DIELECTRIC PROPERTIES OF INSULATING MATERIALS 503 



In the discussion of dipole polarizations it has frequently been 

 pointed out that non-polar materials usually obey the general relation- 

 ship i = n^ whereas for polar materials such as H2O, NH3 and HCl 

 this rule is apparently not obeyed. Water, for example, has n^ = 1.7 

 and e = 78. This apparent discrepancy arises because the refractive 

 inde.x as measured in the visible spectrum is usually compared with 

 the dielectric constant as measured in the electric range of frequencies. 

 Non-polar materials usually have only electronic polarizations and 

 these can form both in the optical and in the electrical frequency 

 ranges, but the dipole polarizations can form and contribute to the 

 dielectric constant only in the electrical frequency range; this is the 

 most frequent source of the above mentioned discrepancy. The 

 general relationship t = v} should apply for any material at any fre- 

 quency provided e and n are measured at the same frequency. The 

 refractive index of water when measured with electric waves,^ for 

 example, at a million cycles, is found to be slightly less than 9, the 

 square of which agrees very well with the observed value e = 78. 

 However, it does not always follow that when e > w^ the molecules of 

 which the material is composed have permanent dipole moments, for 

 this condition can also result from the presence of any slowly-forming 

 or absorptive polarization or of a large atomic polarization. Experi- 

 mental investigations based upon the Debye theory have shown, 

 however, that in the case of water and many other familiar compounds 

 the orientation of dipole molecules actually accounts for the high 

 dielectric constant. 



The Debye theory shows that the magnitude of the dipole polariza- 

 tion of a material is proportional to the square of the electric moment 

 of the molecule, which, as has been pointed out, may be regarded as 

 the vector sum of a number of constituent moments characteristic of 

 the individual atoms or radicals of which the molecule is composed, 

 or alternatively, of the bonds which bind these atoms into molecules 

 or more complex aggregates. The very great amount of experimental 

 study of the Debye theory has shown that the NO2 and CN groups 

 have the largest group moments while CO, OH, NH2, CI, Br, I and 

 CH3 have progressively smaller group moments. The value 34 for the 

 dielectric constant of nitrobenzene (CeHsNOa), as against 5.5 for 

 chlorobenzene (CeHsCl), 2.8 for methyl benzene (C6H5CH3) and 2.28 

 for benzene (CeHe), which is non-polar, are evidence of the large 

 differences in the magnitudes of these group moments and the large 

 part that dipole moments can play in determining the dielectric 

 constant. 



3 Drude, "Physik des Aethers," Stuttgart (1894), p. 486. 



