PROBLEMS. 487 



treme end of the line with a minimum amount of copper ; and 

 (ft) the total cost of feeder copper at 1 6 cents per pound. Ans. 

 (a) First 4,500 feet of feeder 1,980,000 circular mils, next 

 9,900 feet of feeder 1,219,000 circular mils, and remaining 18,900 

 feet of feeder 680,000 circular mils ; (&) 16,400 dollars. 



Note. The resistance of the bonded track, which is used as a return feeder, is 

 very uncertain and it is here to be assumed equal to zero for the sake of simplicity. 



155. (a) Find the position in which the power house should be 

 placed on the railway specified in problem 1 54 in order that the 

 feeder copper may be reduced to a minimum ; (b) find the size of 

 each section of the feeder on the assumption that the two cars on 

 the middle section are in the most unfavorable positions, namely, 

 at the two ends of the section, and on the assumption that the car 

 on the third section is at the extreme end of the section ; and (c) 

 find the total cost of the feeder copper at 16 cents per pound. 

 Total drop to each end of the line to be 75 volts. Ans. (a) 

 10,480 feet from the city end of the railway ; (&) first section 

 441,000 circular mils, city end of second section (1,480 feet) 

 485,200 circular mils, suburban end of second section (9,320 

 feet) 536,700 circular mils, and third section 369,900 circular 

 mils; (c) 7,119 dollars. 



Note. The power house should be placed at the " center of gravity" of a sys' 

 tern in the sense in which this term is defined in Art. 119, Chap. IX., in order that 

 the amount of feeder copper may be a minimum. Similarly, a center of distribution, 

 from which electric lamps are to be supplied by street mains, should be located at the 

 "center of gravity" of the consumers, each consumer being "weighted" in propor- 

 tion to the current delivered to him. 



The feeder on city section of 9, ooo feet is assumed to be of uniform section of 441,. 

 ooo circular mils throughout, but it would be advisable in fact to make the extreme 

 city end of this section of the feeder much smaller than 441,000 circular mils. 



156. A nearly reentrant row of 100 lamps, each taking one- 

 half an ampere, is to be wired in accordance with the return loop 

 scheme, using wire of uniform size. The row is 200 feet long, 

 one end of the row is 50 feet from the service point, and the 

 other end of the row is 60 feet from the service point. The vol- 

 tage at the service point is 115 volts. Find the size of wire to 



