PRACTICAL OPERATION OF DYNAMOS. 195 



through the local circuit formed by the two machines. It is, 

 therefore, very important, in starting a generator which is to be 

 connected in parallel with others already in operation, to be sure 

 that the generator builds up in the proper direction. This con- 

 dition is ensured (a) by leaving the series field coils permanently 

 connected in circuit, as explained below, or (^) by connecting the 

 shunt field windings of the generators, not to the brushes of the 

 respective machines, as shown in Figs, in, 112, 113 and 119, 

 but to the bus bars. This is called " bus excitation." 



Never open the shunt field switch of a generator which is 

 operating in parallel with others. To do this makes the genera- 

 tor armature a short-circuit for the other machines and the arma- 

 ture is likely to be burned out. 



To stop a generator which is operating in parallel with others 

 increase the resistance in its shunt field circuit by manipulating 

 its shunt field rheostat until its load is very small as indicated by 

 the ammeter. Then open the triple pole switch belonging to the 

 generator, and stop the driving engine. 



Feeder control. A central station usually delivers current 

 over a number of feeder circuits to a number of " distributing 

 points," from each of which a small district is supplied with cur- 

 rent through a network of mains. It is desirable, especially in 

 the distribution of current for incandescent lighting, to maintain 

 an unchanging voltage at every distribution point. The impor- 

 tance of this requirement ; s due to the fact that a slight excess 

 of voltage shortens the life of incandescent lamps, and that a 

 slight deficiency of voltage greatly reduces their candle power. 



A constant voltage can be maintained at any one of a number 

 of centers of distribution by proper adjustment of the voltage 

 between the bus bars at the station ; but to maintain constant 

 voltages at two or more independent centers of distribution the 

 voltage between each pair of feeders at the station must be adjusted 

 separately. Let A and B be the constant voltages to be main- 

 tained at two centers a and b, R the resistance of the a feeders, 

 R' the resistance of the b feeders, / the current demanded at a, 



