ppose that, before the B-battery was connected to the plate, each tiny
length of the filament was emitting 1000 electrons each second but was
getting 1000 back each second. There was no net change. Now, suppose
that the B-battery takes away 100 of these each second. Then only 900
get back to the filament and there is a net loss from the filament of
100. Each second this tiny length of filament sends into the vacuum 100
electrons which are taken out at the plate. From each little bit of
filament there is a stream of electrons to the plate. Millions of
electrons, therefore, stream across from filament to plate. That is,
there is a current of electricity between filament and plate and this
current continues to flow as long as the A-battery and the B-battery do
their work.

The negative terminal of the B-battery is connected to the filament.
Every time this battery pulls an electron from the plate its negative
terminal shoves one out to the filament. You know from my third and
fourth letters that electrons are carried through a battery from its
positive to its negative terminal. You see, then, that there is the same
stream of electrons through the B-battery as there is through the vacuum
between filament and plate. This same stream passes also through the
wires which connect the battery to the tube. The path followed by the
stream of electrons includes the wires, the vacuum and the battery in
series. We call this path the "plate circuit."

We can connect a telephone receiver, or a current-measuring instrument,
or any thing we wish which will pass a stream of electrons, so as to let
this same stream of electrons pass through it also. All we have to do is
to connect the instrument in series with the other parts of the plate
circuit. I'll show you how in a minute, but just now I want you to
understand that we have a stream of electrons, for I want to tell you
how it may be controlled.

Suppose we use another battery and connect it between the grid and the
filament so as to make the grid positive. That would mean connecting the
positive terminal of the battery to the grid and the negative to the
filament as shown by the C-battery of Fig. 8. This figure also shows a
current-measuring instrument in the plate circuit.

What effect is this C-battery, or grid-battery, going to have on the
current in the _plate circuit_? Making the grid positive makes it
want electrons. It will therefore act just as we saw that the plate did
and