n is for some of the silver ions to get out
of the solution. They aren't going back to the positive silver plate
from which they just came. They go on toward the negative plate where
the battery is sending an electron for every one which it takes away
from the positive plate. There start off towards the negative plate, not
only the ions which just came from the positive plate, but all the ions
that are in the solution. The first one to arrive gets an electron but
it can't take it away from the silver plate. And why should it? As soon
as it has got this electron it is again a normal silver atom. So it
stays with the other atoms in the silver plate. That's what happens
right along. For every atom which is lost from the positive plate there
is one added to the negative plate. The silver of the positive plate
gradually wastes away and the negative plate gradually gets an extra
coating of silver.

Every time the battery takes an electron away from the positive plate
and gives it to the negative plate there is added to the negative plate
an atom of silver. If the negative plate is weighed before the battery
is connected and again after the battery is disconnected we can tell how
much silver has been added to it. Suppose the current has been perfectly
steady, that is, the same number of electrons streaming through the
circuit each second. Then if we know how long the current has been
running we can tell how much silver has been deposited each second.

The law says that if silver is being deposited at the rate of 0.001118
gram each second then the current is one ampere. That's a small amount
of silver, only about a thousandth part of a gram, and you know that it
takes 28.35 grams to make an ounce. It's a very small amount of silver
but it's an enormous number of atoms. How many? Six billion billion, of
course, for there is deposited one atom for each electron in the stream.

In my next letter I'll tell you how we measure the pull which batteries
can give to electrons, and then we shall be ready to go on with more
about the audion.

LETTER 8

ELECTRON-MOVING-FORCES

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

DEAR YOUNG MAN:

I trust you have a fairly good idea that an ampere means a stream of
electrons at a certain definite rate and hence that a current of say 3
amperes means a stream with three times as many electrons passing along
each second.

In the third and fourth letters you found out why a bat