Chap. 10] ELECTRICAL METHODS 696 



the minima in the ampUtude curve can be determined with sufficient 

 accuracy for the mapping of equipdtential Hnes. The search electrodes 

 are usually copper-jacketed steel rods. For dry ground the Imperial Geo- 

 physical Experimental Survey found a "seK-watering" electrode helpful.^^ 

 Almost any kind of a two- or three-stage amplifier may be used for equi- 

 potential-line surveying, provided that it is light, small, and does not 

 depend on storage batteries for filament supply. Circuit diagrams and 

 descriptions will be found in the reports published by the I.G.E.S. In 

 conducting equipotential-line surveys, several parties may work concur- 

 rently. At least two parties can work in two sections of the area covered 

 by one electrode layout and more can be kept busy with two or more 

 simultaneous electrode arrangements, which may in many cases be sup- 

 plied from the same power source. Two men are required to map equi- 

 potential lines, which should be marked with stakes and be surveyed 

 with a compass (in areas where large declination anomalies are absent) 

 and a 100-foot tape. 



More quantitative survej^s of the potential field require A.C. bridge 

 arrangements. With one of these the voltage ratio and phase difference 

 of adjacent portions of ground are measured. Details are given in sec- 

 tion VI. Another type makes possible a determination of complex poten- 

 tials in reference to the potential and phase of the generator; it therefore 

 requires a "reference" lead from the bridge to the generator. The voltage 

 difference and its phase are measured directly or the potential is split up 

 into its in-phase and quadrature components; thus, in-phase and quadra- 

 ture equipotential lines may be mapped. In the compensator illustrated 

 in Fig. 10-38, the generator is coupled through a power transformer to the 

 primary electrodes and through a phase transformer to the reference lead. 

 The reference voltage is then supplied to the four coils of a variometer in 

 such a manner that two of them (Vi and Vz) are provided with current 

 in phase with the generator current whereas the other pair (F3 and F4), 

 coupled to the reference lead through an air-core transformer, receive 

 current in quadrature. The resultant field is picked up by the secondary 

 coil of the variometer whose position determines the phase of the reference 

 voltage that is taken off on the terminals of the potentiometer P. No 

 sound will be heard in the headphones if the ground potential is com- 

 pensated, in regard to amplitude (potentiometer adjustment) and phase 

 (variometer adjustment) by the reference potential. If this compensator 

 is used for the determination of electromagnetic fields in reference to am- 

 plitude and phase of the generator potential, a search coil is substituted 

 for the ground probes. Data obtained with this compensator msiy be 



' See Edge and Laby, op. cit., p. 265, Fig. 197. 



