greater precautions are necessary for accurate results. There must be no initial direct capacity in arm CD, or a correction will be required. Possibly variable capacity ratio arms would be preferable to resistances.
Null-Impedance Bridge Method for Direct Capacity, Fig. 4
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-
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Fig. 4—Null-Impedance Bridge Method for Direct Capacity
ances R, R and a negative resistance — R/2; the Y has then a null-impedance between corner B and corners A, C connected together[1]. The three terminals 1, 2, 3 of the network to be measured are connected to corners D, C, B and a balance obtained by adjusting the variable standard condenser C′. Then C12 = C′ regardless of the direct capacities associated with C12 and C′, since these capacities either are short-circuited between corners B, A or B, C or are between corners B, D and thus outside of the bridge.
Correct adjustment of the negative resistance may be checked by observing whether there is silence in telephone T2 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 Z1/Z2 by employing a Y made up of impedances Z1, Z2, and — .
- ↑ See appendix, section 4, which also describes a transformer substitute for the Y.
