s.

He had plenty of electrons in it but no way to control them and make
their motions useful. In an audion besides the filament there are two
other things. One is a little sheet or plate of metal with a connecting
wire leading out through the glass walls and the other is a little wire
screen shaped like a gridiron and so called a "grid." It also has a
connecting wire leading through the glass. Fig. 4 shows an audion. It
will be most convenient, however, to represent an audion as in Fig. 5.
There you see the filament, _F_, with its two terminals brought out
from the tube, the plate, _P_, and between these the grid,
_G_.

[Illustration: Fig 5]

These three parts of the tube are sometimes called "elements." Usually,
however, they are called "electrodes" and that is why the audion is
spoken of as the "three-electrode vacuum tube." An electrode is what we
call any piece of metal or wire which is so placed as to let us get at
electrons (or ions) to control their motions. Let us see how it does so.

To start with, we shall forget the grid and think of a tube with only a
filament and a plate in it--a two-electrode tube. We shall represent it
as in Fig. 6 and show the battery which heats the filament by some lines
as at _A_. In this way of representing a battery each cell is
represented by a short heavy line and a longer lighter line. The heavy
line stands for the negative plate and the longer line for the positive
plate. We shall call the battery which heats the filament the "filament
battery" or sometimes the "A-battery." As you see, it is formed by
several battery cells connected in series.

[Illustration: Fig 6]

Sometime later I may tell you how to connect battery cells together and
why. For the present all you need to remember is that two batteries are
in series if the positive plate of one is connected to the negative
plate of the other. If the batteries are alike they will pull an
electron just twice as hard as either could alone.

[Illustration: Pl. IV.--Radiotron (Courtesy of Radio Corporation of
America).]

To heat the filament of an audion, such as you will probably use in your
set, will require three storage-battery cells, like the one I described
in my fourth letter, all connected in series. We generally use storage
batteries of about the same size as those in the automobile. If you will
look at the automobile battery you will see that it is made of three
cells connected in series. That battery would do