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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.

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