ADVANCES IN OIL PROSPECTING 113 
a pickup coil coupled to the generator or the power leads; the object 
_ of this lead being to carry the reference e.m.f. and the generator phase 
to the point of observation. 
b. ORIGINAL GISH-ROONEY APPARATUS 
This apparatus has been described so often in the literature that 
it is hardly necessary to gointo more details. It contains a potentiom- 
eter for measuring the potential difference between the two potential 
electrodes, a milliammeter for measuring the current, and a double 

Fic. 1oa.—First model of CSM Gish-Rocney outfit, built after MSM model. Hand- 
cranked commutator box which contains also batteries, shown in center; milliammeter 
at left and potentiometer at right. 
commutator, the purpose of which is to pass current to the ground 
in alternating direction, yet pass the current and the potential to be 
measured always in the same direction through the meters. Figure 8 
shows a sectional view of the apparatus. 
c. MC COLLUM EARTH-CURRENT METER 
Although this instrument was developed altogether independently 
from the Gish-Rooney apparatus and for a different purpose, it is, 
nevertheless, of practically the same construction as the Gish-Rooney 
apparatus. It is shown in Figure g. It has a voltmeter and milliam- 
meter like the Gish-Rooney apparatus, and also a commutator. It 
differs somewhat from the Gish-Rooney as a number of ranges are 
443 
