352 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



Recent investigations on the conduction process in solid dielectrics 

 have disclosed the fact that in these media Ohm's law is not obeyed. 2 

 The substances investigated were mica, glass, paraffin, shellac and cellu- 

 loid. Excepting paraffin, for which the conductance was so low that the 

 results were uncertain, the conductance was found to increase with the 

 applied potential. The logarithm of the specific conductance increases 

 approximately as a linear function of the potential gradient. In the case 

 of mica, with which substance measurements were made over a large 

 range of potential, the conductance curves are slightly concave toward the 

 axis of potentials. In the case of glass the conductance increase at higher 

 temperatures was found to be noticeably smaller than at lower tempera- 

 tures. 



Since Ohm's law does not hold, it must be assumed either that the 

 number of carriers increases with the applied potential or that the mean 

 speed of the carriers increases. It is not improbable that, under the 

 action of the applied potential, carriers of a type differing from those 

 normally present in the dielectric medium may be formed. It is of par- 

 ticular interest to note that in glass, which is an electrolyte at higher 

 temperatures, the above mentioned results indicate a conduction process 

 differing from that at higher temperatures. 



Compounds of hydrogen with elements which are strongly electro- 

 negative are in general ionized to a slight degree. The most familiar 

 example of this type is water itself, which in the pure state has a specific 

 conductance in the neighborhood of 0.042 X 10~ 7 . 2a Other hydrogen de- 

 rivatives of strongly electronegative groups likewise appear to conduct 

 the current in the pure state, some of them much more readily than 

 water. The specific conductance of formic acid appears to lie in the 

 neighborhood of 10~ 5 . In these cases, however, the process of purification 

 has not been carried to such a point that it can with certainty be stated 

 that the residual conductance is entirely or chiefly due to the ionization 

 of the solvent alone. In the case of hydrogen derivatives, in which the 

 hydrogen is not joined to a strongly electronegative group, the residual 

 specific conductance is as a rule relatively low and it is as yet uncertain 

 to what the residual conductance is due. Acetone, for example, may be 

 purified to a point where its specific conductance is of the order of 10~ 8 , 

 but whether this residual conductance is due to acetone itself or to some 

 impurity is unknown. The same obviously holds true of solvents which 

 contain no hydrogen, such as sulphur dioxide, bromine, etc. 



The hydrogen derivatives of the strongly electronegative groups are 



3 Poole, PUl. Mag. 42, 488 (1921). 



"Kohlrausch and Heydweiller, Ann. d. Phys. 83, 209 (1894). 



