OPERATION OF THE DYNAMO AS A GENERATOR. 79 



usually placed in the circuit so that the station attendant 

 can see when the current has the proper value. In Fig. 

 68 an adjustable resistance B is shown connected in parallel 

 with the field winding ; by varying this resistance, that portion 

 of the fixed current output of the generator which flows through 

 the field winding may be changed at will, and thus the electro- 

 motive force of the generator may be controlled so as to cause it 

 to deliver a constant current in spite of variations in the resistance 

 of the receiving circuit. This hand controlling device is not used 

 in commercial arc lighting plants, and it is only mentioned here 

 to give a clear idea of a possible method for controlling a series 

 generator. 



43. Connections of a shunt or compound generator and its receiv- 

 ing circuit. Fig. 69 is a diagram of connections of a compound 

 generator (short-shunt) with its field rheostat R, main fuses at b 



r*\&- 



\- 



Fig. 69. 



and c, main switch, ammeter A, voltmeter V, and circuit breaker 

 BB. These accessory appliances are always mounted in con- 

 venient positions on a panel of insulating material, slate or marble, 

 called a switch-board. The wires a, b, and c lead from the gen- 

 erator to the switch-board, and the wires d and e lead from the 

 switch-board to the lamps. The wires leading to the individual 

 groups of lamps LL, and to the motor M t in Fig. 69 are, of 

 course, smaller than the mains leading out from the switch-board. 

 It is wise to insert fuses //, as shown in Fig. 69, at every point 



