348 



BELL SYSTEM TECHNICAL JOURNAL 



Assuming that the bridge has been properly calibrated from theo- 

 retical considerations of the constants of the measuring circuit, there 

 are two possible sources of error in the results obtained. These errors 

 are due to the time of operation of the two switching relays W and X. 

 In the case of relay W an error will be caused if its two sets of contacts 

 do not close simultaneously. In the case of relay X the source of 



Fig. 4 



error is due to the time required for the relay to open the charging 

 circuit at its s contact at the end of the operating or releasing time 

 which is being measured. The combined error due to the operation 

 of the two relays may actually be determined by means of the bridge 

 itself and substracted from the results of the measurements. To 

 determine the error, terminal a is connected to ground and terminal h 

 to terminal c. The key K is then operated in the normal manner and 

 a balance secured on the meter. The readings of the dials will then 

 indicate the error of the bridge. 



