ick plating is made. If, therefore, the exact
value of the current is known, the exact amount of silver which will
be deposited on the spoon in a given time can be definitely
calculated.

[Illustration: FIG. 233.--The principle of the galvanometer.]

Current-measuring instruments, or galvanometers, depend for their
action on the magnetic properties of current electricity. The
principle of practically all galvanometers is as follows:--

A closely wound coil of fine wire free to rotate is suspended as in
Figure 233 between the poles of a strong magnet. When a current is
sent through the coil, the coil becomes a magnet and turns so that its
faces will be towards the poles of the permanent magnet. But as the
coil turns, the suspending wire becomes twisted and hinders the
turning. For this reason, the coil can turn only until the motion
caused by the current is balanced by the twist of the suspending wire.
But the stronger the current through the coil, the stronger will be
the force tending to rotate the coil, and hence the less effective
will be the hindrance of the twisting string. As a consequence, the
coil swings farther than before; that is, the greater the current, the
farther the swing. Usually a delicate pointer is attached to the
movable coil and rotates freely with it, so that the swing of the
pointer indicates the relative values of the current. If the source of
the current is a gravity cell, the swing is only two thirds as great
as when a dry cell is used, indicating that the dry cell furnishes
about 1-1/2 times as much current as a gravity cell.

314. Ammeters. A galvanometer does not measure the current, but
merely indicates the relative strength of different currents. But it
is desirable at times to measure a current in units. Instruments for
measuring the strength of currents in units are called ammeters, and
the common form makes use of a galvanometer.

A current is sent through a movable coil (the field magnet and coil
are inclosed in the case) (Fig. 234), and the magnetic field thus
developed causes the coil to turn, and the pointer attached to it to
move over a scale graduated so that it reads current strengths. This
scale is carefully graduated by the following method.

If two silver rods (Fig. 208) are weighed and placed in a solution of
silver nitrate, and current from a single cell is passed through the
liquid for a definite time, we find, on weighing the two rods, that
one has gained in weight