50_specialwave - JOURNAL OF THE BRITISH INSTITUTION OF...

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JOURNA L O F TH E BRITIS H INSTITUTIO N RADI O ENGINEER S THE USE OF SPECIAL WAVEFORMS IN THE STUDY OF LINEAR DIELECTRIC PHENOMENA* by Elizabeth Laverick, B.Sc, Ph.D. (Graduate)} A Paper read before the North-Eastern Section on November 16//?, 1949 SUMMARY A limited amount of work has been published recently on the use of non-sinusoidal waveforms in the study of dielectrics, and it would appear that such methods are advantageous in the study of both linear and non-linear phenomena. In this paper, an attempt has been made to illustrate the usefulness of these methods. First, an outline is given of the steady field and sinusoidal field methods usually used in the measurement of dielectric properties, and of the theories which have been advanced to explain the ensuing results. This is followed by a discussion of the work which has been published recently on the use of special waveforms in dielectric measurements. 1.0. Introduction In general, the electrical properties of a dielectric are described by one quantity, the permittivity. This may be defined as the ratio between the electrical capacitances of a con- denser with the dielectric and with a vacuum as the medium separating the plates respectively. For most materials the permittivity is indepen- dent of the strength of the applied field, for moderate field strengths, but dependent on tem- perature, and in the case of sinusoidal fields on frequency also. The variation of the permittivity with frequency, and the corresponding absorp- tion of energy involved, gives rise to the concept of a complex permittivity, 6 = 6' +76' where E' and e" depend on frequency, and e" is proportional to the dielectric loss. The properties of a dielectric are usually measured in terms of the permittivity (e', or j e | = V( e ' 2 + e" 2 )), and the loss angle (tan 6), over a wide range of frequencies at a series of discrete frequencies. Alternatively, these properties may be measured as a function of temperature. The variations of dielectric properties with frequency and with temperature are generally similar in form. In the case of temperature variations, however, the range in which the variations occur is comparatively small, and in practice * Manuscript received November 27th, 1950. t Formerly at the Physics Department, Durham University; now at the Research Laboratories, The General Electric Co., Ltd., England. U.D.C. No. 621.317.335.3 : 621.3.018.7. a fine control of the temperature is required. The fundamental problem is to explain the experimental facts in terms of the actual mechanism by means of which the absorption of energy takes place in the dielectric. Dielectric losses contribute materially to the attenuation of waves in transmission lines, to insensitive tuning in resonant circuits, and to insulator breakdown in some circumstances. They also form the basis of dielectric heating phenomena.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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50_specialwave - JOURNAL OF THE BRITISH INSTITUTION OF...

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