ADVANCES IN OIL PROSPECTING IOI 
high frequency tests, and on the right are the meters and switches for 
D.C. testing. The bridge.is A.C. operated, and consists of an oscil- 
lator, the frequency of which may be varied within wide limits in the 
high frequency range. A pickup coil with variable coupling transfers 
the energy of the oscillator to the testing circuit, which is essentially 
an A.C. Wheatstone bridge. Non-inductive resistors are used; the 
switches for the 10, 100, 1,000, 10,000 and 100,000 ohm ranges are 
seen on the panel. The same resistors are used when the bridge is 
operated on D.C. The binding posts on the extreme right edge of the 
panel are for making connections to the testing battery or a low fre- 
quency oscillator or commutator.! 
The bridge is only for small samples of regular shape, such as 
diamond drill cores and the like. For larger specimens of consolidated 
rocks, we use a different bridge which consists of a frame with two 
spindles to hold the specimen, and four arms spaced at equal intervals 
which carry contact points at their end. The Gish-Rooney (or Wen- 
ner) method is applied in testing the specimen, and the Gish-Rooney 
equipment as used in the field can be connected directly to this bridge. 
M. W. Pullen (ref. list No. I,) uses also direct and alternating cur- 
rent in his resistivity tester. The resistivities are measured by com- 
paring the resistance of the specimen with a known resistance, that 
is, by switching the current from one circuit containing this speci- 
men over to the other circuit containing the known resistance. The 
electrodes used were chiefly mercury pools, held in place upon the 
specimen by wax dams if the specimen had an irregular shape, or by 
a bakelite cup and a paper collar for drill cores. To eliminate surface 
leakage of current across the specimen, guard rings were applied. In 
using continuous current, the observed resistivity values were found 
to change with time on account of the variable polarization. There- 
fore, discontinuous current was found to be more satisfactory, which 
was obtained by shortening the regular 60-cycle A.C. from a light 
circuit through a potentiometer (for obtaining different test voltages) 
and using a copper oxide rectifier to make the use of a direct current 
galvanometer possible (Fig. 4 a). 
For large specimens of irregular shape, not only the Gish-Rooney 
four-electrode method may be used, but also two like electrodes of 
regular geometric shape may be employed, provided their distance 
is small as compared with the dimensions of the sample. The theory 
1 This resistivity bridge was developed largely after the bridge used by the Radiore 
Company. 
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