The dielectric properties of varnished cloth at low voltage-gradients

Abstract

Measurements of the dielectric properties of typical samples of varnished cloth have been made with the object of studying the laws governing the dissipation of power in the material when it is subjected to an alternating voltage. All the measurements were made under carefully controlled conditions. The samples were conditioned and tested in an enclosure, which was approximately airtight, and the temperature and humidity were regulated as required. An arrangement of mercury electrodes was generally used, but a number of control experiments made with graphite electrodes showed that the properties observed were those of the material itself, and were not peculiar to any one type of electrode. The most important measurements made were those of permittivity and power factor (from which the power loss was calculated), but measurements were also made, in certain cases, of the absorption and leakage currents produced by a constant applied voltage, with the object of correlating the results of alternating- and direct- current measurements. The permittivity and power factor were found to be independent of the voltage for all voltage gradients less than a certain critical value, and the present investigation was confined to this range of voltage gradients, which includes the values commonly employed in practice. The methods of measurement include the Schering bridge, a resistance bridge found to be more convenient in cases where the power factor was very large (values greater than 90 per cent were found), and an electro meter method for the d.c. experiments. The corrections and precautions necessary for high accuracy were studied in detail. The variations of the dielectric properties with atmospheric humidity, temperature and frequency of the applied voltage, have been investigated, and curves representing the more important results are given. Within the frequency range 50 to 4 000 cycles, the majority of the results obey the equations first given by Hopkinson and afterwards by Schweidler, and it is shown that the properties of such samples can be represented by four fundamental constants, the values of which are tabulated. These constants will represent d.c. as well as a.c. observations, but only when the time of electrification is short, i.e. of the same order as the periodic time of the alternating current. The general conclusion reached is that the current responsible for the power loss in varnished cloth is carried by ions or dipoles, the motion of which is opposed by a frictional force depending on the viscosity of the material or some similar property; that the displacement of such ions or dipoles conforms to Hopkinson's superposition principle, and that there is no dielectric hysteresis analogous to magnetic hysteresis. No existing theory gives a completely satisfactory explanation of the equations representing the observations, but the statistical explanation of K. W. Wagner, when applied to several possible theories, will account for them to some extent.