CHARACTERISTIC CURVES. 



385 



plying the ordinates of the given curve by n' jn. This is evident 

 when we consider that the flux <E> has a definite value for a given 

 value of field current, so that the induced electromotive force, 

 Z'n, corresponding to a given value of the field current is pro- 

 portional to the speed. 



Effect of residual magnetism upon the magnetisation curve of 

 a dynamo. When the field current of a dynamo is zero, the 

 armature flux <, and the induced electromotive force <&Z f n, are 

 usually not zero, on account of residual magnetism. This effect 



Volts 



Amperes I x 



6 9 12 



Fig. 18. 



is shown in Figs. 18 and 19. In both of these figures the curves 

 intersect the axis of ordinates at that distance above the origin 

 which represents the value of the induced electromotive force due 

 to the residual magnetism. 



Effect of magnetic hysteresis upon the magnetization curve of a 

 dynamo. On account of magnetic hysteresis, the armature flux 

 4? and the induced electromotive force QZ' n which correspond 

 to a given value of field current are smaller when the field current 

 is increasing than when the field current is decreasing. There- 

 fore the magnetization curve of a dynamo is slightly different for 



25 



