[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.
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