176 0} HY GIsH 
ditions, no evidence of errors from this source has ever been found 
when the ‘“‘guard”’ was in use. 
The foregoing rather technical remarks apply when a commutator 
is used in the resistivity measurements. Resistivity surveys have, 
however, been carried out without employing a commutator but by 
using, instead, simple hand-operated switches. The errors which may 
enter into measurements made by such a method include those which 
have been discussed for the case where a commutatpr is used. Of 
these, that type which arises from the various extraneous potentials 
which act upon the pick-up electrodes is likely to be much more 
serious with this method than with the commutator method. This is 
partly because, even with the best technique, the time required for 
a single setting of the potentiometer and the interval between read- 
ings for reversed current are both so long in comparison with the cor- 
responding cycle in the commutator method that the error in the 
result is likely to be considerably greater on this account alone. On 
the other hand, a single reading obtained by the use of a commutator 
is effectively a mean taken over a considerable number of cycles and 
on this account is of enhanced accuracy in cases where errors of a 
random nature may be appreciable. In the simple switching method, 
errors due to defective insulation may be controlled by the design 
and arrangement of the switches, whereas induction effects will de- 
pend upon the technique of measurement. As far as the last two 
sources are concerned, the results obtained with the simple method 
should be better than those obtained with an unsuitable commutator. 
The writer is, however, of the opinion that if the possibilities of the 
direct-current resistivity method are to be fully realized, suitable 
mechanical means of rapid commutation are required. 
The search for oil often requires exploration to greater depths than 
was required in the earlier applications of geoelectric methods, 
namely, in the search for ores. Views as to the capability of geoelectric 
methods for such deep exploration vary, whether these be based upon 
theory or practice. Obviously the details of structure will become less 
distinct the greater the depth from the surface. Thus, a feature of the 
hidden structure can be detected at the surface only when it is of 
sufficient extent. The necessary relation between depth and extent is 
dependent upon the accuracy of the measurements and upon the mag- 
nitude of the uneliminated influences of the disturbing features of the 
earth. The accuracy of the measurements can be controlled by suit- 
able design and operation of the measuring apparatus. The errors 
506 
