39 



the motor will allow a considerable cur- 

 rent to pass ; but, as the phase of the 

 current lags behind that of the terminal 

 electromotive force by nearly a quarter 

 period, the energy supplied is very small; 

 in fact, only sufficient to overcome the 

 resistance of the armature-circuit and 

 mechanical friction. With an increase of 

 load the armature will be slightly re- 

 tarded, which will bring the point B 

 further away from Bj in the diagram. 

 Simultaneously the lag <P will decrease, 

 and the current which is proportional to 

 the length O A will increase, the increase 

 being slow until the point B Q is reached, 

 and afterwards more and more rapid. 

 Since the mechanical energy given off is 

 proportional to the product of current, 

 and induced electromotive force multi- 

 plied by the cosine of the angle the latter 

 forms with the current, it may be repre- 

 sented by the area of the shaded rectangle, 

 and a series of such rectangles for 

 different positions of the point B on the 

 circle, which represents the induced 

 electromotive force, can be determined. 



