DIELECTRIC PROPERTIES OF INSULATING MATERIALS 507 



material is said to display anomalous dispersion in the range of fre- 

 quencies concerned. However, in a paper published in 1898 Drude ^ 

 applied this term to the decrease of dielectric constant with increasing 

 frequency in the electrical range of frequencies. The justification 

 for this extension of the original application of the term is very direct 

 for electromagnetic theory shows that the dielectric constant and the 

 refractive index of a material are connected by the general relationship 

 e = w^ whatever the frequency of the electromagnetic disturbance. 

 As the dispersion of light by a prism is due to the variation of its re- 

 fractive index with frequency, the use of the expression anomalous 

 dispersion to refer to the decrease of dielectric constant with increasing 

 frequency is consistent and has become generally accepted. 



Interfacial Polarizations 



The polarizations thus far considered are the main types to be 

 expected in a homogeneous material. They depend upon the effect 

 of the applied field in slightly displacing electrons in atoms, in slightly 

 distorting the atomic arrangement in molecules and in causing a slight 

 deviation from randomness in the orientation of polar molecules. 

 The remaining types of polarization are those resulting from the 

 heterogeneous nature of the material and are called interfacial polariza- 

 tions. Interfacial polarizations must exist in any dielectric made up 

 of two or more components having different dielectric constants and 

 conductivities except for the particular case where €172 = 6271, 7 being 

 the conductivity ^ and the subscripts referring to the two components. 

 Heterogeneity in a dielectric may be due to a number of causes, and 

 in the case of practical insulating materials is probably the rule rather 

 than the exception. Impregnated paper condensers and laminated 

 plastics are obvious examples of heterogeneous dielectrics. Paper 

 is itself a heterogeneous dielectric, consisting of water and cellulose. 

 In all probability the plastic resins are also heterogeneous, and cer- 

 tainly so if they contain fillers. Ceramics, being mixtures of crystalline 

 and glassy phases, are also heterogeneous. 



The simplest case of interfacial polarization is that of the two-layer 

 dielectric, that is, a composite dielectric made up of two layers, the 

 dielectric constants and conductivities of which are different. Max- 

 well showed that in such a system the capacity was dependent upon 

 the charging time. This is due to the accumulation of charge at the 

 interface between the two layers, for this charge must flow through a 



^ P. T)T\ide,Ann.d. Physik, 64, 131 (1898), "Zur Theorie der anomalien elektrischen 

 Dispersion." 



* In this expression 7 represents the total a.c. conductivit>-, a quantity which 

 depends on the frequency. 



