2G 



the basis of the exact induction-curve as 

 mapped out from the shape of the poles 

 and armature- coils, it would lead to such 

 difficulties as to make the investigation 

 useless for practical purposes, for which 

 easily understood and simple methods of 

 calculation or construction are wanted. 

 The sine-function has, therefore, as far as 

 the author is aware, always been taken 

 by previous investigators as the basis of 

 their calculations, and in this he pro- 

 poses to follow their example. 



Let, in Fig. 7, the length of the line O 

 I represent the maximum value of the 

 current (crest of the current wave), and 

 let this line revolve in the direction of 

 the arrow around O as the center, with 

 an angular velocity to = Inn, then the 

 projection of O I upon the vertical gives 

 the current at any moment. Let the 

 line O L represent the maximum electro- 

 motive force of self-induction due to the 

 current O I, then the projection of O L 

 will similarly give the electromotive force 

 of self-induction at any moment, the two 

 lines preserving during rotation their 



