ADVANCES IN OIL PROSPECTING seer 
a. GENERAL: POWER SOURCES 
As a source of power, one Radio B battery, or two, may be suff- 
cient, depending on the length of the current basis, of course. If very 
great depth penetration is required, a D.C. generator is used, driven 
by a gasoline engine. When using a Megger, a hand-driven D.C. 
generator is used, which is incorporated in the meter box. Due to the 
fact that polarization must be eliminated, if non-polarized electrodes 
are not used, a commutator is provided in the measuring circuit, 
which brings it about that in reality the current used is not strictly 
-----Double 

Sa) Commutator 
sf ©) 
co 
/ 
Potentiometer 
Milhammerer 
Electrodes making contact with Earth. 
i Golicoaee 
, > * — Fa 
4 Pp is 5 Shy 
/ i, , SUF SNL 
Cs.“ ----------@ —-------- ak! ---- Seep sacs el ee @---~yol} © 
TR TRITIRITT— ~~"" SITTING TIA ISSIR * SISTISITD APS — ~~ TRPEGT IAA 
~ Seen S Beg RoR 5 Soe AEC Ree | ALCREL Ce GES 
E@ of oth 
Fic. 8.—Schematic diagram of Gish-Rooney circuit. 
D.C., but an alternating current of low frequency. Induction effects 
are, therefore, sometimes observed between the current and the 
potential leads; besides, not the ohmic resistance of the ground is 
obtained, but the impedance for the particular frequency employed, 
which for the commutators in Gish-Rooney outfits is approximately 
16, and nearly 50 in Meggers. 
The only way to avoid polarization and induction effects is by 
using non-polarized electrodes, which consist of cups filled with copper 
sulphate and provided with a permeable bottom, made either of 
porous clay or of wood. 
The induction between leads is minimized when using the po- 
tential measuring line at right angles to the current basis, as already 
described. 
441 
