The Dielectric Properties of Insulating Materials 



By E. J. MURPHY and S. O. MORGAN 



This article discusses the variation of dielectric constant and 

 dielectric loss in the radio and power frequency range with the 

 object of giving a simple picture of the t^pe of mechanism which 

 is able to produce anomalous dispersion in this range of frequen- 

 cies. Some of the general characteristics of anomalous dispersion 

 can be demonstrated as well on a simple and arbitrary model of the 

 structure of dielectrics as on the more complex ones which corre- 

 spond more closely to the actual structure of dielectrics. Such a 

 derivation is given here in order to indicate the significance of the 

 dififerent factors which occur in the formulae which have been 

 proposed to account for the variation of dielectric constant and 

 dielectric loss with frequency. This enables a distinction to be 

 made conveniently between the general characteristics which are 

 shared by several types of dielectric polarization and the special 

 characteristics which are peculiar to a restricted class of polariza- 

 tions or to a particular kind of polarization. 



II. Dielectric Polarizability and Anomalous Dispersion 



IN a previous paper ^ the general features of the dependence of 

 dielectric constant on frequency were indicated schematically for 

 the entire range extending from the frequencies used in power trans- 

 mission to those of ultra-violet light. In the range of frequencies 

 below the infra-red (that is, in the electrical range of frequencies) 

 anomalous dispersion is the rule, normal dispersion not having been 

 observed as yet, except for piezo-electric materials, whereas at high 

 optical frequencies normal dispersion is the predominant feature. In 

 the intermediate infra-red region it is not surprising to find a behavior 

 which shows anomalous and normal dispersion in more nearly equal 

 degrees of prominence. 



It will be recalled that anomalous dispersion is the type of frequency- 

 variation in which the dielectric constant decreases with increasing 

 frequency, while normal dispersion is the reverse of this, the dielectric 

 constant or refractive index increasing as the frequency increases. 

 The use of the term anomalous dispersion to describe the dependence 

 of dielectric constant on frequency in the radio and power frequency 

 range is now widespread, and seems quite appropriate, for it brings out 

 the point that the variation of dielectric constant with frequency in 

 ' Murphy and Morgan, B. S. T. J., 16, 493 (1937). 



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