OPERATION OF THE DYNAMO AS A GENERATOR. 8 1 



direction to weaken the residual magnetism, depends only upon 

 the direction of driving of the generator, and upon which terminal 

 of the field winding is connected to a given brush. 



If under given conditions, a generator does build up, it cannot 

 build up if its direction of driving is reversed, or if its field con- 

 nections are reversed ; if, however, its direction of driving and its 

 field connections are both reversed, then it will build up. If, 

 under given conditions, a generator cannot build up, it may build 

 up if its direction of driving is reversed, or if its field connections 

 are reversed. 



A generator driven in a given direction may build up so that 

 either the one or the other of its brushes is the positive brush, 

 according to the initial direction of the residual magnetism. A 

 generator which is repeatedly started and stopped usually builds 

 up each time in the same direction ; but some outside disturbance, 

 such as a momentary current from a lightning discharge, may 

 reverse the residual magnetism of a machine ; in this case the 

 machine will build up in a reverse direction the next time it 

 is started. 



From the above description of the building up process it may perhaps seem that 

 there is no limit to the field strength and voltage reached by a given self-excited 

 generator driven at a given speed. There is however a definite limit in every case, 

 as is explained in Appendix B on characteristics. 



45. Output of a generator, (a] Shunt or compound generator. 

 Shunt and compound generators are used for service in which an 

 approximately constant electromotive force is to be maintained 

 between the terminals of the machine. Under these conditions 

 the current delivered by the machine is inversely proportional to 

 the resistance of the receiving circuit. When the resistance of 

 the receiving circuit is large the current is small, and when the 

 resistance of the receiving circuit is small the current is large, 

 according to Ohm's law. 



(b) Series generator. The relation between the current out- 

 put of a series generator and the resistance of the receiving circuit 

 is complicated by the fact that the -electromotive force of a series 

 6 



