116 C. A. HETEAND 
but from a hand-driven D.C. generator, on the shaft of which a double 
commutator, similar to the one used in the Gish-Rooney apparatus, 
is mounted. 
The most important manufacturers of Megohmers or Meggers 
are: Evershed and Vignoles, Acton, England; James G. Biddle, Phila- 
delphia; and Herman H. Sticht and Company, New York. 
The accompanying figures show a number of Meggers, which are 
all very similar in principle. Details on the construction of these in- 
struments may be obtained by writing to the manufacturers and ask- 
ing for their catalogs. Figure 12 shows wiring diagram and sectional 
view of a Megger manufactured by Biddle, and Figures 13 a and 13 b 
show Meggers manufactured by the Sticht Company. 
III. INTERPRETATION OF RESULTS 
The fundamental principle underlying the interpretation of the 
resistivity results is comparatively simple. 
The depth reached depends on the spacing of the potential electrodes 
(or their distance from a current electrode in methods where station- 
ary current electrodes are used). 
In other words, if the observed resistivity values are plotted as a 
function of the electrode separation (in the 4-terminal method), or 
as a function of the distance of the potential electrodes from the near 
current electrode in single-probe methods, then the electrode separation 
or electrode distance at which marked changes in apparent resistivities 
occur 1s approximately equal to the depth to the resistivity discon- 
tinuities underground. 
This statement is made merely to illustrate the nature of the 
fundamental principle underlying resistivity measurements. In prac- 
tice, however, there are more difficulties, as the presence of more 
than one discontinuity of resistivity is considerably complicating. 
Actual experience shows that this simple depth rule works sur- 
prisingly well in mining when insulated bodies of good conductivity 
are encountered. In application to stratified ground, severe complica- 
tions arise, chiefly with the Gish-Rooney method, and particularly, 
when there is more than one layer present. 
In the practice of resistivity work, therefore, we use several lines 
of attack in the interpretation of results. As in other types of geo- 
physical work, both qualitative and quantitative methods may be 
applied. The qualitative method usually precedes the quantitative 
method in application, and the quantitative method is applied if the 
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