Page:The Bell System Technical Journal, Volume 1, 1922.pdf/23

The excess of one direct capacity C₁₂ over another C₅₅ is readily determines by connecting terminals 1 and 5 to corner D, terminals 3, 4, 7, 8, . . . to corner C or A, and then balance with terminals 2 and 6 on A 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

C₁, and the measurement is repeated with the required direct capacity and standard interchanged. Let R′, R″ be the resistance required in arm A B of the bridge for the first and second balance, then, S being the total slide wire resistane and G₁ the grounded capacity of terminal 1:^[1]

$C_{12}={\frac {R'}{R''}}C'$

$G_{1}={\frac {S-R''}{R''}}C'$

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,

1. See appendix, section 3.