The electrical stability of condensers

The paper gives an account of (a) an investigation into the nature and causes of undesired changes in the capacitance and power factor of condensers, and (b) means of obtaining greater constancy. The principal results and conclusions are as follows:? The changes are almost entirely due to temperature variation. The thermal behaviour of condensers with solid dielectric is in general non-cyclic, and for a representative selection the temperature coefficients of capacitance ranged from ?1800 to +200 parts in 106 per deg. C. The thermal behaviour of air-dielectric condensers is not cyclic in general, but more nearly cyclic than that of solid-dielectric condensers, and cyclic behaviour was obtained in some cases. The coefficients of a representative selection ranged from ?65 to +150 parts in 106 per deg. C. Where the behaviour was cyclic, the capacitance coefficient was between 2 and 3 times that of the linear expansion of the metals used. An analysis of the causes of abnormally large coefficients showed that the variation of capacitance with air density is about 2 parts in 106 per deg. C. With entirely free expansion of metal plates, the coefficient is approximately that of the linear expansion of the metal, in accordance with simple theory. Variation of ?fringe? and ?stray? field with temperature has no appreciable effect on temperature coefficient. Change of residual internal stress in the metal plates and members with change of temperature is not in general a very important factor. Distortion due to temperature-gradients arising from inequalities of thermal characteristics of component parts is likely to be a significant factor in many cases. Variation of elasticity and moment of inertia with temperature is not significant. Unequal expansion of different parts may be a very important factor in producing large coefficients-, particularly if the one set of electrodes is not quite symmetrical with respect to the other (i.e. unequal air-gaps). Even where constraint is in- tended to be avoided by the free sliding of plates in grooves, the frictional constraint is likely to be sufficient to cause distortion. In air-dielectric condensers with equal air-gaps, constraints arising from inequalities of expansion of different parts suffice to account for coefficients of 2 or 3 times the theoretical value for free expansion; with slight inequality of air-gaps (a few mils only) the effect of any small distortion in the plates due to this or any other of the causes listed is very greatly enhanced. An examination of the properties of insulating materials showed that the temperature coefficient of permittivity of solid dielectrics ranges from about ?700 in 106 per deg. C (high-permittivity ceramics) to about +2000 in 106 per deg. C. This temperature coefficient may be significant in variable air-condensers. Ebonites, synthetic plastics, and a specimen of fused silica, showed non-cyclic thermal behaviour. Certain ceramics were very satisfactory in respect of deformation and cyclic behaviour. By comparing a complete and detailed analysis of the thermal characteristics (conduction, radiation, etc.) of the component parts of a condenser with the thermal behaviour of the complete condenser some insight into the reasons for abnormal temperature coefficients can be obtained, and the knowledge thus gained can be applied to design. Minimization of temperature coefficients of capacitance requires elimination of residual stresses and of variation of mechanical constraint, uniformity of effective thermal mass in the various elements, and accurate location of a mechanically suitable insulator with a low temperature-coefficient of permittivity. Among the methods of compensation for temperaturechange, the use of a solid dielectric with negative temperaturecoefficient of permittivity has possibilities, but some serious practical limitations. There are practical objections to the use of ?invar? or copper-plated invar. Bimetallic systems are practicable and effe