DIRECT CAPACITY MEASUREMENT 



23 



The excess of one direct capacity C\2 over another Cm is readily 

 determined by connecting terminals 1 and 5 to corner ^, terminals 

 3, 4, 7, 8, . . . to corner ^''or d*, and then balance with terminals 2 

 and 6 on ^ and C^, respectively, and repeat, with their connections 

 reversed. 



Potentiometer Method, Fig. 3 



The required direct capacity C12 is balanced against one of its 

 associated direct capacities, augmented by a standard direct capacity 



Fig. 3 — Potentiometer Method for Direct Capacity 



C , and the measurement is repeated with the required direct capacity 

 and standard interchanged. Let R\ R" be the resistances required 

 in arm (?^ o[ the bridge for the first and second balance, then, 5 

 being the total slide wire resistance and G\ the grounded capacity 

 of terminal 1:* 



C12 = ^/ C , 



Gi = 



S - R' 

 R" 



C. 



This ratio method requires for the bridge a variable or slide wire 

 resistance and a constant condenser, and it may be employed as an 

 improvised bridge, when sufficient variable capacity is not available 

 for the Colpitts method. Not being a substitution method, however, 



*See appendix, section 3. 



