30 



terminal electromotive force. The elec- 

 trical efficiency is therefore given by the 

 ratio of the lines K E and O E, which is 

 obvious considering the electromotive 

 force O R, absorbed in overcoming the 

 resistance of the armature, is the only 

 purely electrical loss occurring in the 

 armature. 



In alternators used for parallel distri- 

 bution, the object aimed at is generally 

 to keep the terminal pressure constant 

 for all currents, and this can be attained 

 by working with a constant field and 

 suitably varying the speed, or by main- 

 taining a constant speed and adjusting 

 the exciting current so as to suitably 

 vary the electromotive force. The latter 

 plan must be adopted if several machines 

 are worked in parallel; and in this case 

 the diagram, Fig. 7, may be used to 

 determine the limits between which the 

 exciting current must be varied, to keep 

 the terminal electromotive force constant 

 for all loads from no current to the 

 greatest current the armature is intended 

 to pass. The mean induced electromo- 



