When the magnet is plunged into the hollow coil, the lines of force flow through the turns of wire, or are said to cut them. Whenever lines of force cut a coil of wire and they are in motion, electricity is produced. It does not matter whether the coil is slipped over the magnet or the magnet is plunged into the coil, a current will be produced as long as they are in motion. As soon as the magnet or the coil stops moving the current stops. By arranging a coil of wire between the poles of a horse-shoe magnet so that it can be made to revolve, the motion can be made continuous and the current of electricity maintained. Figure 252 shows such an arrangement. Some means of connection with the coil of wire must be established so that the current can be led off. If two metal rings are connected to the ends of the coil, connection can be made by little strips of metal called brushes rubbing against the rings. This scheme is the principle of the telephone magneto and the basis of all dynamos. [Illustration: Fig. 252.—The Principle of the Alternator and the Direct-Current Dynamo.] In the telephone magneto, more than one horseshoe magnet is usually provided. The coil of wire revolves between the poles of the magnets. The coil is wound around an iron frame and together they are called the armature. The end of the armature shaft is fitted with a small spur gear meshing with a larger gear bearing a crank, so that when the crank is turned the motion is multiplied and the armature is caused to revolve rapidly. One end of the coil or armature winding is connected to a small brass pin. This pin connects with a second pin set in the end of the shaft in an insulating brush of hard rubber. The other terminal of the coil is connected to the armature itself. Thus connection can be had to the coil by connecting a wire to the frame of the machine and to the insulated pin. [Illustration: Fig. 253.—Details of the Armature, Commutator, and Brushes.] The armature of a magneto is usually wound with a very fine silk insulated wire, about No. 36 B. & S. gauge in size. This should be carefully removed and wound upon a spool for future use. Replace the wire with some ordinary cotton-covered magnet wire, about No. 24 or 25 B. & S. gauge, winding it on very carefully and smoothly. Connect one end of the winding to the pin leading to the insulated pin by soldering it. This pin is the one at the end of the shaft opposite to that one to which the spur gear is fastened. Connect the other end of the wire to the pin at the same end of the shaft as the gear. This pin is grounded, that is, connected to the frame.