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. It is seldom possi- 

 ble to interpret a set of measurements of dielectric constant or other 

 dielectric properties without encountering some manifestation of 

 anomalous dispersion or of the other characteristic types of behavior 

 which follow as corollaries of it. 



The two catagories, polarizability and dispersion, include a great 

 deal of the dielectric behavior of insulating materials. This paper 

 will deal primarily with anomalous dispersion, but the theory of anom- 

 alous dispersion is not entirely separable from that of the polarizations 

 of which it is an attribute, so it will be necessary to discuss at least 

 briefly the nature of dielectric polarization. 



The Relation between Polarizability and Dielectric Constant 

 For our purposes a dielectric may be thought of as an assemblage of 

 bound charges, where this term is intended to include the electrons and 

 positive cores in atoms and molecules, the ions held at lattice points in 

 ionic crystals and, in general, any assemblage of charged particles which 

 are so bound together that they are not able to drift from one electrode 

 to the other under the action of an applied electric field of uniform 

 intensity. Actual dielectrics, of course, also contain some conduction 

 electrons or ions which are free to drift through the material and dis- 

 charge at the electrodes, producing a direct current conductivity. 

 This conductivity is small at ordinary temperatures in materials 

 classified as dielectrics. 



The positions of these charged particles may be considered to be 

 determined by an equilibrium of forces. When an electric field is 

 applied this equilibrium is disturbed and the bound charges are dis- 

 placed to new positions of equilibrium; then when the applied field is 

 removed they revert to their initial positions. In the equilibrium 

 positions which the charges occupy when a constant electric field has 

 been impressed on the dielectric they have a larger potential energy 

 than in their initial positions. Moreover, they do not revert instantly 

 to their initial positions, and when the retardation is due to friction 

 some of the potential energy of the bound charges is dissipated as 

 heat in the dielectric. 



When an alternating voltage is applied to the dielectric, we may think 

 of the bound charges as moving back and forth with certain amplitudes, 

 a diiTerent amplitude for each different type of bound charge. When 

 the applied electric field is of unit intensity, the sum of the product of 



