DIELECTRIC PROPERTIES OF INSULATING MATERIALS 499 



that the number of molecules, and consequently of bound charges, 

 per unit volume is much greater in the liquid than in the gas. How- 

 ever, the molar polarization, a quantity which is corrected for varia- 

 tions in density, is the same for liquid as for gaseous nitrogen. 



The time required for the applied field to displace the electrons 

 within an atom to new positions with respect to their nuclei is so short 

 that there is no observable effect of time or frequency upon the value 

 of the dielectric constant until frequencies corresponding to absorption 

 lines in the visible or ultra-violet spectrum are reached. For con- 

 venience in this discussion the frequency range which includes the 

 infra-red, visible and ultra-violet spectrum will be called the optical 

 frequency range while that which includes radio, audio and power 

 frequencies will be called the electrical frequency range. For all fre- 

 quencies in the electrical range the electronic polarization is indepen- 

 dent of frequency and for a given material contributes a fixed amount 

 to the dielectric constant, but at the frequencies in the optical range 

 corresponding to the absorption lines in the spectrum of the material, 

 the dielectric constant, or better the refractive index, changes rapidly 

 with frequency, and absorption appears. (The justification for using 

 refractive index n and dielectric constant e interchangeably for the 

 qualitative discussion of the properties of dielectric polarizations fol- 

 lows from the relation, e = n^, which is known as Maxwell's rule. 

 This is a general relationship based upon electromagnetic theory and 

 is applicable whenever e and n are measured at the same frequency 

 no matter how high or low it may be.) 



The electronic polarization of a molecule may be regarded as an 

 additive property of the atoms or of the atomic bonds in the molecule, 

 and may be calculated for any dielectric of known composition with 

 sufficient accuracy for most purposes. Within any one chemical class of 

 compounds such as, for example, the saturated hydrocarbons or their 

 simple derivatives, in which all of the bonds are C — H, C — CorC — X, 

 the calculated values agree with the measured to within a few per 

 cent. For other classes of compounds — for example, benzene, in 

 which there are both single and double bonds such calculations must be 

 corrected for the fact that some of the valence electrons have their 

 binding forces and hence their polarizabilities altered in the double 

 bond as compared to the single bond. Such values of electronic 

 polarization, usually called atomic refractions, have been determined 

 for all of the different types of bonds from the vast amount of experi- 

 mental study of refractive indices of organic and inorganic compounds. 



In some materials the electronic polarization is the only one of 

 importance. For example, in benzene the dielectric constant is the 



