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 travel as far as _b_ through the wire of the new instrument. They do so and the pointer swings around to some new position. Opposite that we mark the number of volts which the standard battery told us there was between _n_ and _b_. [Illustration: Fig 15]