40 W. D. KELLER 
Mexico, Missouri; Jones and Turner, mining contractors, Linn, 
Missouri; and the physics department, University of Missouri, the 
writer borrowed and assembled a set of equipment, and obtained 
data which would otherwise not be available. Professor W. A. Tarr, 
of the department of geology, University of Missouri, offered sug- 
gestions and criticisms. 
APPARATUS AND METHOD 
The method followed was the simple Wenner 4-electrode hook-up, 
described in some of the handbooks? on geophysical prospecting. In 
addition, a supplementary potential electrode, as proposed by Lee 
and Swartz,‘ was frequently used for tapping in the center of the cir- 
cuit. As a ready-made resistivity measuring set was not available with 
the funds at hand for the work, a modified Gish-Rooney set was as- 
sembled by using a standard o—75 milliammeter, student potentiom- 
eter, go-volt radio B battery, and a locally constructed circuit re- 
versing commutator driven by a 6-volt motor. 
Ordinary surveyor’s tapes were used to measure the electrode 
separation. They were laid off in feet, and as a convenience the com- 
putations of resistivity were reported, not in ohms per centimeter 
cube, the usual method, but in a field unit, ohms per foot cube. At 
each observation the electrode spacing in feet was selected, the cur- 
rent in milliamperes and the potential in millivolts read, and the field 
resistivity unit, 7, computed from the equation: 
E 
r=a— 
Jf 
where a is the electrode separation in feet, and E and J, millivolts and 
milliamperes, respectively. It is obvious that the computation was 
made with the greatest ease in the field at once, and was available for 
use there on the ground. Since an intelligent selection of the location 
or procedure of the next set-up may be governed to a large extent by 
the results at the previous one, and since this advantage is lost if 
calculations are delayed until the office is reached, the advantage of a 
simple and easily calculated unit is apparent. 
In accordance, all data embodied in this report are in terms of the 
aforementioned field resistivity unit. 
3A. S. Eve and D. A. Keys, Applied Geophysics, London, Cambridge University 
Press (1929), p. 83. Geophysical Prospecting, 1920, Amer. Inst. Min. Met. Eng., New 
York, p. 54. 
4F. W. Lee and J. H. Swartz, “Resistivity Measurements of Oil-Bearing Beds,” 
U.S. Bur. Mines Tech. Paper 488 (1930). 
562 
