welcome. And
the moment a couple of them start off on that errand along comes another
sulphate ion in the solution and lands two more electrons on the plate.
That's how the battery keeps on discharging.

We mustn't let it get too much discharged for the lead sulphate is not
soluble, as I just told you, and it will coat up that plate until there
isn't much chance of getting the process to reverse. That's why we are
so careful not to let the discharge process go on too long before we
reverse it and charge. That's why, when the car battery has been used
pretty hard to start the car, I like to run quite a while to let the
generator charge the battery again. When the battery charges, the
process reverses and we get spongy lead on the negative plate and lead
peroxide on the positive plate.

You've learned enough for one day. Write me your questions and I'll
answer and then go on in my next letter to tell how the audion works.
You know about conduction of electricity in wires; that is, about the
electron stream, and about batteries which can cause the stream. Now you
are ready for the most wonderful little device known to science: the
audion.

LETTER 5

GETTING ELECTRONS FROM A HEATED WIRE

DEAR SON:

I was pleased to get your letter and its questions. Yes, a proton is a
speck of electricity of the kind we call positive and an electron is of
the kind we call negative. You might remember this simple law; "Like
kinds of electricity repel, and unlike attract."

The word ion[2] is used to describe any atom, or part of a molecule
which can travel by itself and has more or less than its proper number
of electrons. By proper number of electrons I mean proper for the number
of protons which it has. If an ion has more electrons than protons it is
negative; if the inequality is the other way around it is positive. An
atom or molecule has neither more nor less protons than electrons. It is
neutral or "uncharged," as we say.

No, not every substance which will dissolve will dissociate or split up
into positive and negative ions. The salt which you eat will, but the
sugar will not. If you want a name for those substances which will
dissociate in solution, call them "electrolytes." To make a battery we
must always use an electrolyte.

Yes, it is hard to think of a smooth piece of metal or a wire as full
of holes. Even in the densest solids like lead the atoms are quite far
apart and there are large spaces between the nu