On the variation with frequency of the conductivity and dielectric constant of dielectrics for high-frequency oscilla­tions

Abstract

Although we have a certain amount of knowledge regarding the variation of the conductivity of dielectrics with frequency for comparatively low frequencies, within the telephonic range, say, up to 5000 per second, where the conductivity is in general a linear function of the frequency, it cannot be said that any information exists at present as to what happens when we extend the range of frequencies up to those employed in radiotelegraphic work. That energy is dissipated in condensers used in oscillation circuits has been known since 1861, when W. Siemens pointed out that the glass of a Leyden jar became heated on charge and discharge. Threlfall, extending the early experiments of Arno, working with a rotating electrostatic field, found that under these conditions there was no hysteresis loss at 10 7 ∼ per second in the dielectrics he employed: ebonite, glass, and sulphur. At somewhat lower frequencies of the order of a million a second, several observers have made measurements of the energy dissipated, and find that condensers have an appreciable decrement. Reference may be made to the following: W. Hahnemann and L. Adelmann, G. Dupreux, J. J. Stockley, M. Wien, J. A. Fleming and G. B. Dyke, L. W. Austin, E. F. W. Alexanderson. Most of the measurements were made at working voltages, so it is impossible to say how much of the energy loss is due to brush discharges and how much to a true dielectric conductivity. Moreover, the measurements have generally been confined to some particular frequency. The object of the experiments to be described below was to measure the conductivity of the dielectric over a wide range of frequency, employing continuous oscilla­tions of sine wave form, and of low voltages.