tery drives
electrons around a conducting circuit. You also found that there are
several different kinds of batteries. Batteries differ in their
abilities to drive electrons and it is therefore convenient to have some
way of comparing them. We do this by measuring the electron-moving-force
or "electromotive force" which each battery can exert. To express
electromotive force and give the results of our measurements we must
have some unit. The unit we use is called the "volt."

The volt is defined by law and is based on the suggestions of the same
body of scientists who recommended the ampere of our last letter. They
defined it by telling how to make a particular kind of battery and then
saying that this battery had an electromotive force of a certain number
of volts. One can buy such standard batteries, or standard cells as they
are called, or he can make them for himself. To be sure that they are
just right he can then send them to the Bureau of Standards and have
them compared with the standard cells which the Bureau has.

I don't propose to tell you much about standard cells for you won't have
to use them until you come to study physics in real earnest. They are
not good for ordinary purposes because the moment they go to work
driving electrons the conditions inside them change so their
electromotive force is changed. They are delicate little affairs and are
useful only as standards with which to compare other batteries. And even
as standard batteries they must be used in such a way that they are not
required to drive any electrons.

[Illustration: Fig 12]

Let's see how it can be done. Suppose two boys sit opposite each other
on the floor and brace their feet together. Then with their hands they
take hold of a stick and pull in opposite directions. If both have the
same stick-motive-force the stick will not move.

Now suppose we connect the negative feet--I mean negative terminals--of
two batteries together as in Fig. 12. Then we connect their positive
terminals together by a wire. In the wire there will be lots of free
electrons ready to go to the positive plate of the battery which pulls
the harder. If the batteries are equal in electromotive force these
electrons will stay right where they are. There will be no stream of
electrons and yet we'll be using one of the batteries to compare with
the other.

That is all right, you think, but what are we to do when the batteries
are not just equal in e. m. f.? (e.