DIELECTRIC PROPERTIES OF INSULATING MATERIALS 501 



frequency ranges merge, atomic polarizations contribute a constant 

 amount to e(or w^) for a given material. The atomic polarization is 

 determined as the difference between the polarization which is meas- 

 ured at some low infra-red or high electric frequency and the electronic 

 polarization as determined from refractive index measurements in the 

 visible spectrum. 



The electronic and atomic polarizations are considered to comprise 

 all of the so-called instantaneous polarizations; that is, the polariza- 

 tions which form completely in a time which is very short as compared 

 with the order of magnitude of the periods of applied fields in the 

 electrical range of frequencies. 



The Debye Orientational Polarization 

 The remaining types of polarization are of the "absorptive" kind, 

 characterized by relaxation-times corresponding to "relaxation- 

 frequencies" in the electrical range of frequencies. These polariza- 

 tions include the important type which is due to the effect of the applied 

 field on the orientation of molecules with permanent electric moments, 

 the theory of which was developed by Debye. Among the other 

 possible polarizations of the absorptive type are those due to inter- 

 facial effects or to ions which are bound in various ways. 



Debye,^ in 1912, suggested that the high dielectric constant of water, 

 alcohol and similar liquids was due to the existence of permanent 

 dipoles in the molecules of these substances. The theory which Debye 

 based upon this postulate opened up a new field for experimental 

 investigation by providing a molecular mechanism to explain dielectric 

 behavior which fitted into and served to confirm the widely held 

 views of chemical structure. Debye postulated that the molecules 

 of all substances except those in which the charges are symmetrically 

 located possess a permanent electric moment which is characteristic 

 of the molecule. In a liquid or gas these molecular dipoles are oriented 

 at random and therefore the magnitude of the polarization vector is 

 zero. When an electric field is applied, however, there is a tendency 

 for the molecules to align themselves with their dipole axes in the 

 direction of the applied field, or, put in another way, to spend more 

 of their time with their dipole axes in the direction of the field than 

 in the opposite direction. This dipole polarization is superimposed 

 upon the electronic and atomic polarizations which are also induced by 

 the field. The theory as developed by Debye accounts for the ob- 

 served difference between the temperature and frequency dependence 

 of the dipole polarizations and the instantaneous polarizations. While 

 the latter are present in all dielectrics, the dipole polarizations can 

 2 P. Debye, Phys. Zeit., 13, 97, (1912); Verh. d. D. phys. Ges., 15, 777 (1913). 



