680 



ELECTRICAL METHODS 



[Chap. 10 



in the vicinity of power plants, trolley lines, and the like, and discharge 

 them again in others. Current is collected in the negative zones and 

 leaves the conductor in the anodic or positive zones. It is again in the 

 latter that the destruction of metal occurs. The effects described are 

 related as cause and effect. In the former, chemical action is the cause 



S£LF-POT£NriAL SURV£y of A/ffAnear KATANGA 



Fig. 10-29. Location of folds of graphitic slates in Katanga (after Poldini). 



Pipe Colltctinq 

 Currtnt 



Pipe Discharging Currenf 

 [Corrosion Zonf) 



Fig. 10-30. Location of corrosion zones in pipes by self-potential measurements 



(after Schlumberger). 



of the current; in the latter, extraneous currents produce the chemical 

 reactions. 



Since anodic zones are generally located in areas of low resistivity, sys- 

 tematic studies of the resistivity of soils around pipe lines have been made. 

 Another line of attack is to measure the acidity of soils to determine their 

 corrosiveness. Self-potential methods arc used for the determination of 



