344 BELL SYSTEM TECHNICAL JOURNAL 



best adjustment and the most desirable circuit condition for the 

 operation of a relay or similar device. With the bridge, this requires 

 very little time, especially since the results of the individual measure- 

 ments are immediately available to guide the work. With the oscillo- 

 graph, several hours may be required and the results obtained are 

 available only after developing and analyzing the oscillograms. 



The calibration of the bridge is determined by the values of certain 

 capacities and resistances in the measuring circuit. These may 

 usually be selected with sufficient accuracy during manufacture so that 

 the bridge requires no further calibration after it has been constructed. 

 In fact, it has been found practicable to design the bridge so that the 

 steps on a standard decade resistance box correspond to decimal 

 fractions of a second. 



Jl 



f °V\MaAM/o- 



Fig. 1 



The principles underlying the measurement of an interval of time 

 will be explained in connection with Fig. 1. Two condensers Ci and C^ 

 of unequal capacity are charged from a common battery Bz. The 

 condenser Ci, which has a larger capacity than C2, is charged through an 

 adjustable high resistance R\ during the time elapsing between the 

 operation of relay W and the subsequent operation of relay X. This 

 elapsed time is the interval of time to be measured and the charge 

 accumulated on the condenser is an accurate means for doing so. 

 The second condenser C-> is used merely for comparison purposes. It is 

 charged through a fairly low resistance Ri and acquires its full charge 

 in a relatively small interval of time. 



After the completion of the charging interval, relay Y is operated 

 and the two condensers are discharged simultaneously through a 

 differential meter circuit. If the two charges are equal, the meter will 

 show no deflection, but if they are unequal, it will show a momentary 

 deflection, the direction of which will indicate whether the charge on C\ 

 is too high or too low. By repeating the charging and discharging 

 process a few times and adjusting the value of resistance R\ in series 

 with the first condenser, the charges on the two condensers can be made 

 equal. When this condition is obtained, the interval of time during 



