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BELL SYSTEM TECHNICAL JOURNAL 



Long Cable Method ^ 

 Referring to Fig. 14, assume that the wires under test are non- 

 loaded and that a testing current of very low frequency is used so 

 that the wires are electrically short. Calling the capacitance and the 



2 2 "^ 2 2 



Fig. 16 — -First equivalent circuit — long cable method for locating a series 

 resistance unbalance. 



conductor resistance of the length {T — D) of each wire, Ci and (r — x), 

 respectively, and of the length D of each wire, C2 and x, respectively, 

 the bridge circuit of Fig. 14 is practically equivalent to that of Fig. 16. 

 When the bridge is balanced so that there is no current through the 

 detector, the impedance Zi looking into the upper branch of the net- 



£zA 111 rtx r+x 



2 2 F 2 2 



Fig. 17 — Second equivalent circuit — long cable method for locating a series 

 resistance unbalance. 



work must be equal to the impedance Z2 looking into the lower branch. 



At the balance point the bridge circuit is practically equivalent to 



that shown in Fig. 17, in which the network up to the point of fault, 



as seen from the bridge terminals of the lower branch, is replaced by 



a single resistance-capitance network. 



' Credit for the long cable method is given to Capt. F. Reid in the paper, "Cable 

 Testing," by E. S. Ritter, loc. cit. 



