ADVANCES IN OIL PROSPECTING 103 
of the bridge; the fixed comparison coils are in the other arm of the 
_ bridge; both connect to the variable slide wire and the secondary of 
the buzzer coil. The comparison coils are made up of three fixed re- 
sistances of 10, 100, and 1,000 ohms. An additional resistance of 100 
ohms may be used in series with the cup when its resistance is low. 
The balance is adjusted on the slide wire, with proper setting of the 
fixed resistances, and the reading on the slide wire is multiplied by 
the resistance of the comparison coil used. 
The use of the cup is very convenient as not only soils, but also 
electrolytic solutions may be tested in it. It may be standardized by 
using solutions of known concentrations. The results must be re- 

Fic. 5.—Shepard earth-resistivity meter. 
ferred to a normal temperature. The publication by R. O. E. Davis 
(ref. list No. I,) gives a number of tables on resistivities of soils and 
solutions which will be found valuable when using this bridge. 
b. DETERMINATION OF RESISTIVITIES ON OUTCROPS 
On account of the difficulties which are inherent in the accurate 
determination of the resistivity of samples and which have been dis- 
cussed before, determinations of the resistivities on outcrops, or, 
generally speaking, on the ground surface are ordinarily much more 
satisfactory. 
Quite a few methods may be used for this purpose. For approxi- 
mate tests, the Shepard earth-resistivity meter may be employed 
(Fig. 5). It is an instrument working on D.C.; the ohmmeter and the 
battery are mounted on one probe; the probes are made of iron, and 
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