it. If only the extra
electrons on the negative zinc plate could get around to the positive
copper plate. They can if we connect a wire from one plate to the other.
Then the electrons from the zinc stream into the spaces between the
atoms of the wire and push ahead of them the electrons which are
wandering around in these spaces. At the other end an equal number of
electrons leave the wire to satisfy the positive copper plate. So we
have a stream of electrons in the wire, that is, a current of
electricity and our battery is working.

That's the sort of a battery I used to play with. If you understand it
you can get the general idea of all batteries. Let me express it in
general terms.

At the negative plate of a battery ions go into solution and electrons
are left behind. At the other end of the battery positive ions are
crowded out of solution and join the plate where they cause a scarcity
of electrons; that is, make the plate positive. If a wire is connected
between the two plates, electrons will stream through it from the
negative plate to the positive; and this stream is a current of
electricity.

[Illustration: Pl. III.--Dry Battery for Use in Audion Circuits
(Courtesy of National Carbon Co., Inc.). Storage Battery (Courtesy of
the Electric Storage Battery Co.).]

LETTER 4

THE BATTERIES IN YOUR RADIO SET

(This letter may be omitted on the first reading.)

MY DEAR YOUNG MAN:

You will need several batteries when you come to set up your radio
receiver but you won't use such clumsy affairs as the gravity cell which
I described in my last letter. Some of your batteries will be dry
batteries of the size used in pocket flash lights.

These are not really dry, for between the plates they are filled with a
moist paste which is then sealed in with wax to keep it from drying out
or from spilling. Instead of zinc and copper these batteries use zinc
and carbon. No glass jar is needed, for the zinc is formed into a jar
shape. In this is placed the paste and in the center of the paste a rod
or bar of carbon. The paste doesn't contain sulphuric acid, but instead
has in it a stuff called sal ammoniac; that is, ammonium chloride.

The battery, however, acts very much like the one I described in my last
letter. Ions of zinc leave the zinc and wander into the moist paste.
These ions are positive, just as in the case of the gravity battery. The
result is that the electrons which used to associate with a zinc ion