Direct Capacity Measurement' 



By GEORGE A. CAMPBELL 



Synopsis: Direct capacity, direct admittance and direct impedance are 

 defined as the branch constants of the particular direct network which is 

 equivalent to any given electrical system. Typical methods of measuring 

 these direct constants are described with especial reference to direct ad- 

 mittance; the substitution 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. Propo.sed 

 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 com- 

 parable 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 " measurement; this was a distinct advance, since the trans- 

 mission 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 com- 

 pletely how telephone circuits will behave under all possible con- 

 nections. 



For making these direct capacity measurements alternating cur- 

 rents of telephone frequencies were introduced so as to determine 

 more exactly the effective value of the capacity in telephonic trans- 

 mission, 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 associa- 

 tion with other capacities; and these associated capacities may be 

 much larger than the particular direct capacity which it is neces- 



1 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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