ELECTRIC DISTRIBUTION AND WIRING. 273 



a field rheostat. Similarly the magnetic flux out of N 1 (and into 

 S f ) produces the voltage, a to c, and this flux is produced by the 

 exciting coils, BB and BB, the current in which can be controlled 

 by a second field rheostat. 



The tendency of the armature current to increase the flux out 

 of N' (and into S f ) and to decrease the flux out of N (and into 

 S) may be to some extent counteracted by connecting the field 

 coils, AA and AA, between brushes, a and b, and connecting the 

 field coils, BB and BB, between brushes, c and b. The best 

 method, however, for counteracting this tendency is to place a 

 field winding in series with either of the outside mains, I or 2, so 

 that it acts to increase the flux out of N (and into S) and to de- 

 crease the flux out of N' (and into S'). 



(&) The double -current generator. The synchronous converter, 

 or rotary converter as it is sometimes called, is an ordinary direct- 

 current dynamo with the addition of two or more collector rings 

 tapped into equidistant points of the armature winding. When 

 such a machine is driven by mechanical power it can supply direct 

 current from the brushes that rub on the commutator, or alternat- 

 ing current from the brushes that rub on the collector rings, or it 

 can supply direct current and alternating current simultaneously. 

 Therefore such a machine is called a double-current generator. 

 This machine is fully described in the second volume of 

 this text 



An arrangement, due to Dobrowolsky, for using a double- 

 current generator for supplying direct current to an Edison three- 

 wire system is also described in the second volume of this text. 



(c) The use of rlieostats for balancing a three-wire system. If 

 a three- wire system is kept exactly balanced, an ordinary 220- 

 volt generator having two brushes can be used to supply the sys- 

 tem. When such a generator is used, approximate balance is 

 maintained by arranging the lights in and near the station in 

 groups which may be transferred by the station attendant from 

 the ^4-set to the .#-set or vice versa, at will, by throwing switches 

 on the main switch-board, and the approximate balance so ob- 

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