268 



ELEMENTS OF ELECTRICAL ENGINEERING. 



of copper by using a high voltage * is evident when we consider 

 that the delivery of a certain amount of power can be accom- 

 plished by half as much current when the voltage is doubled, 

 that the doubling of the voltage permits in general a double vol- 

 tage-drop in the line wires, and that this double voltage-drop with 

 half as much current means that the wires may be one quarter as 

 heavy as before. 



The disadvantage of the series-grouping of lamps is that each 

 group must be turned off and on as a unit, unless a special de- 

 vice is used to connect an equivalent resistance in place of a lamp 

 which is to be turned off. 



The grouping of incandescent lamps in series on high- 

 voltage supply mains is exemplified in the lighting of streets by 

 incandescent lamps from a 1 , 100- or 2, 200- volt alternating-current 

 supply. In this case the lamps are arranged in series-groups of 

 10 or 20 lamps each and provision is made for automatically in- 

 serting an equivalent resistance in place of a lamp that happens 

 to break down. 



C 



C 



000^ 



0< 



C 



000 



000 



Fig. 155. 



112, The Edison three-wire system of distribution. Fig. 

 155 shows a number of no-volt lamps connected, in series- 

 groups of two lamps each, to 22O-volt mains, and supplied with 

 current from two iio-volt generators connected in series; and 

 Fig 156 shows an arrangement which is the same as Fig. 155, 



*See equation (43), Art. 117. 



