We have seen that a current-carrying wire has a magnetic field and acts like a magnet; so it will be easily seen that if a magnet be held near the wire it will be either attracted or repelled, the motion depending upon the poles that come near each other. As shown in the figure, the N pole of the magnet repels the field of the wire, causing it to revolve. We see that this action is just the reverse to that in galvanometers, where the coil is fixed, and the magnet, or magnetic needle, is allowed to move. As soon as the part of the wire, marked A in Fig. 225, gets a little distance from the pole, the opposite side of the wire, B, begins to be attracted by it, the attraction getting stronger and stronger, until it gets opposite the N pole. If the N pole were still held in place, B would vibrate back and forth a few times, and finally come to rest near the pole. If, however, as soon as B gets opposite N the S pole of the magnet be quickly turned toward B, the coil will be repelled and the rotary motion will continue.
Fig. 228.
Figs. 229 to 231.
Fig. 232.
Fig. 233.
Let us now see how this helps to explain electric motors. We may consider the wire of Fig. 225 as one coil of an armature, and the plates, C and Z, as the halves of a commutator. In this arrangement, it must be noted, the current always flows through the armature coil in the same direction, the rotation being kept up by reversing the poles of the field-magnet. In ordinary simple motors the current is reversed in the armature coils, the field-magnets remaining in one position without changing the poles. This produces the same effect as the above. The current is reversed automatically as the brushes allow the current to enter first one commutator bar and then the opposite one as the armature revolves. The regular armatures have many coils and many commutator bars, as will be seen by examining the illustrations shown.