THE MEASUREMENT OF SMALL CAPACITIES 

 By E. S. Bieler, M.Sc. 



Presented by Dr. A. S. Eve, C.B.E., F.R.S., F.R.S.C. 

 (Read May Meeting, 1920). 



In connection with some tests on high-tension porcelain insulators 

 of the suspension type, it was found necessary to measure the capacity 

 of one or more of these in series. Special precautions had to be taken 

 in the measurement, as the total capacity usually amounted to less than 

 25 micro-microfarads (22.5 cm.). 



Several methods were considered. Commutator and tuning-fork 

 methods have become obsolete since the advent of generators of fairly 

 high-frequency alternating-current of pure sine-wave form. The 

 charge-sharing method used by Mr. Norman Campbell^ for ionization 

 measurements, although well adapted for measuring the capacity of 

 electrometers, is not so convenient in the case of other small capacities. 



It was soon found that some bridge method must be adopted. The 

 question of bridge measurements with alternating currents has been very 

 ably treated by Mr. G. A. CampbelP in an article entitled "Shielded 

 Balance," but the arrangement of apparatus he describes, although well 

 suited for institutions where measurements of capacity and inductance 

 are continually being made, necessitates too great an outlay to be worth 

 installing for a single research. 



Two variations of a bridge method were used. It was considered 

 that the description of these methods would be of general interest. 

 In the second method, in particular, the use of a guard-ring condenser 

 with alternating current by double balance should invite special attention . 



I .-^Comparison with a Muirhead Air Condenser 



The two end terminals A and B (Fig. 1) of a Kelvin- Varley Slide 

 Potentiometer were connected by means of a twin cable with an earthed 

 sheath to a Vreeland Oscillator giving a pure sine-wave of current at 

 about 12 volts and any frequency from 400 to 1,500. 



The insulators whose capacity was required, were suspended from 

 the ceiling by a rope, about 1 ft. from the Slide. The top of the string 



^ Norman Campbell, "A Note on a Method of Determining Capacities in Measure- 

 ments of Ionization", Phil. Mag., 1911, Vol. 21, p. 42. 



2 G. A. Campbell, Electrical World, 1904, Vol. 43, pp. 633, 647. 



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