LOCATION Of OPENS IN TOLL TELEPHONE CABLES 



49 



viz., two bridge potentials ninety degrees apart and a field current 

 lagging one bridge potential forty degrees, a complete set of sensitivity 

 curves was calculated for different lengths of line from zero to eighty 

 miles. These are shown in Fig. 17. It should be noted that the 

 sensitivity for detecting an unbalance in i? is a maximum at the 

 desired length of fifty miles. The sensitivity for an unbalance in ;' 



.2-0 



CO C 





CO i> 







10 



20 



30 40 50 bO 70 80 

 Length of Line in Miles 



Fig. 20 — Observed galvanometer sensitivities which are comparable to the 

 calculated sensitivities of Fig. 17. Sensitivity for unbalance in the component: 

 I — R with the phase of testing potential applied for balancing R. 

 11 — r with the phase of testing potential applied for balancing r. 

 Ill — R with the phase of testing potential applied for balancing r. 

 IV — r with the phase of testing potential applied for balancing R. 



is low at the shorter lengths of line, but increases as the length of line 

 increases. It may be noted that in both Figs. 16 and 17 the sensi- 

 tivity curve for changes in R passes through zero when the testing 

 potential is applied for balancing r. Likewise the sensitivity curve 

 for changes in r passes through zero when the testing potential is 

 applied for balancing R. This point is where the field current and 

 galvanometer unbalance current are ninety degrees out of phase. 

 Naturally this point coincides with the point of maximum sensitivity 

 for the normal potential arrangement. As stated above, it would be 

 ideal if these "reverse sensitivities" could be zero and the "true 

 sensitivities" could be maximum throughout the entire range of 

 lengths. Reference to Fig. 17 will show that the reverse sensitivity 



