so BELL SYSTEM TECHNICAL JOURNAL 



for r is practically zero throughout the range of lengths. This fact in 

 itself is significant. Assuming r to be set on zero, R could be varied 

 to secure an approximate balance, using the proper testing potential. 

 This balance would be fairly accurate since the reverse sensitivity for 

 r is quite low throughout. Shifting bridge potentials ninety degrees, 

 r could be adjusted almost to the proper point since R is practically 



Fig. 21 — Commercial design of motor-driven commutator which is used to supply 

 4-cycle alternating potentials for the impedance bridge. 



correct. The balance of R and r can then be refined as often as is 

 necessary to secure a perfect balance. Since r is not used in any 

 calculation, and since its effect on R is small once an approximate 

 balance of R and r is obtained, the need for an accurate balance of r 

 is small. 



A series of observations made with experimental apparatus arranged 

 as shown in Fig. 18 gave the sensitivity curves shown in Figs. 19 and 

 20. The observed sensitivity characteristics shown by these curves 

 agree very favorably with the theoretical values shown by the curves 



of Figs. 16 and 17. 



By way of summarization, it may be observed that in the process 

 of developing a suitable method for the location of faults in telephone 

 cables a definite sequence of steps has been taken to provide an effec- 

 tive treatment of the problem : 



1. In establishing requirements for a suitable method, a study was 



