=160. Currents and Motion.= We have seen, Chapter XII., that when coils of wire are rapidly moved across a strong magnetic field, a current of electricity is generated. We have now to deal with the opposite of this; that is, we are to study how _motion_ can be produced by allowing a current of electricity to pass through the armature of a machine. [Illustration: Fig. 224.] [Illustration: Fig. 225.] Fig. 224 shows, by diagram, a coil H, suspended so that it can move easily, its ends being joined to a current reverser, and this, in turn, to a dry cell D C. A magnet, H M, will attract the core of H when no current passes. When the current is allowed to pass first in one direction and then in the opposite direction, by using the reverser, the core of H will jump back and forth from one pole of H M to the other. There are many ways by which motion can be produced by the current, but to have it practical, the motion must be a rotary one. (See "Study," Chapter XXVI., for numerous experiments.) [Illustration: Fig. 226.] =161. The Electric Motor= is a machine for transforming electric energy into mechanical power. The construction of motors is very similar to that of dynamos. They have field-magnets, armature coils, commutator, etc.; in fact, the armature of an ordinary direct current dynamo will revolve if a current be passed through it, entering by one brush and leaving by the other. There are many little differences of construction, for mechanical and electrical reasons, but we may say that the general construction of dynamos and motors is the same. Fig. 225 shows a coil of wire, the ends of which are connected to copper and zinc plates. These plates are floated in dilute sulphuric acid, and form a simple cell which sends a current through the wire, as shown by the arrows. [Illustration: Fig. 227.] 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. [Illustration: Fig. 228.] [Illustration: Figs. 229 to 231.] [Illustration: Fig. 232.] [Illustration: Fig. 233.]