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EXPLORATION GEOPHYSICS 



Field work of this type is relatively slow, both as regards tracing out 

 the points of equipotential, and the time required for surveying-in the 

 equipotential stations with their irregular locations. However, if the field 

 work is carefully done, the method often is more accurate than that 

 described in the following- section. 



Measurements of Earth Potentials at Regularly Spaced Intervals. — 



The equipment required for the direct measurement of potentials is some- 

 what more involved than that described 

 for locating points of equipotential. It 

 comprises a potentiometer, two non- 

 polarizing electrodes, and a long insul- 

 ated flexible connecting wire which is 

 wound on a reel to facilitate handling. 

 A photographic view of the complete 

 equipment and operators is shown in 

 Figure 267. 



The direct current potentiometer 

 should have a range of to 1000 milli- 

 volts, A double - pole double - throw 

 switch is provided for reversing the 

 input. The entire apparatus should be 

 mounted in a hardwood or metal water- 

 proofed box equipped with suitable 

 carrying straps. A short, sturdy tripod 

 for the potentiometer and a camp stool 

 for the operator facilitates the field 

 work. 



Direct Current Potentiometer. — 



The potentiometer is a basic instrument 

 used in many types of quantitative 

 direct current measurements. The 

 theory of the potentiometer is given 

 in any standard text on general physics 

 or electricity. Constant potentials of 

 any value within the range of the in- 

 strument may be obtained by proper 

 combination of a dial or drum setting, 

 which controls a variable resistor and 

 a tap switch. Photographic views of 

 the exterior and interior of a potentiometer are shown in Figure 268. 



Fig. 268. — Interior and exterior views of 

 potentiometer showing: a, control dial; b, 

 calibrated scale; c, variable and d, fixed re- 

 sistors; and e, standard cell. 



Field Procedure for Potential Measurements 



The measurements are conducted to determine the relative potential 

 and polarity between certain pre-selected stations in the area. Usually, the 



