and the other has lost. If the current is
allowed to flow twice as long, the amount of silver lost and gained by
the electrodes is doubled; and if twice the current is used, the
result is again doubled.

As a result of numerous experiments, it was found that a definite
current of electricity will deposit a definite amount of silver in a
definite time, and that the amount of silver deposited on an electrode
in one second might be used to measure the current of electricity
which has flowed through the circuit in one second.

A current is said to be one ampere strong if it will deposit silver on
an electrode at the rate of 0.001118 gram per second.

[Illustration: FIG. 234.--An ammeter.]

In marking the scale, an ammeter is placed in the circuit of an
electrolytic cell and the position of the pointer is marked on the
blank card which lies beneath and which is to serve as a scale (Fig.
235). After the current has flowed for about an hour, the amount of
silver which has been deposited is measured. Knowing the time during
which the current has run, and the amount of deposit, the strength of
the current in amperes can be calculated. This number is written
opposite the place at which the pointer stood during the experiment.

The scale may be completed by marking the positions of the pointer
when other currents of known strength flow through the ammeter.

[Illustration: FIG. 235.--Marking the scale of an ammeter.]

All electric plants, whether for heating, lighting, or for machinery,
are provided with ammeters, such instruments being as important to an
electric plant as the steam gauge is to the boiler.

315. Voltage and Voltmeters. Since electromotive force, or voltage,
is the cause of current, it should be possible to compare different
electromotive forces by comparing the currents which they produce in a
given circuit. But two voltages of equal value do not give equal
currents unless the resistances met by the currents are equal. For
example, the simple voltaic cell and the gravity cell have
approximately equal voltages, but the current produced by the voltaic
cell is stronger than that produced by the gravity cell. This is
because the current meets more resistance within the gravity cell
than within the voltaic cell. Every cell, no matter what its nature,
offers resistance to the flow of electricity through it and is said to
have internal resistance. If we are determining the voltages of
various cells by a comparis