504 BELL SYSTEM TECHNICAL JOURNAL 



equal to the initial conductivity obtained by extrapolating the ap- 

 parent d-c conductivity towards the instant of applying the measuring 

 voltage. We believe that this relationship considerably simplifies the 

 description of the meaning of certain types of measurements upon 

 dielectrics. 



In spite of the fact that several terms are already used to distinguish 

 different conductivities, there remains some ambiguity in the meaning 

 of these terms. For example, the physical meaning of the term d-c 

 conductivity when applied to a dielectric is vague. Moreover, it will 

 be evident in the later discussion that the initial conductivity will 

 depend upon free ions for some materials and upon polar molecules for 

 other materials. To avoid this confusion we have found it convenient 

 to use two terms which refer to the nature of the conduction processes 

 rather than to the method of measurement : these are free ion conduc- 

 tivity and polarization conductivity. The first is the ordinary conduc- 

 tivity due to the drift of free electrons or ions to the electrodes; the 

 second is a conductivity determined by the energy dissipated as heat 

 by the polarization currents in the dielectric. The latter bears the 

 same relation to the neutral polarizable aggregates in the material, 

 which carry the polarization currents, as does the free ion conductivity 

 to the free ions in the dielectric. The terms free ion conductivity and 

 polarization conductivity, or some other terms having approximately 

 the same meaning, are essential to the discussion as they refer unam- 

 biguously to two distinct properties of the material, while the terms 

 initial, final, true, infinite-frequency, a-c and d-c conductivity all refer 

 to different methods of measuring these two properties of the material. 



The current flowing in a dielectric to which a constant potential is 

 applied often decreases with time for periods of the order of a few 

 minutes or longer measured from the time of applying the potential. 

 This decreasing current is variously referred to as a residual charging 

 current, an absorption current, an anomalous conduction current or an 

 irreversible absorption current, depending upon the interpretation given 

 to the phenomenon. We have already indicated that these residual 

 currents complicate the measuring technique in the determination of 

 the d-c conductivity of insulating materials. The most definite kinds 

 of residual currents are those which are simply a manifestation of the 

 structural characteristics which give rise to anomalous dispersion of the 

 dielectric constant. These residual currents and the residual charges 

 associated with them will be referred to here as the direct-current 

 counterparts of anomalous dispersion to indicate that they are not 

 independent properties of the material, but necessary requirements of 

 the existence of anomalous dispersion occurring at sufficiently low 



