Fig. 143.
128. Induced Currents by Rotary Motion. The motions of the coils in straight lines are not suitable for producing currents strong enough for commercial purposes. In order to generate currents of considerable strength and pressure, the coils of wire have to be pushed past magnets, or electromagnets, with great speed. In the dynamo the coils are so wound that they can be given a rapid rotary motion as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them.
Fig. 144.
129. Field-Magnets; Armature; Commutator. What we need then, to produce an induced current by a rotary motion, is a strong magnetic field, a rotating coil of wire properly placed in the field, and some means of leading the current from the machine.
Fig. 145.
Fig. 146.
If a loop of wire, Fig. 140, be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. This agrees with the experiments suggested in § 127, when the current generated in a coil passed in one direction during its motion toward the strongest part of the field, and in the opposite direction when the coil passed out of it. A coil must be cut by lines of force to generate a current. A current inside of the machine, as in Fig. 140, would be of no value; it must be led out to external conductors where it can do work. Some sort of sliding contact is necessary to connect a revolving conductor with outside stationary ones. The magnet, called the field-magnet, is merely to furnish lines of magnetic force. The one turn of wire represents the simplest form of armature.