. American telephone practice . run through the center of the cable,so that at least one good wire may always be available in no good wire is available, a separate wire may be strung tobe used as the return in this test. If the lead wires, from the instruments to the faults wire, haveappreciable resistance, this should be measured, and deducted fromthe value of X. After this the distance to the fault may be readilyobtained from the resistance per foot of the conductor. TESTING. 879 A second Varley method, quite as good as this, and very valuableas a check method, is the following
. American telephone practice . run through the center of the cable,so that at least one good wire may always be available in no good wire is available, a separate wire may be strung tobe used as the return in this test. If the lead wires, from the instruments to the faults wire, haveappreciable resistance, this should be measured, and deducted fromthe value of X. After this the distance to the fault may be readilyobtained from the resistance per foot of the conductor. TESTING. 879 A second Varley method, quite as good as this, and very valuableas a check method, is the following: The apparatus is arranged asin Fig. 642, and it will be noted that the conditions are similar tothose of the first method, and that only a reversal, at the set, of thegood and bad wires is required. With this arrangement, the bridge equation, when a balance isreached, must be the letters referring to the same things as before. again calling L the loop resistance of the good and bad wires, we have L = X+Y+C,ovC+Y — L — SAD WlfTE X / V qoop vyi^E Q FAUUTi FIG. 642.—VARLEY LOOP TEST—CHECK METHOD. Substituting this in the second member of the last form of the equa-tion, we have A (R+ L—X)—BX, whence X= A + B This result also is independent of the resistance of the fault, andit may be said of both Varley methods that the fault resistance maybe one which changes its value from moment to moment, withoutchanging the final result. Also the potentials of the two groundconnections with reference to the bad wire may change from mo-ment to moment and not harm the result. Indeed, it is possiblesometimes to locate a fault with no battery at all, as the differenceof potential between the two grounds is frequently great, wheregrounded trolley systems exist. These two methods ordinarily will be applied to pairs of wires 880 AMERICAN TELEPHONE PRACTICE. forming a loop of which the two sides are about alike; this is the casein testing cable conductors for a ground on one or more,
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