CHARACTERISTIC CURVES. 



393 



tangent of is equal to R r f/f, which is equal to R r . When the 

 resistance R r has so far increased that the point of intersection 

 is near the knee of the curve at K, then either a very slight addi- 

 tional increase of R r , or a momentary slowing down of the gen- 

 erator (which reduces the ordinates of the characteristic curve) 

 will lead to a state of affairs in which the generator characteristic 

 and the receiving circuit characteristic do not intersect at all. 

 The resistance R k , corresponding to the point K, Fig. 26, is 

 called the critical resistance of the series generator for the given 

 speed, inasmuch as the generator when driven at the given speed 

 cannot build up, or if it is already built up, it cannot maintain its 



Fig. 26. 



Fig. 27. 



field magnetism, if the resistance of the receiving circuit is greater 

 than R k . The critical resistance is proportional to the speed at 

 which the generator is driven. The current 7 ft , corresponding to 

 the point K, is called the critical current of the generator. The 

 critical current does not vary perceptibly with the speed of the 

 generator, but has a definite value for a given generator. 



The points P and /*, Fig. 27, represent the two presumably 

 possible operating conditions of a given series generator, running 

 at a given speed, and supplying current to charge a storage bat- 

 tery of which the counter-electromotive force is e. In fact the 



