actions. The steady stream of electrons in the fine wire is due to
the e. m. f. of the battery _B_, that is to the pull of the
positive terminal and the shove of the negative.

If the wire is uniform, that is the same throughout its length, then
each inch of it requires just as much e. m. f. as any other inch. Two
inches require twice the e. m. f. which one inch requires. We know how
much e. m. f. it takes to keep the electron stream going in the part of
the wire from _n_ to _b_. It takes just the e. m. f. of the
standard cell, _S_, because when that had its feet braced at
_n_ it pulled just as hard at _b_ as did the big battery
_B_.

Suppose the distance _n_ to _d_ (usually written _nd_) is
twice as great as that from _n_ to _b_ (_nb_). That means
that battery _X_ has twice the e. m. f. of battery _S_. You
remember that _X_ could exert the same force through the length of
wire _nd_, as could the large battery. That is twice what cell
_S_ can do. Therefore if we know how many volts to call the e. m.
f. of the standard cell we can say that _X_ has an e. m. f. of
twice as many volts.

If we measured dry batteries this way we should find that they each had
an e. m. f. of about 1.46 volts. A storage battery would be found to
have about 2.4 volts when fully charged and perhaps as low as 2.1 volts
when we had run it for a while.

That is the way in which to compare batteries and to measure their e. m.
f.'s, but you see it takes a lot of time. It is easier to use a
"voltmeter" which is an instrument for measuring e. m. f.'s. Here is how
one could be made.

First there is made a current-measuring instrument which is quite
sensitive, so that its pointer will show a deflection when only a very
small stream of electrons is passing through the instrument. We could
make one in the same way as we made the ammeter of the last letter but
there are other better ways of which I'll tell you later. Then we
connect a good deal of fine wire in series with the instrument for a
reason which I'll tell you in a minute. The next and last step is to
calibrate.

We know how many volts of e. m. f. are required to keep going the
electron stream between _n_ and _b_--we know that from the e.
m. f. of our standard cell. Suppose then that we connect this new
instrument, which we have just made, to the wire at _n_ and
_b_ as in Fig. 15. Some of the electrons at _n_ which are so
anxious to get away from the negative plate of battery _B_ can now
t