DIELECTRIC PROPERTIES OF INSULATING MATERIALS 509 



cellulose in such a way as to form a reticulated pattern which may 

 correspond to the pattern formed by the micelles or to some feature 

 of it. An interesting feature of this structure is that the conductance 

 of the aqueous constituent can be changed by varying the moisture 

 content or the salt content of the material and the efifect on the di- 

 electric constant observed.^" 



Frequency Dependence of Dielectric Constant 



As has been pointed out, each of the different types of polarization 

 may contribute to the dielectric constant an amount depending upon 

 the polarizability and its time of relaxation. The upper curve in Fig. 

 1 shows schematically the variation of the dielectric constant (or of the 

 square of the refractive index) for a hypothetical material possessing 

 an interfacial polarization with relaxation-frequency in the power 

 range, a dipole polarization with relaxation frequency in the high 

 radio frequency range and atomic and electronic polarizations with 

 dispersion regions in the infra-red and visible respectively. If polariza- 

 bility were plotted, instead of e (or w^), the curves would be of the same 

 general form but of different magnitudes, because of a relationship 

 between the two given earlier. 



At the low-frequency side of Fig. 1, the dielectric constant curve 

 has its highest value, usually called the static or zero-frequency 

 dielectric constant. Here all of the polarizations have time to form 

 and to contribute their full amount to the dielectric constant. With 

 increasing frequency e begins to decrease as the relaxation-frequency 

 of the interfacial polarization is approached and reaches a constant 

 lower value (called the infinite-frequency dielectric constant) when 

 the applied frequency is sufficiently above the relaxation-frequency of 

 the polarization that it has not time to form appreciably. It is this 

 decrease of e with frequency which is called anomalous dispersion. The 

 horizontal arrows across the top of Fig. 1 indicate the frequency region 

 in which the various types of polarizations indicated are able to form 

 and contribute to the dielectric constant. 



At still higher frequencies we see that e again decreases as the 

 relaxation-frequency of the dipole polarization is approached, and 

 again reaches a constant lower value as the frequency becomes too 

 high for the field to affect appreciably the orientation of dipoles. 

 This second region of anomalous dispersion is similar to the first, 

 which was due to interfacial polarizations. It has been shown as 

 occurring at a higher frequency, but it should be emphasized that the 

 frequency ranges chosen to illustrate anomalous dispersion in Fig. 1 



" Murphy and Lowry, Jour. Phys. Chem., 34, 594 (1930). 



