632 ELECTRICAL METHODS [Chap. 10 



polarity, and the liquid becomes polarized. Since matter is transported, 

 differences in concentration result, which in turn give rise to a potential 

 difference. This opposes the potential difference causing the electrical 

 field and is known as the polarization counter e.m.f., E'. Hence, Ohm's 

 law, as applied to the passage of D.C. through polarized electrolytes, is 

 often written in the form I = (E — E')/R. 



Owing to the slow speed with which the ions travel, the counter e.m.f. 

 reaches its maximum value but gradually. After the concentration gra- 

 dient between the electrodes has become linear, stationary conditions are 

 reached. The time required for its establishment depends on the distance 

 between the electrodes and the diffusion constant of the electrolyte. The 

 counter e.m.f. cannot exceed definite values for given conditions and solu- 

 tions, but the electrical field can be increased. Hence, in determining 

 resistivities of rocks containing electrolytes, more reliable values can be 

 obtained with high fields. The difficulty mentioned may, of course, be 

 avoided by alternating current. 



B. Metallic and Electrolytic Current Conduction 



From the viewpoint of molecular physics the following kinds of current 

 conduction may be distinguished: (1) electronic conduction, (2) electrolytic 

 conduction, and (3) dielectric conduction. The first is identical with 

 metallic conduction and is due to the movement of free electrons; the 

 second results from the transport of ions in electrolytes. In the third no 

 free electrons are available. Under the influence of an electrical field the 

 effective centers of electrons and nuclei are displaced. Current propaga- 

 tion results from changes of this polarization or changes in the electrical 

 flux with time and is called displacement current. It will be discussed 

 separately in section c. 



Metallic conductivity is associated with virtually all minerals of metallic 

 luster, ores composed of those minerals, and impregnations of metallic 

 minerals in crystalline and metamorphic rocks. The best conductors are 

 the sulfides, a few of the oxides, and graphite. The conductivity of ores 

 and mineral deposits depends largely on the continuity of the conducting 

 particles. Details are given below. Numerical values for the conductivi- 

 ties of minerals and ores will be found in section g. 



The fundamental difference between electronic and electrolytic conduc- 

 tion lies in the fact that in the solids no matter is transferred whereas in 

 the electrolytes the current propagation is invariably accompanied by a 

 transport of matter and hence by chemical transformation. In solids 



1 See B. McCollum and K. H. Logan, Bur. Stand. Tech. Paper, 25 (1914); M. W. 

 Pullen, U.S. Bur. Mines Circ. No. 6141 (1929). 



