Alternating Current (ac) Generators
In an electrical generator, the galvanometer mentioned above would be replaced by some electrical device. For example, in an automobile, electrical current from the generator is used to operate headlights, the car radio, and other electrical systems within the car. The ends of the coil are attached not to a galvanometer, then, but to slip rings or collecting rings. Each slip ring, in turn, is attached to a brush, through which electrical current is transferred from the slip ring to an external circuit.
As the metal coil passes through the magnetic field in a generator, the electrical power produced constantly changes. At first, the generated electric current moves in one direction (as from left to right). Then, when the coil reaches a position where it is parallel to the magnetic lines of force, no current at all is produced. Later, as the coil continues to rotate, it cuts through magnetic lines of force in the opposite direction, and the electrical current generated travels in the opposite direction (as from right to left).
Thus, a spinning coil in a fixed magnetic field of the type described here will produce an alternating current, one that travels in one direction for a moment of time, and then the opposite direction at the next moment of time. The rate at which the current switches back and forth is known as its frequency. The current used for most household devices, for example, is 60 hertz (60 cycles per second).
The efficiency of a generator can be increased by substituting for the wire coil described above an armature. An armature consists of a cylindrical iron core around which is wrapped a long piece of wire. The longer the piece of wire, the greater the electrical current that can be generated by the armature.