Page:The Bell System Technical Journal, Volume 1, 1922.pdf/24

24 greater precautions are necessary for accurate results. There must be no initial direct capacity in arm, or a correction will be required. Possibly variable capacity ratio arms would be preferable to resistances.

Assuming that the electron tube supplies the means of obtaining an invariable true negative resistance, Fig. 4 shows a method which determines any individual direct capacity from a single bridge setting. The bridge arms are replaced by a Y network made up of two resist-

ances R, R and a negative resistance — R/2; the Y has then a null-impedance between corner and corners  connected together. The three terminals 1, 2, 3 of the network to be measured are connected to corners and a balance obtained by adjusting the variable standard condenser C′. Then C$12$ = C′ regardless of the direct capacities associated with C$12$ and C′, since these capacities either are short-circuited between corners or  or are between corners  and thus outside of the bridge.

Correct adjustment of the negative resistance may be checked by observing whether there is silence in telephone T$2$ after the balance has been obtained. Assuming invariable negative resistance, this test need be made only when the bridge is set up, or there is a change in frequency. The bridge may be given any ratio Z$1$/Z$2$ by employing a Y made up of impedances Z$1$, Z$2$, and — $$Z_ Z_/(Z_ + Z_)$$.