ELECTRICAL METHODS 519 



Five-Electrode Method Using Four Equal Spacings (Figure 313b) 

 _ 3 , { Vp-Vn \ - 1_ ( Vp-Vr \ 



-A r 



A 



or 



L^£? Partitioning Method (Figure 313c) 

 Pa= A-rta (—^ — -) = 47ra {—^ — -) 



Unsymnictrical Method : Energising Electrodes Separated by a 

 Finite Distance (Figure 313d) 



27r\ a L — aJ 2-rr\2a L — 2a/ 



1 , 1 



.+ 



2a L —2a L — a 



Unsymmetrical MetJiod : One Energizing Electrode at an Infinite 

 (Distance Figure 313e) 



The power electrode 6^2 is assumed to be at infinity. (In practice, 

 this assumption is justified when the separation SiSo is 5 to 10 times that 

 of the farthest potential electrode from Si.) 



2 IT a 2 TT 2a 

 or 



Vp-Vn 



Pa 



= AttG 



At sufficiently great spacings of the power electrodes, S\ and S^, the 

 angle 6 may take on any value and the formula will be correct. If the 

 distance S1S2 is not much greater than SiR, however, the effect of S2 

 may not be neglected, and the formula must be modified. 



