ravel 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]

If we move the end of the wire from _b_ to _d_ the pointer
will take a new position. Opposite this we mark twice the number of
volts of the standard cell. We can run it to a point _e_ where the
distance _ne_ is one-half _nb_, and mark our scale with half
the number of volts of the standard cell, and so on for other positions
along the wire. That's the way we calibrate a sensitive
current-measuring instrument (with its added wire, of course) so that it
will read volts. It is now a voltmeter.

If we connect a voltmeter to the battery _X_ as in Fig. 16 the
pointer will tell us the number of volts in the e. m. f. of _X_,
for the pointer will take the same position as it did when the voltmeter
was connected between _n_ and _d_.

There is only one thing to watch out for in all this. We must be careful
that the voltmeter is so made that it won't offer too easy a path for
electrons to follow. We only want to find how hard a battery can pull an
electron, for that is what we mean by e. m. f. Of course, we must let a
small stream of electrons flow through the voltmeter so as to make the
pointer move. That is why voltmeters of this kind are made out of a long
piece of fine wire or else have a coil of fine wire in series with the
current-measuring part. The fine wire makes a long and narrow path for
the electrons and so there can be only a small stream. Usually we
describe this condition by saying that a voltmeter has a high
resistance.

[Illustration: Fig 16]

Fine wires offer more resistance to electron streams than do heavy wires
of the same length. If a wire is the same diameter all along, the longer
the length of it which we use the greater is the resistance which is
offered to an electron stream.

You will need to know how to describe the resistance of a wire or of any
part of an electric circuit. To do so you tell how many "ohms" of
resistance it has. The ohm is the unit in which we measure the
resistance of a circuit to an electron stream.

I can show you what an ohm is if I tell you a simple way to measure a
resistance. Suppose you have a wire or coil of wire and want to know its
resistance. Connect it in series with a battery and an ammeter as shown
in Fig. 17.