486 ELEMENTS OF ELECTRICAL ENGINEERING. 



and cost, when the size is reduced in steps so as to give the 

 specified voltage-drop with a minimum amount of copper. The 

 cost of copper is to be taken at 16 cents per pound. Ans. (a) 

 260,300 circular mils, 946.5 pounds, 151.2 dollars, (&) first 

 section 3 19,700 circular mils, second section 267,000 circular mils, 

 third section 157,400 circular mils, 868 pounds, 139 dollars. 



Note. In estimating the distance, L,, of the ' ' center of gravity ' ' of the consumers 

 in the above problem, one may use one ampere as the unit instead of one lamp. 



154. An electric railway 33,300 feet in length is divided into 

 three sections of which the lengths, 9,000 feet, 10,800 feet and 

 13,500 feet, are proportional to the schedule speeds of cars on 

 the respective sections so that a car running from end to end of 

 the line traverses each section in 10 minutes. Four cars are 

 always on the first section five minutes apart going each way, two 

 cars are always on the second section ten minutes apart going 

 each way, and a single car is always on the third section making 

 the round trip in 20 minutes. Owing to frequent stops on the 

 first section the cars take an average current of 125 amperes each, 

 on the second section the stops are less frequent and each car 

 takes an average of 105 amperes, and on the third section the stops 

 are least frequent and the car that is always on this section takes 

 an average of 95 amperes. 



The " center of gravity " of the four cars that are always on 

 the first section is at the middle of the section, the " center of 

 gravity " of the two cars that are always on the second section is 

 at the middle of that section, and the most unfavorable position 

 of the single car that is always on the third section is when it is 

 at the extreme end of the line. Assume, therefore, that 500 

 amperes are delivered continuously at the middle of the first 

 section (4,500 feet from the city end), that 210 amperes are de- 

 livered continuously at the middle of the second section (14,400 

 feet from the city end), and that 95 amperes are delivered con- 

 tinuously at the extreme end of the line. If the power house is 

 located at the city end of the line, find : (a) The size of each 

 section of the feeder to give a total drop of 75 volts at the ex- 



