Direct Capacity Measurement[1]
Synopsis: Direct capacity, direct admittance and direct impedance are defined as the branch constants of the particular network which is equivelant to any given electrical system. Typical methods of measuring these direct constants are described with especial reference to direct admittance; the subsitution alternating current bridge method, due to Colpitts, is the preferred method, and for this suitable variable capacities and conductances are described, and shielding is recommended. Proposed methods are also described involving the introduction of electron tubes into the measuring set, which will reduce the measurement to a single setting or deflection. This gives an alternating current method which is comparable with Maxwell's single null-setting cyclical charge and discharge method. Special attention is drawn to Maxwell's remarkable method which is entirely ignored by at least most of the modern text-books and handbooks.
THE object of this paper is to emphasize the importance of direct capacity networks; to explain various methods of measuring direct capacities; and to advocate the use of the Colpitts substitution method which has been found preeminently satisfactory under the wide range of conditions arising in the communication field.
About thirty years ago telephone engineers substituted the so-called "mutual capacity" measurement for the established "grounded capacity" measurementl this was a distinct advance, since the transmission efficiency is more closely connected with mutual capacity than with grounded capacity. Mutual capacity, however, can give no information respecting crosstalk, and accordingly, about twenty years ago, I introduced the measurement of "direct capacity" which enabled us to control crosstalk and to determine more completely how telephone circuits will behave under all possible connections.
For making these direct capacity measurements alternating currents of telephone frequencies were introduced so as to determine more exactly the effective value of the capacity in telephonic transmission, and to include the determination of the associated effective direct conductances which immediately assumed great importance upon the introduction of loading.
Telephone cables and other parts of the telephone plant present the problem of measuring capacities which are quite impossible to isolate, but which must be measured, just as they occur, in association with other capacities; and these associated capacities may be much larger than the particular direct capacity which it is neces-
- ↑ This article is also appearing in the August issue of the Journal of the Optical Society of America and Review of Scientific Instruments. An appendix is added here giving proofs of the mathematical results.
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