73 6 



THE POPULAR SCIENCE MONTHLY. 



on both generator and motor, it is evident that the armature coils 

 of the motor will be traversed by successive positive and negative 

 electrical impulses at just the right time, if the armature rotates 

 in unison with that of the generator, as both armatures then pass 

 through like portions of their magnetic fields during the same 

 current phase. If these alternating current impulses are not, how- 

 ever, properly timed, they will interfere with each other and the 

 motor armature will not rotate. It is possible, then, to utilize the 

 alternating-current dynamo as a motor, but only on the con- 

 dition that it runs synchronously with the generator. Evi- 

 dently it must first be brought up to the speed of the generator 

 before the conditions are realized that will keep it in motion. 

 As a practical motor it has therefore the fatal defect that it will 

 not start of itself, and it has the further one that it is readily 

 thrown out of synchronism by a slight excess of load, and is then 

 speedily brought to a standstill. 



Clearly an apparatus so sensitive as this could not be relied 

 upon for commercial work nor expected to stand as a solution of 



Fig. G. — Diagram illustrating Principle of Tesla Motor. 



the alternating-current motor problem. When Mr. Tesla took up 

 the question he sought for a new principle of action and found 

 it in what has since come to be known as the multiphase current. 

 He conceived that by providing the armature of his generator 

 and the field of his motor with two more sets of coils, connected 

 so as to form distinct circuits, he would be able to produce a pro- 

 gressive shifting of the magnetic poles of the motor field, and 

 thus drag around an armature capable of magnetic induction and 

 placed within the sphere of influence of his rotating field. This 



