108 C. A. HEILAND 
(1) Wenner’s arrangement, also called simply the 4-terminal method. (This 
is the electrode arrangement most widely used.) The potential electrodes 
are arranged in line with the current electrodes, and the spacing of all 4 
electrodes is equal. That is, the distance between the outside terminals is 
3a if a is the electrode interval. 
(2) Single-current-probe method: One current electrode is far apart from the 
other, and the two potential electrodes are used in the vicinity of one 
electrode. If their interval is a, they are so used in the vicinity of the cur- 
rent electrode that the distance of the current electrode from the potential 
electrode next to it is also always kept equal to a. The potential electrodes 
need not be in a straight line with the current electrodes. 
(3) This third possible electrode arrangement is practically identical with 
that described in No. 2, except that the second current electrode is no 
longer in infinity, and that the potential electrodes are now in a straight 
line with the current electrodes. 
(4) The fourth method may be called the single-potential-probe method. 
Again two current electrodes are used, but only one potential electrode, 
the position of which is varied between the two current electrodes. 
(5) The fifth method is identical with the fourth, but this time one current 
electrode is at infinity. This method may thus be called the single-current 
and the potential-electrode method. 
Two more methods are possible and have been used, both being 
modifications of the second and the first method already described. 
The first of these is the single-probe method by Eve and Keys 
and is similar to the second method (Fig. 7, JZ a), with the exception 
that the interval between the two potential and one current electrode 
is no longer equal; only the spacing between the potential electrodes 
remains constant. 
The second of these additional methods is a modification of Wen- 
ner’s and Gish-Rooney’s 4-terminal method. An additional electrode 
is provided between the potential electrodes; measurements take 
place as before, that is on four electrodes. Two sets of results, how- 
ever, are now obtained: one (at the right), using the electrodes R and 
S; the second (at the left), using the electrodes P and S. This is called 
Lee’s method (see ref. list No. III, 37) of partitioning." 
1 A third additional method may be mentioned here, which is the “Potential center 
displacement” method of S. H. Williston and C. R. Nichols, described by F. H. 
Lahee in the third (1931) edition of his Field Geology (p. 694). The electrode arrange- 
ment is that shown in IV in Figure 7. The object of the method is the measurement of 
the displacement of the potential center from its geographic center between the two 
primary electrodes which it would occupy in homogeneous ground. In surveying an 
area, the whole contacting arrangement is stepped forward as in resistivity mapping, 
the electrode base remaining constant. The depth of penetration is roughly equal to 
half the length of this base; an area may be covered with different exploration depths 
as in resistivity mapping. The potential center displacements are at a maximum over the 
lateral edges of resistant bodies; zero displacements between two opposite maxima 
would thus indicate the crest of an anticline, etc. 
438 
