31 



tive force e is found by Formula 1 and 

 its maximum value is obviouly — 



E, = V~2 e (9). 



The maximum value of the electromotive 



force of self-induction is given by — 



E* = 2 n n L r . . . * . . . (10), 



where L is the coefficient of self-induc- 

 tion, which can be determined by various 

 well-known methods ; or by running the 

 armature in a constant field, and measur- 

 ing the terminal electromotive force with 

 and without a current passing through 

 the armature. The electromotive force 

 lost in resistance is simply the product 

 of the current I and the resistance R of 

 the armature. With the knowledge of 

 these quantities it is now easy to deter- 

 mine the strength of field required for 

 any current, since, according to Formula 

 1, the field is proportional to the induced 

 electromotive force. For this purpose 

 the diagram, Fig. 7, may be used, or the 

 modified form shown in Fig. 8. Here O 

 A represents the terminal electromotive 

 force, which is to be kept constant. A 



