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 generated in the armature coils, and this current, as it traverses the field coils as well as those of the armature, causes the polarity of both parts to increase. As a result 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 that the current, which starts with an infinitesimal strength, soon rises to the maximum capacity of the machine.