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 previously described for the motor.

[Illustration:  FIG. 239.—­A modern electrical machine.]

The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction.  Such alternating currents are unsatisfactory for many purposes, and to be of service are in many cases transformed into direct currents; that is, current which flows steadily in one direction.  This is accomplished by the use of a commutator.  In the construction of the motor, continuous motion in one direction is obtained by the use of a commutator (Section 310); in the construction of a dynamo, continuous current in one direction is obtained by the use of a similar device.

322.  Powerful Dynamos.  The power and efficiency of a dynamo are increased by employing the devices previously mentioned in connection with the motor.  Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up of many coils wound on soft iron.  The speed with which the armature is rotated influences the strength of the induced current, and hence the armature is run at high speed.