on of the respective currents produced, the
result will be true only on condition that the resistances in the
various circuits are equal. If a very large external resistance of
fine wire is placed in circuit with a gravity cell, the _total_
resistance of the circuit (made up of the relatively small resistance
in the cell and the larger resistance in the rest of the circuit) will
differ but little from that of another circuit in which the gravity
cell is replaced by a voltaic cell, or any other type of cell.

With a high resistance in the outside circuit, the deflections of the
ammeter will be small, but such as they are, they will fairly
accurately represent the electromotive forces which produce them.

Voltmeters (Fig. 236), or instruments for measuring voltage, are like
ammeters except that a wire of very high resistance is in circuit with
the movable coil. In external appearance they are not distinguishable
from ammeters.

[Illustration: FIG. 236.--A voltmeter.]

The unit of electromotive force is called the _volt_. The voltage of a
dry cell is approximately 1.5 volts, and the voltage of a voltaic cell
and of a gravity cell is approximately 1 volt.

316. Current, Voltage, Resistance. We learned in Section 287 that
the strength of a current increases when the electromotive force
increases, and diminishes when the electromotive force diminishes.
Later, in Section 288, we learned that the strength of the current
decreases as the resistance in circuit increases.

The strength of a steady current depends upon these two factors only,
the electromotive force which causes it and the resistance which it
has to overcome.

317. Resistance. Since resistance plays so important a role in
electricity, it becomes necessary to have a unit of resistance. The
practical unit of resistance is called an ohm, and some idea of the
value of an ohm can be obtained if we remember that a 300-foot length
of common iron telegraph wire has a resistance of 1 ohm. An
approximate ohm for rough work in the laboratory may be made by
winding 9 feet 5 inches of number 30 copper wire on a spool or
arranging it in any other convenient form.

In Section 299 we learned that substances differ very greatly in the
resistance which they offer to electricity, and so it will not
surprise us to learn that while it takes 300 feet of iron telegraph
wire to give 1 ohm of resistance, it takes but 39 feet of number 24
copper wire, and but 2.2 feet of number 24