RESISTANCE AND ELECTROMOTIVE FORCE. 59 



Note. High-voltage direct-current power transmission is not much used in Amer- 

 ican practice. 



52. A motor using 100 kilowatts of power is 10 miles from 

 the generator. Line wires 200 mils in diameter are to be used. 

 What electromotive force is required at the generator in order 

 that the line loss may be only 5 per cent, of the output of the 

 generator ? Ans. 7,602 volts. 



53. A direct-reading voltmeter V, Fig. 27, having 16,000 

 ohms resistance, is connected from main A to earth. The volt- 

 meter gives a reading of 2.6 volts and the electromotive force 

 between the mains is 1 10 volts. 



Find the insulation resistance 

 between main B and the earth 

 on the assumption that the in- 

 sulation resistance of main A 

 is : (a) infinite ; (^) the same as 

 that of main B ; (c ) one tenth 

 of that of main B. Ans. (a) 

 660,900 ohms; (ff) 644,900 Fig 2? 



ohms ; (c) 500,900 ohms. 



54. Three resistances A, B and C of which the values are 

 500 ohms, 200 ohms and 1.2 ohms, respectively, are connected 

 to a battery of negligible resistance, the electromotive force of the 

 battery being 2 volts. The connections are made so that the 

 whole current produced by the battery flows through A, then 

 divides and passes through B and C in parallel, and returns to 

 the battery. Calculate the total resistance of the circuit, the total 

 current, the current in B, the current in C, the electromotive 

 force between the terminals of A, and the electromotive force 

 between the terminals of B (or C). Ans. 501.19 ohms, 

 0.00399 ampere, 0.000024 ampere, 0.003966 ampere, 1.995 

 volts, 0.005 volt, in order. 



55. Three resistances of 4, 4 and 2 ohms respectively are con- 

 nected in parallel ; and two resistances of 6 and 3 ohms in parallel. 

 The first combination is connected in series with the second, and 



