Chap. 10] 



ELECTRICAL METHODS 



643 



Fig. 10-7. Clamp for rock resistivity 

 determinations (after Sundberg). 



being suitable for testing drill cores. 

 Drill cores may be dipped in solder 

 of low melting point to reduce con- 

 tact resistance. Specimens sus- 

 pected of being anisotropic should 

 be measured in more than one direc- 

 tion. In any event, it is a good 

 policy to reverse the specimen and 

 to make at least two determinations. 



Ejects of polarization are dij05cult to eliminate. In D.C. measurements the 

 direction of current should be reversed in as brief periods as possible and 

 high voltages should be used. Owing to the difficulties with direct cur- 

 rent, A.C. methods are now in more prevalent use. 



2. Continuous D.C. methods. For moderate requirements of accuracy, 



the direct reading ohm- 

 meters of radio and uni- 

 versal testing sets are 

 quite satisfactory; they 

 are conveniently carried 

 in the field for rapid 

 checks on unprepared 

 specimens of approxi- 

 mately regular shape. 

 For measurements on out- 

 crops, the Shepard earth 

 resistivity tester may be 

 apphed. This consists of 

 two long steel electrodes 

 with the ohmmeter and 

 self-contained battery 



Fig. 10-8. Substitution method of resistivity deter- 

 mination with mercury electrodes (after Pullen). 



mounted on one of the rods. The surface of the rod 

 acting as the cathode is greater than that of the anode, 

 in order to reduce polarization. As may be expected, 

 Wheatstone bridge methods are widely applied for resis- 

 tivity determinations by continuous' or commutated 

 D.C. and low-frequency A.C. Well suited for measure- 

 ment of high^ resistance is the bridge shown in Fig. 

 10-10. Rz is a comparison resistance of high order, 

 from 10,000 to 1,000,000 ohms; the ratio n/ri is adjusted 

 in steps up to 1:1000. Then Rx = Rz-n/rz . Resist- 

 ances up to 10 ohms may be measured with this bridge. 

 Likewise convenient for rock resistivity measurement 



Mercurif 



£uord Ring 



Fig. 10-9. Mer- 

 cury electrode 

 for drill cores 

 (after Pullen). 



