CHARACTERISTIC CURVES. 



395 



ditions the battery and the generator work together to produce 

 an excessive current, and the power delivered by the battery and 

 by the generator is all used in heating the circuit. 



When a street car is coasting down hill it may drive the series 

 dynamos, which are geared to its axles, as generators, if the con- 

 nections between field and armature are reversed. The difficulty 

 in getting these dynamos to build up in the proper direction to 

 charge the line, as it were, thus returning power to the system, is 



Volts 



\e Amperes 



O 



Fig. 28. 



precisely the same as that which is described above. The line volt- 

 age, between trolley and rail, always causes the series machines 

 to build up in the wrong direction to accomplish the desired 

 result. The action which does take place is that the two series 

 machines build up as generators in the direction such as to help 

 the trolley voltage force an excessive current through the circuit 

 including the machines and the rheostats. 



The curious see-saw action of a series generator which supplies 

 current to a motor of which the field magnet is separately excited, 

 is closely related to the above described behavior of a series gen- 

 erator connected to a storage battery. Let a series generator 



