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BELL SYSTEM TECHNICAL JOURNAL 



then 



d2 = i {Gi + G2 — G\i) 



which indicates a method by which large grounded capacities can be 

 balanced against three variable capacities, only one of which need be 

 calibrated, and that one need be no larger than the required direct 

 capacity. 



Two-setting methods, as illustrated by the Colpitts and potentio- 

 meter methods, rest upon the possibility of connecting one of the 

 associated direct capacities between opposite corners of the bridge 

 where it is without influence on the balance, and not altering any 

 associated direct capacity introduced into the working arms of the 

 bridge. Numerous variations of these methods have been considered 

 which may present advantages under special circumstances. Thus, 

 if conductors 1, 2, 3 of Fig. 7 are in commercial operation, and it is 

 not permissible to directly connect two of them together, a double 

 bridge might be employed with a testing frequency differing from 



Fig. 7 — Double Bridge for Direct Capacity 



that of operation. A telephone is shown for each ear, and a constant 

 total direct capacity is divided between the three branches in the 

 proportion required to silence both telephones. 



One-setting methods attained ideal simplicity in the Maxwell 

 discharge method, but we found it necessary to use alternating current 



