LOCATION OP OPENS IN TOLL TELEPHONE CABLES 39 



Type of Cable Circuit Average Percentage Error 



Extra Light or Non-Loaded 19-Gauge Cable 0.083% 



Medium Heavy or Heavy Loaded 19-Gauge Cable 0.033 



Extra Light or Non-Loaded 16-Gauge Cable 0.022 



Medium Heavy or Heavy Loaded 16-Gauge Cable 0.090 



Since these errors, at a frequency of four cycles, are for the maximum 

 length of line which may be encountered in practice, it was considered 

 that they might be neglected in comparison with the importance of 

 securing a frequency of testing potential which would be high enough 

 to give an impedance bridge a suitable sensitivity to unbalance. 

 Hence, a computation of the sensitivity appeared to be the next step 

 in the selection of a suitable frequency of testing potential. 



1000 ohms 



Fig. 8. 



Fig. 9. 



Fig. 8 — Bridge used in the location of opens. E represents the low-frequency 

 source, and Rp a protective resistance. The rheostat R and the 1000-ohm resistance 

 may be regarded as the ratio arms of the bridge. The impedance of the line is 

 represented by Z l, a resistance and capacitance in series. The rheostat r is used in 

 balancing the resistance component of the line impedance, so that the impedance 

 angle of Zb will equal the impedance angle of Z l- 



A very sensitive electrodynamometer, or a galvanometer equipped 

 with an electromagnetic field, was used as a detector in the impedance 

 bridge network shown in Fig. 8. The sensitivities for several fre- 

 quencies of testing potential were computed for this modified form 

 of the De Sauty bridge. The condition for balance in this impedance 

 bridge network is 



RZ, = lOOOZyj 

 or 



R{a - jb) = 1000(r - jXb). 



