630 ELECTRICAL METHODS [Chap. 10 



potentials due to differences in solution pressure P' and (2) differences in 

 concentration : 



AJ5 = 1.98. 10"'. - logio ^^, volts. (10-lc) 



n Ci • i 2 



With this equation, voltages are calculated which are in better agree- 

 ment with those actually observed. Under the assumption that there are 

 no appreciable differences in concentration of solution between the upper 

 and the lower parts of the ore body, eq. (10-lc) retains the form AE = 

 c • logio P'x/P'i , which gives potentials sufficient to account for the anomalies 

 measured at the surface. Pyrite in undisintegrated form is likely to have 

 a solution pressure of around 10^ atm. and in altered form (such as limo- 

 nite, and the like, in the gossan) probably not more than lO"^'' atm. The 

 equation in its last form makes it possible to allow for equivalent series 

 connections of altered and unaltered portions of an ore body. 



Electrode potentials occur when metal probes are placed in contact with 

 the ground in order to measure potential differences between them. Con- 

 tact potentials amounting to several millivolts and even several tens of 

 millivolts may be observed and may be avoided by the use of so-called 

 nonpolarizable electrodes. The latter consist of electrodes (of the same 

 metal for a pair) immersed in a saturated solution of one of its salts (copper 

 in copper sulfate, zinc in zinc sulfate, and so on). The solutions are 

 carried in vessels made of a permeable substance (porous clay or beef gut) 

 so that they may filter through and make contact with the ground solu- 

 tions. Inasmuch as identical metals and identical solutions are used for 

 both electrodes, the electrode potentials are of equal sign and cancel when 

 no current is flowing. 



Potentials arising from direct contact of metal with soil solutions of 

 different concentrations are avoided by the interposition of a concentrated 

 solution. To obviate diffusion potentials (see paragraph h), some investi- 

 gators have proposed the use of two chambers, an inner one with the 

 saturated solution and an outer chamber with a more dilute solution. 

 Upon the passage of current, the polarization (see paragraph 2) of these 

 electrodes is negligible; since metals in solutions containing their ions 

 belong to the "reversible" systems, a passage of current will form no new 

 chemical compounds. What polarization occurs is due merely to concen- 

 tration changes in the solution near the electrodes. In the practice of 

 electrical prospecting this concentration polarization is small because of 

 the small current densities involved. Nonpolarizable electrodes are neces- 

 sary in all D.C. methods of electrical exploration; in A.C. procedures 

 ordinary metal electrodes are satisfactory. 



(6) Diffusion (osmotic) potentials occur in wells in connection with po- 



