, but the
deflection is in the opposite direction, showing that a current
exists, but that it flows in the opposite direction. We learn,
therefore, that a current may be induced in a coil by moving a magnet
back and forth within the coil, but that a magnet at rest within the
coil has no such influence.

An electric current transforms a coil into a magnet. A magnet in
motion induces electricity within a coil; that is, causes a current to
flow through the coil.

A magnet possesses lines of force, and as the magnet moves toward the
coil it carries lines of force with it, and the coil is cut, so to
speak, by these lines of force. As the magnet recedes from the coil,
it carries lines of force away with it, this time reducing the number
of the lines which cut the coil.

[Illustration: FIG. 238.--As long as the coil rotates between the
poles of the magnet, current flows.]

320. A Test of the Preceding Statement. We will test the statement
that a magnet has electric properties by another experiment. Between
the poles of a strong magnet suspend a movable coil which is connected
with a sensitive galvanometer (Fig. 237). Starting with the coil in
the position of Figure 228, when many lines of force pass through it,
let the coil be rotated quickly until it reaches the position
indicated in Figure 238, when no lines of force pass through it.
During the motion of the coil, a strong deflection of the galvanometer
is observed; but the deflection ceases as soon as the coil ceases to
rotate. If, now, starting with the position of Figure 238, the coil is
rotated forward to its starting point, a deflection occurs in the
opposite direction, showing that a current is present, but that it
flows in the opposite direction. So long as the coil is in motion, it
is cut by a varying number of lines of force, and current is induced
in the coil.

_The above arrangement is a dynamo in miniature_. By rotation of a
coil (armature) within a magnetic field, that is, between the poles of
a magnet, current is obtained.

In the _motor_, current produces motion. In the _dynamo_, motion
produces current.

321. The Dynamo. As has been said, the arrangement of the preceding
Section is a dynamo in miniature. Every dynamo, no matter how complex
its structure and appearance, consists of a coil of wire which can
rotate continuously between the poles of a strong magnet. The
mechanical devices to insure easy rotation are similar in all respects
to those pre