518 MAGNETISM AND ELECTRICITY the north pole, which is repelled again; thus the motor is kept running. The armature (rotating loops of wire) is called the rotor, while the Fig. 255. A simple laboratory demonstration motor. (Courtesy of the Central Scientific Company.) stationary electromagnets are spoken of as the stator, or field. Motors are such compact machines that they can be directly attached to the machines which they are designed to operate, thus eliminating belts, shafts, or chains necessary to steam-driven machinery. Motors require little care and are ready to operate at any instant at the turn of a switch. By using motors of different sizes, no more than the minimum power necessary is used. The reader would find it a profitable exercise to compile a list of home appliances which utilize electric motors. The average very small motor which is used in vacuum cleaners, sewing machines, stirrers, vibrators, and other household appliances will run on either direct or alternating current. Such motors are called "universal" motors. Two more efficient types of motors are known as the synchronous and induction alternating-current motors, most large alternating-current motors being of the latter type. The speed of synchronous motors depends upon the number of cycles per second of the alternating current supplied. Electric clocks are run by small synchronous motors, which are very simple in construction. The accuracy of electric clocks depends upon a continuous supply of current and proper regulation of the alternations in the current at the powerhouse. The fact that electric clocks are now in such wide use is an excellent testimony of the service furnished by most large electric-power companies. Fig. 256. A simple motor which uses electromagnets instead of permanent magnets. (Courtesy of the Central Scientific Company.)
ELECTROMAGNETIC INDUCTION 519 Alternating Current May Be Changed to Direct Current by Several Methods. A direct current may be generated by running a direct-current generator with an alternating-current motor. This is one of the best ways to produce direct current when only alternating current is available. Several types of rectifiers, such as the tungar battery-charger, the kenotron radio tube, and the thyratron, or grid-glow tube, act as one- FlG. 257. A battery-charging motor-generator set. (Courtesy of the General Electric Company.) way electron valves. The principle of these tubes will be explained in the next Section of this Unit. Copper-copper oxide rectifiers depend upon the fact that electrons can flow readily from the metal into and through the oxide film but cannot flow in appreciable amounts in the opposite direction. A cell containing a plate of aluminum and a plate of lead or some other metal dipping into a solution of some substance such as borax shows a similar rectifying action at the surface of the aluminum electrode. The mercury-arc rectifier is another common type of rectifier that was formerly widely used for charging batteries and is the most important method today for converting alternating current to direct current.