1891.] 



on Electro-magnetic Bepulsion. 



315 



nating current, and are placed on either side of a drum armature 

 wound over with three sets of insulated wire coils, the terminals of 

 the coils coming to insulated sections of a commutator. The func- 

 tions of this commutator are to keep one coil, B, on short circuit 

 during the time when it is in such positions relatively to the field 

 coils C that the induced current in the closed coil causes it to be 

 repelled by the field coils, and as each successive coil on the armature 

 becomes in turn the active coil, rotation is kept up. A motor, made 

 by Prof. Thomson, based on these principles,^but with some additions, 

 is on the table, and on turning the current into it it speedily starts 

 and gets up considerable speed. The details of the actual construc- 

 tion are a little less simple than in the diagram shown, because the 



Fig. 19. 



Diagram illustrating elementary form of alternating-current motor. 



motor is made to start by sending the external current into the 

 armature by means of a commutator and brushes. When, however, 

 the proper speed is attained, the armature coils are automatically 

 short-circuited, and the motor continues to run in virtue of the 

 current induced by the field-magnet in the armature coils. This is 

 by no means, however, the last word said on this portion of the 

 applications, and I think we may shortly look to Prof. Thomson to 

 give us further and more perfect methods of utilising these facts in 

 the construction of self-starting alternating motors. 



§ 18. For the opportunity of exhibiting to you this evening these 

 remarkable experiments, I am personally indebted to Prof. Elihu 

 Thomson, both for constructing and sending the apparatus we have 

 used. Part of these appliances have now an historical value, and 

 have been presented by him to the Eoyal Institution. For the use 



