through which the current enters and leaves are held stationary; keeping this fact in mind, it can be seen at once that in Fig. 12 the current will flow from the brush a through the two sides of the armature wire to brush b, hence all the coils on the right of the vertical line will be traversed by the current in the same direction—that is, either to or from the center of the shaft—and in the coils on the left the direction will be opposite, which is just the same order as was explained in connection with Fig. 11.
An electric motor can be turned into an electric generator by simply reversing the direction in which the armature rotates—that is, any electric machine is either a generator or a motor. This fact can be illustrated by means of Figs. 13 and 14, both of which show the armature and the poles of the field magnet. The first figure represents an electric motor, and, as can be seen, the pull between the N pole of the armature and the P pole of the field is in the direction of arrow b, hence the armature will rotate in the same direction, as indicated by arrow a. To obtain the polarity of the armature and field it is necessary to pass an electric current through both—that is to say, we must expend electrical energy to obtain power from the machine. As soon as the current ceases to flow, the polarity of the armature and field dies out, and the rotation of the former comes to an end. The magnetism, however, does not die out entirely; a small residue is always left, although it is never sufficient to produce rotation, and even if it were it could only cause the armature to revolve through one quarter of a turn. If, after the current has been shut off, the armature shaft is rotated in the reverse direction, as indicated by arrow a in Fig. 14, the motion will be against the pull of the magnetism; therefore, although the poles may be very weak, an amount of power sufficient to overcome their attraction must be applied to the pulley, otherwise rotation can not be accomplished. In consequence of the backward rotation a current is Fig. 13.Fig. 14. Figs 13, 14.—Diagrams illustrating the Difference between an Electric Motor and a Generator. generated in the armature coils, and this current, as it traverses the field coils as well as those of the armature, of the increased polarity the resistance to rotation is increased, and more power has to be applied to the pulley. The increase in the strength of the poles results in increasing the current generated, and this in turn further increases the pole strength, so that one effect helps the other, the result being