very well for the
filament circuit.

By the way, do you know what a "circuit" is? The word comes from the
same Latin word as our word "circus." The Romans were very fond of
chariot racing at their circuses and built race tracks around which the
chariots could go. A circuit, therefore, is a path or track around which
something can race; and an electrical circuit is a path around which
electrons can race. The filament, the A-battery and the connecting wires
of Fig. 6 form a circuit.

[Illustration: Fig 7]

Let us imagine another battery formed by several cells in series which
we shall connect to the tube as in Fig. 7. All the positive and negative
terminals of these batteries are connected in pairs, the positive of one
cell to the negative of the next, except for one positive and one
negative. The remaining positive terminal is the positive terminal of
the battery which we are making by this series connection. We then
connect this positive terminal to the plate and the negative terminal to
the filament as shown in the figure. This new battery we shall call the
"plate battery" or the "B-battery."

Now what's going to happen? The B-battery will want to take in electrons
at its positive terminal and to send them out at its negative terminal.
The positive is connected to the plate in the vacuum tube of the figure
and so draws some of the electrons of the plate away from it. Where do
these electrons come from? They used to belong to the atoms of the plate
but they were out playing in the space between the atoms, so that they
came right along when the battery called them. That leaves the plate
with less than its proper number of electrons; that is, leaves it
positive. So the plate immediately draws to itself some of the electrons
which are dodging about in the vacuum around it.

Do you remember what was happening in the tube? The filament was
steadily going on emitting electrons although there were already in the
tube so many electrons that just as many crowded back into the filament
each second as the filament sent out. The filament was neither gaining
nor losing electrons, although it was busy sending them out and
welcoming them home again.

When the B-battery gets to work all this is changed. The B-battery
attracts electrons to the plate and so reduces the crowd in the tube.
Then there are not as many electrons crowding back into the filament as
there were before and so the filament loses more than it gets back.

Su