Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ed a sliding orwiping contact is thusestablished and thecurrent is conducted tothe external circuit. During the first halfrevolution of the loop,ABCD, Fig. 307, thedirection of current inAB is from B towardA, and from brush, M,which is therefore posi-tive to the external cir-cuit, composed of in-candescent lamps inparallel. From thelamp terminals thecurrent flows back tobrush P, the negativebrush, and around the loop from C to D, to B, etc. Now con-sider the second half revolution, F
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ed a sliding orwiping contact is thusestablished and thecurrent is conducted tothe external circuit. During the first halfrevolution of the loop,ABCD, Fig. 307, thedirection of current inAB is from B towardA, and from brush, M,which is therefore posi-tive to the external cir-cuit, composed of in-candescent lamps inparallel. From thelamp terminals thecurrent flows back tobrush P, the negativebrush, and around the loop from C to D, to B, etc. Now con-sider the second half revolution, Fig. 308. The direction ofcurrent in the wires AB, and CD, is reversed, Flemings rule,and current flows from D to C, through brush P, now positive,then through the lamps in the opposite direction to that inFig. 307, and through brush M, now negative, to AB, to D,etc. In every revolution the polarity of the brushes changestwice, or there are two alternations of current per revolution inthe external circuit. The number of alternations per minute inany alternator equals the speed in revolutions per minute mul-. JLANLPS Fig. 307.—Simple Alternating Current Dynamo. At the instant depicted in the revolutionbrush M is positive. DYNAMO ELECTRIC MACHINES. 339 tiplied by the number of poles. The number of times whichthe alternations occur is usually expressed simply as the fre-quency, thus, a frequency of 7200 alternations per minutemeans 7200 reversals of current. The frequency is usuallyexpressed for the period of one minute. Students Experimental Dynamo and Motor.—A simple appa-ratus for studying the principles of induction and commutation in adynamo, is illustrated in Fig. 309, and consists of a horseshoe electro-magnet fitted with cast-iron pole pieces and mounted on a woodenbase. A rectangular coil of No. 26 copper wire is mounted on a hard-wood sbaft, provided withpointed steel ends and sus-pended between two brass Vcentres, suitably supportedby a brass framework ex-tending fro
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