The Dielectric Properties of Insulating Materials

N 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 transmission 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 frequencyvariation 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 i M u r p h y and Morgan, B. S. T. J., 16, 493 (1937). 640 I DIELECTRIC PROPERTIES OF INSULATING MATERIALS 641 the radio and power range is in certain respects the same type of phenomenon as optical anomalous dispersion. Anomalous dispersion plays a very important part in the behavior of dielectrics in the electrical range of frequencies.