of
the battery and away from the negative plate. That's what the battery
does to them for being away from home; it drives them along the wire.
There's a regular stream or procession of them from the negative end of
the wire toward the positive. When we have a stream of electrons like
this we say we have a current of electricity.

We'll need to learn more later about a current of electricity but one of
the first things we ought to know is how a battery is made and why it
affects these wandering electrons in the copper wire. That's what I
shall tell you in my next letter.[1]

[Footnote 1: The reader who wishes the shortest path to the construction
and operation of a radio set should omit the next two letters.]

LETTER 3

HOW A BATTERY WORKS

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

MY DEAR BOY:

When I was a boy we used to make our own batteries for our experiments.
That was before storage batteries became as widely used as they are
to-day when everybody has one in the starting system of his automobile.
That was also before the day of the small dry battery such as we use in
pocket flash lights. The batteries which we made were like those which
they used on telegraph systems, and were sometimes called "gravity"
batteries. Of course, we tried several kinds and I believe I got quite a
little acid around the house at one time or another. I'll tell you about
only one kind but I shall use the words "electron," "proton," "nucleus,"
"atom," and "molecule," about some of which nothing was known when I was
a boy.

We used a straight-sided glass jar which would hold about a gallon. On
the bottom we set a star shaped arrangement made of sheets of copper
with a long wire soldered to it so as to reach up out of the jar. Then
we poured in a solution of copper sulphate until the jar was about half
full. This solution was made by dissolving in water crystals of "blue
vitriol" which we bought at the drug store.

Blue vitriol, or copper sulphate as the chemists would call it, is a
substance which forms glassy blue crystals. Its molecules are formed of
copper atoms, sulphur atoms, and oxygen atoms. In each molecule of it
there is one atom of copper, one of sulphur and four of oxygen.

When it dissolves in water the molecules of the blue vitriol go
wandering out into the spaces between the water molecules. But that
isn't all that happens or the most important thing for one who is
interested in making a battery.