43 



speed, and the only adjustment which 

 must be left to the attendant is that of 

 the exciting current. The question now 

 arises, within what limits an error in this 

 adjustment may be safe. To this ques- 

 tion the diagram, Fig. 10, gives the 

 answer. 



In this diagram the circle E, repre- 

 sents as before the terminal electro- 

 motive force, and the line Oa the direc- 

 tion of the resultant of the electromotive 

 force of self-induction, and that lost in 

 overcoming the resistance of the arma- 

 ture. The coefficient of self-induction 

 depends upon the density of lines within 

 the armature core, being obviously the 

 smaller, the more nearly the core is 

 magnetized to saturation. For a reason 

 which will be explained presently, the 

 iron in the armature of alternators is 

 never magnetized to any considerable ex- 

 tent, the induction seldom exceeding 

 about 7,000 C. G. S. lines, and as at this 

 low figure the permeability is not much, 

 if at all, affected, it can be assumed, 

 without any great error, that the self- 



