Chap. 10] 



ELECTRICAL METHODS 



723 



basis. Fig. 10-58 shows the ratio of apparent 

 and true resistivity as a function of the ratio 

 of electrode separation and length of spread for 

 a polarized doublet. A distinct change in pjp 

 occurs. This is of interest because the potential 

 curve along the same line exhibits a smaller varia- 

 tion. 



D. Procedure; Equipment 



oj 0.2 03 0.4 Of as 



/ 



Fig. 10-58. Curve of 

 apparent resistivity over 

 polarized doublet (after 

 Hummel). 



For measurement of apparent resistivity, 

 widely different procedures, electrode arrange- 

 ments, and equipment are used. The latter fall into the following groups: 

 (1) D.C. commutator method (Wenner-Gish-Rooney) with separate meas- 

 urement of voltage and current; (2) Megger method, likewise with 

 commutator and with direct resistance measurement by dual-coil indicators; 

 (3) D.C. measurement with nonpolarizable electrodes; (4) A.C. method. 



1. D.C. commutator (Gish-Rooney) method. A scheme of the equipment 

 is given in Fig. 10-11. Ci and C2 are the external or current electrodes; 

 Pi and P2 the potential electrodes. Current is measured with a milliam- 

 meter between Ci and C2 , while the potential difference is observed be- 

 tween Pi and P2 on a potentiometer. A section of a double commutator 

 is interposed in each circuit so that the current flows in the same direction 

 through the measuring instruments while it is reversed periodically in the 

 ground (about sixteen times a second). Much care must be devoted to 

 the proper design of the commutator to insure correct and steady meter 

 readings. Inasmuch as it takes the current a certain amount of time to 

 build up to its equilibrium value (an effect which increases with increasing 

 electrode separation, see formula [10-45a]), the segments on the potenti- 

 ometer commutator are offset with respect to those on the current 

 commutator. 



Because of the insulating segments, the current read on the milliam- 

 meter is lower and hence the resistance is greater than its actual value. 

 This difference may be corrected out by determining the value of a known 

 resistance with commutator in motion and with commutator at rest (com- 

 mutator factor). Unsteadiness of the galvanometer needle of the poten- 

 tiometer can be avoided by placing a condenser of large capacity in series 

 with one of the potential leads. Leakage between high- and low-voltage 

 circuits in the instrument may be eliminated by the use of a grounded 

 guard ring between the two sections of the commutator. A customary 

 commutator design is shown in Fig. 10-59b. 



2. Megger method. The "Megger" (abbreviation for "megohmer") in- 

 strument was developed primarily for the use of power and communication 



