DIELECTRIC PROPERTIES OF INSULATING MATERIALS 531 



is also a polarization conductivity. However, if we are dealing with a 

 macroscopically non-homogeneous dielectric such as a two-layer dielec- 

 tric or a material which has a high resistance blocking layer at one of 

 the electrodes 70 is a free ion conductivity. This is evident from the 

 previous discussion of the significance of 700. 



These relationships are of interest in connection with the difficulties 

 encountered in the interpretation of d-c conductivity data which 

 were mentioned in the introduction. They apply especially to the 

 methods recommended by Joffe ^^ and by Richardson. ^^ 



When a constant potential difference is maintained between the 

 plates of a condenser containing a solid dielectric the current observed 

 does not in general remain constant but usually decreases with time. 

 This decrease may continue for several minutes or hours. The study 

 of these residual currents is of importance in connection with the 

 interpretation of conductivity data on dielectrics. The question 

 arises as to how much of the observed behavior of the residual currents 

 which flow in dielectrics under constant potential may be explained as 

 the d-c counterparts of anomalous dispersion. Many materials of 

 practical importance as insulators exhibit a more complicated type of 

 variation with frequency than is indicated by equations (17) and (19) 

 which refer only to the simplest observed type of dispersion. The more 

 complicated types of behavior observed are usually attributed to the 

 presence of polarizations possessing a wide distribution of relaxation- 

 times. However, other processes may also contribute to the deviation 

 of the experimental curves from the theoretical. One of these is 

 electrolysis which produces changes in the composition, and conse- 

 quently in the conductivity of the material. Another effect which 

 may contribute is a possible lack of constancy of the relaxation-time. 

 It is evident, therefore, that the d-c counterparts of anomalous dis- 

 persion due to a polarization of a single relaxation-time should not be 

 expected to explain all of the observed residual phenomena, particularly 

 in solid dielectrics. However, the quantitative relations derived as 

 the d-c counterparts of anomalous dispersion are applicable to some 

 materials, and for those to which they are not quantitatively applicable, 

 may serve as a useful guide in the interpretation of the residual cur- 

 rents. As another section of this paper is planned in which the in- 

 fluence of residual currents upon conductivity measurements will be 

 discussed further, we have included in the appendix some relationships 

 which are useful in the interpretation of the behavior of these currents. 



i^A. Joffe, Ann. d. Physik, 72, 481 (1923); "Phvsics of Crystals," New York 

 {\92S);Zeits.f. Physik, 62, 730 (1930). See also Sinjelnikoff and Walther, Zeits. f. 

 Physik, 40, 786 (1927). 



"S. W. Richardson, Proc. Roy. Soc, 107 A, 102 (1925). 



