ELECTRIC DISTRIBUTION AND WIRING. 



269 



except that a third main, CD, is added as shown. The addition of 

 this third main makes it possible to turn single lamps on and off, 

 and if the lamps in use by each consumer are judiciously placed, 

 some in the ^4-set and some in the j5-set, there will always be 

 nearly the same number of lamps in each set, even when entire 

 freedom is allowed in the turning off and on of single lamps, so 

 that the middle main need never carry much current. In fact 

 the current in the middle main will be a small incoming current 

 when the ^4-set contains a few more lamps than the ^-set, or a 

 small outgoing current when the ^-set contains a few more lamps 

 than the ^4-set. 



6 Q O C 



> c 



O O C 



o 6 c 



o C 



) 



Fig. 156. 



The arrangement shown in Fig. 156 is called the Edison three- 

 wire system of distribution. In practice the middle main is usually 

 made of the same size wire as each outside main, and each out- 

 side main need be only one quarter as heavy as would be required 

 to supply the same number of lamps in the simple parallel sys- 

 tem using 1 10 volts ; therefore to supply a given number of lamps 

 in the Edison three-wire system requires only three-eighths as 

 much copper as would be required in the simple system with the 

 same per cent, drop of voltage. 



When the number of lamps in the ^4-set in Fig. I 56 is different 

 from the number of similar lamps in the .#-set the system is said 

 to be unbalanced. When the system is unbalanced the middle 

 main carries current, and the effect of the voltage-drop in the 



