Abstract :
The possibilities of cellulose acetate as a dielectric have received attention recently from several investigators, at home and abroad. This material has been under consideration by the Electrical Research Association for the past two years, not only from the point of view of its industrial applications, but as a medium for the investigation of fundamental phenomena. The policy of the Electrical Research Association in regard to protracted researches of this nature is to release as much information as possible in the form of interim reports, rather than waiting until the final conclusions are available. The report which follows is one of a series on dielectric phenomena which, for convenience, are related to materials or groups of materials widely used in industry. Other reports in this series relate to ebonite, paper, and varnished cloth. The dielectric properties of cellulose acetate have been studied over the temperature range 25? C. to 90? C, using alternating voltages at frequencies covering the range 50 to 4000 cycles per second, and also direct voltage. Preliminary tests showed that the power factor of commercial cellulose acetate may vary from 2 percent to about 15 percent. The detailed investigation was confined to samples of low power factor. The material was found to absorb water from the atmosphere, and its power factor was reduced by drying. Samples which were dried at a temperature of 90? C. were regarded as completely dry, and such samples were used for most of the measurements. Other samples dried at 25? C. possessed a slightly higher power factor and therefore presumably contained traces of water. Measurements were made also on these samples in order to find out the effect of the water. Measurements of permittivity and power factor were made by means of the Schering bridge fitted with a Wagner earth connection. Mercury electrodes were used. It was found that as long as the voltage gradient does not exceed 3 kV/mm. (75 volts per mil), the permittivit- y and power factor are practically independent of the voltage, and the power dissipated is therefore proportional to the square of the voltage. The power loss (W) was found to increase with increasing frequency (f) according to the law W = G0 + Af? where G0, A and ?, are constants. The index ? was greater than unity. This law may be derived from Hopkinson´s equations for dielectric absorption. Within the range of this investigation, the power dissipated on the application of a definite alternating voltage diminished with rise of temperature up to 90? C. Samples containing traces of moisture and tested at the lowest frequency (50 cycles), and a sample under very high-voltage gradients (17 kV/mm.), formed the only exception to this rule. The true dielectric constant of the material increases with rise of temperature, but the component of the permittivity dueto absorption, like the power loss, diminishes with rise of temperature. Measurements were made under direct-current conditions of the absorption current flowing on charge and discharge, and also of the final leakage current obtained after a very long charge. Both the final leakage current and the absorption current flowing after times of charge of 10 seconds or greater, were increased by a rise of temperature. Such absorption currents obeyed Hopkinson´s superposition principle approximately, but they appear to have no very close connection with the currents flowing in the a.c. experiments. The alternating currents are probably determined almost entirely by the absorption currents flowing during the first thousandth of a second of the charge. When the voltage gradient in the material is increased beyond 3 kV/mm., the power loss increases more rapidly than the square of the voltage, i.e. the power factor and the a.c. conductance increase. This increase may be very large, the normal (low voltage) power factor being trebled in one case at 17 kV/mm. Several samples of the material broke down under voltage gradients
