DIELECTRIC PROPERTIES OF INSULATING MATERIALS 511 



electrical frequency range, is due to the dissipation of the energy of the 

 field as heat because of the friction experienced by the bound charges 

 or dipoles in their motion in the applied field in forming the polariza- 

 tions. The theory of dispersion shows that the dielectric constant and 

 absorption are not independent quantities but that the absorption 

 curve can be calculated from the dielectric constant vs. frequency 

 curve and vice versa. The absorption maximum is greatest for those 

 materials showing the greatest change in dielectric constant in passing 

 through the dispersion region. Thus a material having a high di- 

 electric constant must have a large dielectric loss at the frequency at 

 which e has a value half way between its low and high-frequency values. 

 Though the quantum theory is necessary for the explanation of many 

 optical and electrical phenomena a simple explanation, sufficient for 

 our purposes, of the general form of the curves of dielectric constant vs. 

 frequency in the infra-red and visible spectrum may be given in terms 

 of the Lorentz theory of optical dispersion. In this theory the form 

 of the dispersion curves depends upon the variation with frequency of 

 the relative importance of the inertia of the typical electron and of the 

 frictional forces and restoring forces acting upon it. For electronic 

 polarizations the frictional or dissipative force is negligible, except 

 in the narrow frequency interval included in the absorption band, and 

 the inertia and restoring force terms predominate. For the atomic 

 polarizations the frictional force is larger and the absorption region 

 extends over a wider interval of frequencies. P'or dipole and inter- 

 facial polarizations the influence of inertia is entirely negligible as 

 compared with the frictional or dissipative forces so that in effect these 

 polarizations may be thought of as aperiodically damped. 



Temperature Dependence of Dielectric Constant 

 The dielectric constant of a material is a constant only in the ex- 

 ceptional case. Besides the variation with frequency which has been 

 considered the dielectric constant varies with temperature. Elec- 

 tronic polarizations may be considered to be unaffected by the tempera- 

 ture. The refractive index does indeed change with temperature 

 but this is completely accounted for by the change of density, and the 

 molar refraction is independent of temperature. The atomic and 

 ionic vibrations are, however, affected by temperature, the binding 

 force between ions or atoms being weakened by increased temperature. 

 This factor of itself would yield a positive temperature coefficient for 

 the atomic polarizations but the decrease in density with the increase 

 in temperature acts in the opposite direction. The result is that 

 calculation of the temperature coefficient of atomic polarizations 



