LOCATION OF OPENS IN TOLL TELEPHONE CABLES 



45 



A frequency of four cycles was selected as the optimum as regards 

 the size of hyperbolic error discussed above, the sensitivity available, 

 and the amount of phase shift with increase in length of line. The 

 curve showing the change of hyperbolic error with variation in fre- 

 quency (Fig. 7) shows four cycles to be at or near the critical point of 

 the curve. The sensitivity at four cycles has the advantage of being 

 sufficient but not excessive. To a large extent, the condition of phase 

 shift (with increase in length of line) governs the ease of securing a 

 balance over the range of line lengths. The ideal arrangement would 

 be one in which the field current could always be placed in phase with 

 the component of the bridge unbalance current it was desired to 

 eliminate. Such a quality is not characteristic of the type of bridge 

 used; however, a desirable approximation of such an arrangement is 

 obtained when a four-cycle frequency of testing potential is used. 



10 20 50 40 50 60 



Length of Line in Miles^ 



Fig. 14 — Calculated variation in the angle between unbalance current and bridge 

 voltage for unbalance in the capacitance component R and for unbalance in the 

 resistance component r of the impedance of non-loaded 19-gauge cable. Testing 

 potential of 4-cycles. 



In order to check the assumption (stated above) that the bridge 

 could also be balanced by keeping Xb constant and varying R and r, 

 without materially changing the conditions of balance, another set of 

 galvanometer unbalance currents was calculated with Xb constant 

 and R and r varied respectively 10% from the condition of balance. 



