[Illustration: Figs. 599 and 600.--Varley loop method of fault location with Leeds and Northrup fault finder. This method may be used as a check on the Murray methods. Connect the faulty wire to 1, and measure the resistance of the loop. Then throw switches as shown in the fig. 600. Let: a = resistance to fault, d = distance to the fault in miles, M = resistance of the faulty wire per mile, r = resistance of the loop, R = resistance of the coil R, or 100 ohms, T = A ÷ (1,000 - A) to be read from the table. From the Wheatstone bridge relation: a = (r - 100T) ÷ (T + 1), and d = (r - 100T) ÷ (T + 1)M. EXAMPLE--A wire having a resistance of 16.46 ohms per mile is grounded. This was looped with a wire of unknown resistance and the resistance of the loop was found to be 54.07 ohms. Connections were made as in the figure, and the reading A was found to be 234. From the table T = .3055, and substituting: d = (54.07 - 30.55) ÷ (1.3055 × 16.46) = 1.094 + miles.] Special Loop.--This method may be used to advantage where the length of the cable or faulty wire only is known and where there are two other wires which may be used to complete the loop. It is not necessary that the resistance of the faulty wire and the length and resistance of the other wires be known. Figs. 601 to 604 show the connections and method of testing. EXAMPLE.--All the wires in a cable 10,852 ft. long were found to be grounded so that none of them could be used as good wires. Two wires were selected out of another cable going to the same place by a different route and securely joined to one of the grounded wires at the distant end. This grounded wire and one of the good ones were connected as shown in figs. 601 and 602 and the reading A was found to be 307. Connections were then made as shown in figs. 603 and 604 and A was found to be 610. What is the value of d? According to formula d = AL/A = (307 × 10,853)/610 = 5,461 ft. [Illustration: Figs. 601 and 602.--Special loop test with Leeds and Northrup fault finder. For the first measurement connect the faulty wire to 2, either of the good wires, as Z, to 1, the post Gr to ground, and short circuit the coils R and E by closing switches U and V as in the figures. Balance in the usual way and call the dial reading A. For the second measurement, connect the post Gr. (disconnected from ground), to the other good wire y as shown in figs. 603 and 604, and get another balance; call this reading A'. The distance d, to the fault is determined from the simple formula d = AL ÷ A' where L is the length of the cable or faulty wire.]