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BELL SYSTEM TECHNICAL JOURNAL 



The wires 1-2 and 3-4 form the pairs of a quad containing a series 

 unbalance of resistance, F. The total length of the faulty wire is T, 

 and the length of the portion of the faulty wire between the distant 

 end of the cable and the fault is D. The bridge has equal ratio arms, 

 A, and a balancing resistance, R. The audible frequency generator 

 is a buzzer or other source of relatively low frequency current. 



The bridge is balanced first with the distant ends of wires 1, 2, 3 

 and 4 open, and then with the distant ends of wires 1, 2, 3 and 4 con- 

 nected together. The location of the unbalance from the distant end 

 can be calculated from the formula: 



D = T 



Re 



where Ro and Re are the balancing resistances for the measurements 

 with the distant end open and the distant end short-circuited, re- 

 spectively. This test is suitable for use only on non-loaded cable, up 

 to a few miles in length. 



Reversing 

 switch 



Fig. 14 — Schematic circuit — long cable method for locating a series resistance 



unbalance. 



The bridge arrangement for applying the test to long (either loaded 

 or non-loaded) cables differs from that for short cables in that the wires 

 of each pair, 1-2 and 3-4, are connected together at the distant end 

 when measuring Rq, and a testing current of very low frequency is 

 used. A battery, reversed either manually or by means of a motor- 

 driven commutator, provides a satisfactory source of current, as in- 

 dicated in Fig. 14. 



With the wires of each pair, 1-2 and 3-4, connected together at the 

 distant end as shown, the balancing resistance is adjusted to a value 



