52 



BELL SYSTEM TECHNICAL JOURNAL 



by reversing a testing battery. The device developed for this purpose 

 is a 4-cycle, motor-driven commutator shown in Fig. 21. By studying 

 the curves used in the selection of a suitable frequency of testing 

 potential, it may be observed that the selected value of four cycles is 

 not critical, in fact a variation of ± 25 per cent may be allowable in 



Fig. 23 — Panel assembly of the impedance bridge. 



different machines. However, in practical application, the accuracy 

 of results obtained depends upon a comparison or ratio of two im- 

 pedance measurements which cannot be made simultaneously. Since 

 impedance varies with frequency, it is important that the frequency 

 of testing potential should remain constant while the two measure- 

 ments are being made. This requirement is met for the short time 

 required for two measurements. The assembly of the 4-cycle com- 

 mutator with associated apparatus is shown in Fig. 22. 



