[Electric engineering.] . s on the dynamo D, 34 ELECTRIC TRANSMISSION. 15 Example.—The insulation resistance of an alectric-light main wastested by means of a Weston voltmeter having a resistance of18,000 ohms. When connected across the lines, the voltmeter gave areading of 110 volts. When one line was connected to ground throughthe voltmeter, the reading was only 4 volts. What was the insulationresistance of the other line ? Solution.—We have by formula 1, D (110 - 4) 18,000R = 4 ? 106 X 18,000 = 477,000 ohms. Ans. Note.—The insulation resistance of lines is usually expressed inmegohms, 1 meg
[Electric engineering.] . s on the dynamo D, 34 ELECTRIC TRANSMISSION. 15 Example.—The insulation resistance of an alectric-light main wastested by means of a Weston voltmeter having a resistance of18,000 ohms. When connected across the lines, the voltmeter gave areading of 110 volts. When one line was connected to ground throughthe voltmeter, the reading was only 4 volts. What was the insulationresistance of the other line ? Solution.—We have by formula 1, D (110 - 4) 18,000R = 4 ? 106 X 18,000 = 477,000 ohms. Ans. Note.—The insulation resistance of lines is usually expressed inmegohms, 1 megohm being equal to 1,000,000 ohms. The resistance ofthe line in this case would therefore be .477 megohm. TESTS EOR GROUNDS OR CROSSES. 41. Vailey Ix>op Test.—One of the commonest methodsfor locating a ground or cross is by means of the Varleyloop test. In Fig. 37, G is a sensitive galvanometer con-nected across the arms of a Wheatstone bridge in the ordi-nary manner; A B and A C are the ratio arms and CD the. F- FIG. 37. rheostat or balance arm of the bridge. D E is the faultyline and F the location of the fault. The two lines shouldbe connected together at E and the ends of the loop BED,so formed, connected across the terminals of the bridge as 15 ELECTRIC TRANSMISSION. 35 the unknown resistance. Call y the resistance of the loopfrom B to F and x the resistance from D to F. With thebattery connected between A and D, as in the ordinarymethod of using the Wheatstone bridge, balance the will give, by working out the unknown resistance inthe usual manner, a resistance R equal to the sum of theresistances of the two wires forming the loop; that is, R = y + x. Or, the resistance R of the whole loop may be calculated,because the length and size of the line wire are known.
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