Chap. 10] ELECTRICAL METHODS '629 



tration near the surface and below, and (b) to differences in ore material 

 above and below; (2) in the interference potentials which originate on 

 metal electrodes when the latter make contact with the ground at points 

 where the concentrations of electrolytic solutions are different; (3) in the 

 construction of nonpolarizable electrodes. 



Potential differences resulting from the contact of two electrodes with 

 solutions of different concentration may be calculated from the potential 

 of a metal against a solution, which is dependent only on the concentration 

 of the ions of the particular metal. The potential difference of solution 

 minus metal is 



where R is the gas constant, or 8.309 joule per degree C; n is the valency; 

 F is Faraday's constant of electrolysis, that is, the quantity of electricity 

 liberating one gram equivalent, or 96,494 coulombs (so that the ratio 

 R/F is 8,610 e.m.u.'s or 0.861- 10"* volts); t is the absolute temperature, 

 or 273° + e, P' is constant for the metal involved (its electrolytic solution 

 pressure); and C is the ion concentration. If two electrodes of the same 

 metal are immersed in two solutions of different concentration, C2 and Ci 

 (which may be connected by a syphon bridge to afford a return circviit 

 with negligible diffusion potential), the potential difference between the two 

 electrodes is 



A^ = 1 .98 . 10"* . - logio ^ volts, (10-16) 



n Li 



where the numerical factor contains the ratio R/F, the modulus of the 

 natural logarithms, and the conversion of e.m.u.'s into volts. An evalua- 

 tion of the above equation shows that for e — 18°C. the voltage difference 

 is AE = 1/n- 0.0577 -logio C2/C1 volts, so that for w = 1 and a concentra- 

 tion ratio of 10, the voltage difference is 0.058 volts and, for a ratio of 100, 

 0.115 volts. 



The voltage gradients observed on polarized ore bodies are normally 

 from 1 to 2 millivolts per foot, which amounts to anomalies of the order of 

 0.1 volt to a maximum of 1.5 volts. This is comparable to the voltage 

 delivered from a wet cell; it is, therefore, probable that not only differences 

 in concentration but differences in the composition of the ore near the 

 surface (gossan!) and unoxidized portions below, where the solutions are 

 less acid and poorer in H ions, play a part in causing such potentials. To 

 explain the phenomenon on ore bodies showing spontaneous polarization, 

 equation (10-16) should be written in a form allowing for (1) electrode 



