684 



ELECTRICAL METHODS 



[Chap. 10 



JL 



12 3 4 5 6b 



Fig. 10-32. Fraction of current penetrating below 

 depth d, as a function of the ratio of depth and base 

 length (after Knaebel). 



As previously stated, 

 a determination of the 

 direction of current lines 

 is impracticable since 

 there is but little change 

 in potential away from 

 the direction of maxi- 

 mum current (see Fig. 

 10-33). In the direction 

 X of maximum potential 

 difference, the electrical 

 field is —dY jdx. In 

 any other direction r, it 

 is —dV/dr. Since r = 

 x/cos a, 



-? = E = 



dr 



Ex cos a, (10-22) 



so that the vector, for all 

 values of a, is repre- 

 sented by a figure eight. 

 This explains the sharp- 

 ness of the nulls observed 

 in the location of equi- 



potential lines, a condition which is strictly true for direct current only. 



Equipoitnfiol 



Line 

 1 



Fig. 10-33. Relation of amplitude curve, and directions of equipotential 

 and current lines. 



