The Dielectric Properties of Insulating Materials, III Alternating andDirect Current Conductivity

T 1 HE preceding paper 1 dealt with the dielectric constant, showing mainly how it varies with the frequency of the applied alternating voltage for those dielectrics which behave in the simplest manner, and indicating the general character of the structural features responsible for this behavior. The discussion is extended here to the conductivity, Murphy and Morgan, B. S. T. J., 17, 640 (1938). 502 DIELECTRIC PROPERTIES OF INS ULA TING MATERIALS 503 which is not less important than the dielectric constant as a property of an insulating material. Though general aspects of the conductivity will be described for the sake of completeness, we wish mainly to show that materials which possess the property of anomalous dispersion may be considered to have two quite definite conductivities: one of these is the ordinary d-c conductivity due to free ions or electrons; the other is a special value of the a-c conductivity which will be discussed in this paper. We believe that the recognition of the existence in many materials of two conductivities instead of one is of considerable advantage, particularly in interpreting the behavior encountered in direct-current conductivity measurements on insulating materials, a subject upon which there has existed a considerable divergence of opinion. The measurement of the direct-current conductivity of an insulating material is usually complicated by the fact that the current which flows when a constant potential is applied does not remain constant but decreases with time.