524 BELL SYSTEM TECHNICAL JOURNAL 



conductivity for this system has the simple value 



Too = l/47rCoi? = Ti 



as shown in Item 3<f, Table I. Here 71 is the free ion conductivity of 

 the main part of the dielectric, R is its resistance and 47rCo(= Ajd) is 

 the ratio of thickness to length of specimen. (In Table I of the pre- 

 ceding paper the symbol C«> is used in place of Cn.) The infinite- 

 frequency conductivity in a non-homogeneous system of this type is a free- 

 ion conductivity. 



Bound ions may also be distributed with macroscopic uniformity in a 

 dielectric. An example of this type of bound ion conductivity ^^ is 

 one due to conducting particles dispersed uniformly in a relatively 

 non-conducting medium. This is the case referred to in Item 3e of 

 Table I. Macroscopic uniformity is obtained in this case by the 

 random distribution of a large number of particles. However, there 

 are some general experimental indications that the distribution of 

 bound ions may in some materials depend upon the basic internal 

 structure of the dielectric and involve some regular geometrical 

 configuration repeated throughout the material. 



In certain dielectrics which absorb an appreciable amount of water 

 when in a humid atmosphere, conduction takes place in aqueous 

 conduction paths permeating the solid. Examples of these materials 

 are cotton, paper, silk and wool. This property is probably shared by 

 many other polymeric substances. The water in these materials is 

 distributed in minute capillaries, the dimensions and other character- 

 istics of which probably determine the form and distribution of the 

 conduction paths. ^^ There exists in these materials a condition capable 

 of producing a bound ion conductivity inasmuch as there are indica- 

 tions that the conducting paths are not of uniform cross-sectional area. 

 Evidence for the existence of a bound ion conductivity in the kind of 

 material to which we have just referred is provided, for example, by 

 conductivity measurements on cotton. ^^ Raw cotton contains salts 

 which can be removed by extraction leaving the material otherwise 

 practically unchanged. These salts are likely to be distributed in the 

 material with macroscopic uniformity as they form part of its natural 

 structure. The fact that the removal of these salts decreases the 



" As already mentioned we shall call any infinite-frequency conductivity which is 

 caused by a macroscopically uniform distribution of bound ions a bound ion conduc- 

 tivity. In some places this term will be applied to any conductivity due to bound ions 

 irrespective of whether or not that conductivity is the limiting high-frequency value. 



1^ One of the basic structural units of cellulose and other similar materials is the 

 micelle. This usually contains a large number of molecules and the capillaries we 

 refer to may correspond to the intermicellar spaces. 



1^ Murphy, Journal of Physical Chemistry, 33, 200 (1929). 



