738 THE POPULAR SCIENCE MONTHLY. 



the ring an eighth, of a revolution. They will be shifted another 

 eighth when the generator armature reaches the position shown 

 in the last diagram, and will be successively displaced around the 

 ring R as this armature revolves until a complete revolution has 

 been made, when the parts are in their original position and ready 

 to repeat the same cycle of operations. 



The principle of the rotation of the magnetic poles has been 

 applied by Mr. Tesla to a great variety of constructions. He has 

 designed machines in which the field magnetism remains fixed 

 and that of the armature is shifted, and others again in which 

 there is a progressive shifting of the magnetic poles of both 

 the field and armature in opposite directions. He has also found 

 that the motor armature may consist of sets of closed coils, cur- 

 rents being developed in them by induction, and by making the 

 induced portion of the generator stationary and the field revolv- 

 ing he has been able to produce apparatus free from all movable 

 electrical contacts. In operating motors of this character Mr. 

 Tesla usually employed a generator with multiple armature cir- 

 cuits as described above; but in the course of his experiments 

 he discovered that the ordinary continuous or direct current 

 machine could by slight alterations be made to furnish an alter- 

 nating multiphase current as well as and in addition to the 

 direct current. To accomplish this he found it was only neces- 

 sary to add to the machine a pair of collector rings for each 

 circuit of the multiphase current, and connect them with the 

 proper armature coils. If, for instance, he desired to produce 

 a two-phase current requiring two circuits from his generator 

 to his motor, one circuit would include a set of coils in the arma- 

 ture of the generator that were passing through the position in 

 which the maximum current was being produced, and the other a 

 set of coils in which at the same time the minimum current was 

 being generated. The phases of the current would then follow each 

 other in the same order as in the previous machines with distinct 

 circuits on the armature. With this form of machine a multiple- 

 phase alternating current, it will be seen, can be taken off from 

 the collector rings, while a direct current can be taken from the 

 commutator, and a part or the whole of this direct current be 

 sent through the field coils to energize them and then put to any 

 use for which such currents are suitable. 



This machine was later developed into what has come to be 

 known as a rotary transformer. Instead of being driven by 

 power it is driven by one of the forms of current which it is 

 capable of furnishing, the other current being taken off and 

 utilized. For example, if a multiple-phase current is passed 

 into the machine by the collector rings it will be driven as a 

 motor and generate direct or continuous currents. If, on the 



