Chap. 10] 



ELECTRICAL METHODS 



726 



than the resistivities determined with either the Gish-Rooney commutator 

 or porous-pot resistivity equipment. 



3. D.C. resistivity measurements with porous pots. By the use of porous 

 pots, polarization on the potential electrodes and -hence the commutator 

 can be eliminated, although it is still necessary to provide for a reversing 

 switch to eliminate effects of stray currents and leaks from the current into 



P.C. Generator 



Ohms 



Cumufatar 



Jame* Q. Biddlt Co. 

 Fig. 10-60. Schematic Megger circuit. 



<d> 



<E> 



Ph 



Fig. 10-61. Measurement of apparent resistivity with four-terminal A.C. 

 method (adapted from Wenner). 



the potential circuit. The Shepard earth resistivity meter and the Lee 

 "geoscope" are representative of instruments using porous-pot electrodes. 

 In the latter the galvanometer of the potentiometer is used with resistances 

 in such a manner that the current is kept adjusted to a predetermined 

 value. This value is made numerically equal to 2ira, (= 191, if a is in 

 feet) so that the potentiometer readings give directly the value of the 

 apparent resistivity. 



