LOCATION OF OPENS IN TOLL TELEPHONE CABLES 



41 



This impedance bridge can be balanced in two ways: by varying r 

 and Xb, keeping R constant, or by varying R and r, keeping Xb 

 constant (Fig. 8). The effect is essentially the same in either case. 

 When R is varied instead of Xb, the only difference is that the balance 

 of the bridge is disturbed for both the real and imaginary components. 

 This fact necessitates a correction of r each time R is changed in 

 securing a balance of b against Xb- If Xb is varied, the balances of r 

 against a, and Xb against b, are independent functions. In practice, 

 it is easier to vary R and keep Xb constant, but for the purpose of 

 theoretical discussion it lends clarity to consider Xb to be variable 

 from the condition of balance. 



From the equation (1) above the impedances of different lengths of 

 line may be computed. For the purpose of designing a suitable 

 impedance bridge arrangement, it is sufficient to consider the 19-gauge, 

 non-loaded cable only, as the effect of loading on the general line 

 characteristic is small at the frequencies employed. A number of 

 impedance values representing different lengths of 19-gauge, non-loaded 



8§ 12 

 o Q 



So 

 tn CO 



E e 

 o o 



.S.£ 9 



10 



0.0.' 



c3c3 6 



10 20 ;30 40 50 60 



Length of Line in Miles. 



Fig. 10 — Characteristics of the resistance component and capacitance component 

 of the impedance of lengths 19-gauge, non-loaded cable. 



cable up to sixty miles, at three frequencies, viz., twenty, eight and four 

 cycles, were selected and the condition of balance of the impedance 

 bridge calculated for each case from equations (4) and (5). Curves 

 of these impedances are shown in Fig. 10, where the reactances and 

 resistances at the three chosen freciuencies are plotted. 



