36 



in this fashion: Given an alternator, with 

 excited field and running at a proper 

 speed, and a pair of terminals from which 

 any desired strength of current can be 

 obtained at a constant pressure, and hav- 

 ing a frequency approximately corre- 

 sponding to the speed of the alternator, 

 what will be the condition under which 

 the alternator will fall into step with the 

 supply-current, and what will be the 

 relation between the current passing 

 and the mechanical energy given off? 

 Also, how will this relation be affected by 

 variations in the strength of the field ? 

 Let, in Fig. 9, the circle E, represent the 

 terminal electromotive force, and the cir- 

 cle E the electromotive force induced in 

 the armature; and assume a certain 

 current under which the electromotive 

 force of self-induction is O L and that 

 lost in resistance O R. Since both 

 quantities are proportional to the cur- 

 rent, their resultant O A will for all 

 currents form the same angle with 

 the axes, the point A being simply 

 shifted further out on the line 0« if the 



