DIELECTRIC PROPERTIES OF INSULATING MATERIALS 525 



dielectric loss indicates that the material possessed a bound ion 

 conductivity before the salts were removed. 



Some of the materials belonging to the class of dielectrics which we 

 have just discussed are closely related in chemical and in physical 

 structure to compounds which are important biologically and in the 

 study of plant and seed structure. Many are also of commercial 

 importance as insulating materials. 



It is not necessary that the conduction paths be composed of aqueous 

 solutions: in some materials plasticizers or products of pyrolysis are 

 sufficiently conducting for this purpose. The dielectric behavior of 

 certain plastics may be interpreted as evidence for the existence of such 

 non-aqueous conduction paths in the material, producing a free ion 

 conductivity, a bound ion conductivity and a contribution to the 

 dielectric constant. Imperfections of structure occurring in crystals 

 are able to produce a bound ion conductivity and there is experimental 

 evidence that these imperfections do occur. ^^ The regular lattice ions 

 in an ionic crystal have too high a binding energy, and dissipate too 

 little energy in their motions in a radio frequency electric field to 

 produce a bound ion conductivity. 



The polarization which is responsible for the bound ion conductivity 

 is of the interfacial, or Maxwell-Wagner, type. This type of polariza- 

 tion may be of importance in materials with a cellular structure 

 and in materials which may be described as interstitially conducting 

 dielectrics. 



In the above discussion we have outlined the character of three 

 widely different types of infinite-frequency conductivity : 



(a) An orientational conductivity depending upon the small changes 

 which an applied field produces in the average orientation of polar 

 molecules. 



(b) A bound ion conductivity depending upon the displacement of 

 uniformly distributed bound ions. 



(c) An infinite-frequency conductivity which is proportional to the 

 free ion conductivity of one of the constituents of a dielectric consisting 

 of two or more layers of widely difi^erent conductivities. 



The Relaxation-Time 



The relaxation-time is closely related to the infinite-frequency 

 conductivity. This may be seen by reference to equation (20), which 

 shows that the relaxation-time is given by 



4tToo 



"See, for example, A. Smekal, Zeits.f. techn. Physik, 8, 561 (1927). 



