POLYPHASE CURRENTS.

41

poles one-half, and thereby doubling the speed of any givenmotor.

It is obvious that the Tesla electrical transformers which haveindependent primary currents may be used with the generatorsdescribed. It may also be stated with respect to the deviceswe now describe that the most perfect and harmonious actionof the generators and motors is obtained when the numbers of thepoles of each are even and not odd. If this is not the case, therewill be a certain unevenness of action which is the less appreci-able as the number of poles is greater; although this may be in ameasure corrected by special provisions which it is not herenecessary to explain. It also follows, as a matter of course, thatif the number of the poles of the motor be greater than that ofthe generator the motor will revolve at a slower speed than thegenerator.

In this chapter, we may include a method devised by Mr.Tesla for avoiding the very high speeds which would be neces-sary with large generators. In lieu of revolving the generatorarmature at a high rate of speed, he secures the desired result bya rotation of the magnetic poles of one element of the generator,while driving the other at a different speed. The effect is thesame as that yielded by a very high rate of rotation.

In this instance, the generator which supplies the current foroperating the motors or transformers consists of a subdividedring or annular core wound witli four diainetrically-oppositecoils, e e 7 , Fig. 33. Within the ring is mounted a cylindricalarmature-core wound longitudinally with two independent coils,f it, the ends of which lead, respectively, to two pairs of insu-lated contact or collecting rings, r> d' g g 7 , on the armature shaft.Collecting brushes d d' g g' bear upon these rings, respectively,and convey the currents through the two independent line-cir-cuits m m 7 . In the main line there may be included one or moremotors or transformers, or both. If motors be used, they are ofthe usual form of Tesla construction with independent coils orsets of coils j ,j 7 , included, respectively, in the circuits m m 7 .These energizing-coils are wound on a ring or annular field or onpole pieces thereon, and produce by the action of the alternatingcurrents passing through them a progressive shifting of the mag-netism from pole to pole. The cylindrical armature h of themotor is wound with two coils at right angles, which form inde-pendent closed circuits.