery is first connected to the condenser there is lots of
space in the waiting-rooms so there is a great rush or surge of
electrons into one plate and away from the other. Just at this first
instant the charging current, therefore, is large but it decreases
rapidly, for the moment electrons start to pile up on one plate of the
condenser and to leave the other, an e. m. f. builds up on the
condenser. This e. m. f., of course, opposes that of the battery so that
the net e. m. f. acting to move electrons round the circuit is no longer
that of the battery, but is the difference between the e. m. f. of the
battery and that of the condenser. And so, with each added electron, the
e. m. f. of the condenser increases until finally it is just equal to
that of the battery and there is no net e. m. f. to act.

What would happen if we should then disconnect the battery? The
condenser would be left with its extra electrons in the negative plate
and with its positive plate lacking the same number of electrons. That
is, the condenser would be left charged and its e. m. f. would be of
the same number of volts as the battery.

[Illustration: Fig 26]

Now suppose we connect a short wire between the plates of the condenser
as in Fig. 26. The electrons rush home from the negative plate to the
positive plate. As fast as electrons get home the e. m. f. decreases.
When they are all back the e. m. f. has been reduced to zero. Sometimes
we say that "the condenser discharges." The "discharge current" starts
with a rush the moment the conducting path is offered between the two
plates. The e. m. f. of the condenser falls, the discharge current grows
smaller, and in a very short time the condenser is completely
discharged.

[Illustration: Fig 27]

That's what happens when there is a short conducting path for the
discharge current. If that were all that could happen I doubt if there
would be any radio communication to-day. But if we connect a coil of
wire between two plates o