Chap. 10] ELECTRICAL METHODS 681 



the zones.of egress when the corrosion is of a chemical nature or is produced 

 by stray currents not due to industrial plants. Measurements are made 

 in the customary manner with nonpolarizable electrodes at short intervals 

 to locate zones of positive potential. Fig. 10-30 shows measurements 

 made in this manner along 2300 feet of pipe line in Paris, at intervals of 

 some 65 feet, establishing two corrosion zones by their positive potential. 

 C. and M. Schlumberger^* have described these and other methods for the 

 location of corrosive zones in more detail. 



IV. EQUIPOTENTIAL-LINE AND POTENTIAL-PROFILE 



METHODS 



A. Conditions in Stationary Fields 



When electric energy is appUed to two points at the ground surface, an 

 electric current will flow between them because of their difference in po- 

 tential. If the medium between the two electrodes is homogeneous, the 

 current and potential distribution is regular and may be calculated. When 

 good or poor conductors are imbedded in this homogeneous medium, a 

 distortion of the electrical field occurs. Good conductors have a tendency 

 to attract the current lines toward them while poor conductors force them 

 away. Theoretically it should be possible to detect bodies of different 

 conductivity by measuring the geometric disposition of these current lines. 

 In practice this cannot be done with suflficient accuracy; it is necessary to 

 determine the direction in which no current flows by locating points which 

 have no potential difference. Using a null method has the advantage of 

 both accuracy and ease of procedure. Quantitative measurements of po- 

 tential difference are not required when the lines of identical potential, or 

 "equipotential lines," are traced. For homogeneous ground the potential 

 variation in both a horizontal and a vertical plane is illustrated in Fig. 

 10-31. The potential gradient is not uniform; it is greatest in the vicinity 

 of the electrodes. The "current lines" are concave to the surface because 

 of the repulsion of adjacent current fibers. Equipotential lines, at right 

 angles to the current lines, are circles only in the immediate vicinity of the 

 electrodes. Elsewhere in the horizontal plane and vertical section, they 

 are curves of the fourth degree. 



For a stationary field the potential distribution in homogeneous ground 

 can be calculated. By a stationary field is meant here a field which does 

 not change with time and is produced by direct current after equilibrium 

 has been reached. The results obtained apply also to A.C. fields if skin 

 effect and elliptical polarization are neglected (quasi-stationary fields). A 



2* A.I.M.E. Tech. Pub. No. 476. The trade journals covering oil and gas trans- 

 portation currently carry articles dealing with pipe corrosion. 



