you tell me what is going
to happen to the stream of electrons in the plate circuit? Remember that
just at the instant when we closed the switch the grid was neither
positive nor negative. We were at the point of zero volts on the audion
characteristic of Fig. 35. When we close the switch the current in the
plate circuit starts to jump from zero mil-amperes to the number of
mil-amperes which represents the point where Zero Volt St. crosses
Audion Characteristic. But this jump in plate current makes the grid
positive as we have just seen. So the grid will help the plate call
electrons and that will make the current in the plate circuit still
larger, that is, result in a larger stream of electrons from _a_ to
_b_.

This increase in current will be matched by an increased effect in the
coil _cd_, for you remember how you and "Brownie" behaved. And that
will pull more electrons away from plate 1 of the condenser and send
them to the waiting-room of 2. All this makes the grid more positive and
so makes it call all the more effectively to help the plate move
electrons.

[Illustration: Pl. V.--Variometer (top) and Variable Condenser (bottom)
of the General Radio Company. Voltmeter and Ammeter of the Weston
Instrument Company.]

We "started something" that time. It's going on all by itself. The grid
is getting more positive, the plate current is getting bigger, and so
the grid is getting more positive and the plate current still bigger. Is
it ever going to stop? Yes. Look at the audion characteristic. There
comes a time when making the grid a little more positive won't have any
effect on the plate-circuit current. So the plate current stops
increasing.

There is nothing now to keep pulling electrons away from plate 1 and
crowding them into waiting-room 2. Why shouldn't the electrons in this
waiting-room go home to that of plate 1? There is now no reason and so
they start off with a rush.

Of course, some of them came from the grid and as fast as electrons get
back to the grid it becomes less and less positive. As the grid becomes
less and less positive it becomes less and less helpful to the plate.

If the grid doesn't help, the plate alone can't keep up this stream of
electrons. All the plate can do by itself is to maintain the current
represented by the intersection of zero volts and the audion
characteristic. The result is that the current in the plate circuit,
that is, of course, the current in coil _ab_, becomes gr