PRACTICAL OPERATION OF DYNAMOS. 197 



(or negative) bus bar, and the other wire is connected to a change- 

 over switch, so that it may be connected at will to either of the 

 two negative (or positive) bus bars. Arrangements must also be 

 provided for quickly altering the connections on the switch board 

 so that any given generator may be operated with either set of 

 generators at will. 



(3) The booster method. A third method of feeder control is 

 to connect an auxiliary generator in series with each feeder cir- 

 cuit at the station, so that by changing the field excitation of this 

 generator the feeder voltage may be raised to any desired extent 

 above the bus bar voltage. This method of feeder control is not 

 adapted to a direct current lighting station where the feeder vol- 

 tages differ from the bus bar voltage by only a few volts, inas- 

 much as the auxiliary generators would be too expensive and the 

 losses of power would be excessive, much greater perhaps than 

 in the simple rheostat method of feeder control. The booster 

 method is, however, frequently used in electric railway power 

 stations. For example, suppose that a power station is erected 

 for supplying current to four or five miles of electric railway, and 

 suppose that it becomes necessary to supply current from this 

 station to an extension of the railway. In such a case either an 

 excessive amount of capital must be invested in a heavy copper 

 feeder, or a very large drop amounting to one or two hundred 

 volts must be permitted. The latter alternative is usually chosen 

 and it is desirable to make up for the excessive voltage drop by 

 increasing the feeder voltage at the station. This is done by con- 

 necting the armature of a one or two hundred volt generator in 

 the feeder circuit at the station. This auxiliary generator, which 

 may be either engine or motor driven, is called a booster. The 

 booster used as here described would have series field excitation, 

 so that its electromotive force would be roughly proportional to 

 the current flowing through it, thus keeping the voltage at the 

 distant end of the feeder circuit approximately constant irrespec- 

 tive of the amount of current delivered. 



The use of the booster as outlined in the above example would 



