m. f. is code for electromotive
force). I'll tell you, because the telling includes some other ideas
which will be valuable in your later reading.

[Illustration: Fig 13]

Suppose we take batteries which aren't going to be injured by being made
to work--storage batteries will do nicely--and connect them in series as
in Fig. 13. When batteries are in series they act like a single stronger
battery, one whose e. m. f. is the sum of the e. m. f.'s of the separate
batteries. Connect these batteries to a long fine wire as in Fig. 14.
There is a stream of electrons along this wire. Next connect the
negative terminal of the standard cell to the negative terminal of the
storage batteries, that is, brace their feet against each other. Then
connect a wire to the positive terminal of the standard cell. This wire
acts just like a long arm sticking out from the positive plate of this
cell.

[Illustration: Fig 14]

Touch the end of the wire, which is _p_ of Fig. 14, to some point
as _a_ on the fine wire. Now what do we have? Right at _a_, of
course, there are some free electrons and they hear the calls of both
batteries. If the standard battery, _S_ of the figure, calls the
stronger they go to it. In that case move the end _p_ nearer the
positive plate of the battery _B_, so that it will have a chance to
exert a stronger pull. Suppose we try at _c_ and find the battery
_B_ is there the stronger. Then we can move back to some point, say
_b_, where the pulls are equal.

To make a test like this we put a sensitive current-measuring instrument
in the wire which leads from the positive terminal of the standard cell.
We also use a long fine wire so that there can never be much of an
electron stream anyway. When the pulls are equal there will be no
current through this instrument.

As soon as we find out where the proper setting is we can replace
_S_ by some other battery, say _X_, which we wish to compare
with _S_. We find the setting for that battery in the same way as
we just did for _S_. Suppose it is at _d_ in Fig. 14 while the
setting for _S_ was at _b_. We can see at once that _X_
is stronger than _S_. The question, however, is how much stronger.

Perhaps it would be better to try to answer this question by talking
about e. m. f.'s. It isn't fair to speak only of the positive plate
which calls, we must speak also of the negative plate which is shooing
electrons away from itself. The idea of e. m. f. takes care of both
these