Chap. 10] ELECTRICAL METHODS 701 



cates of those occurring in nature. Model experiments with equipotential 

 lines and potential profiles are usually made in tanks filled with a weakly 

 electrolytic solution. Model ore bodies are generally made of some readily 

 available metal. Fujita^^ used a vacuum tube oscillator, line electrodes of 

 bare copper wire, a copper sulfate solution (33 mg copper per 100 cm^), 

 and phones (no amplifier). The secondary electrodes were made of glass 

 tubes filled with mercury in which a coiled platinum wire was immersed 

 and made contact with the surface of the solution. The model ore body 

 consisted of a copper sheet 5 mm thick and 100 mm square. 



Fujita's results may be summarized as follows: (1) The field between 

 the Une electrodes was uniform only in the central third portion. (2) For 

 a conductive body the maximum distortion was observed when its strike 

 was at right angles to the electrodes. (3) For a nonconductive body the 

 maximum distortion resulted when its strike was parallel with the line 

 electrodes. (4) The dip of a model ore body could be determined from 

 the difference in the disturbed area on either side of the suboutcrop. 

 (5) The position most favorable for indications was in the center between 

 the electrodes. (6) The greatest area of distortion was obtained when the 

 length of the line electrodes was five times the length of the ore body and 

 their distance three times. (7) The depth reached increased in direct 

 proportion to the length of the ore body, the detectable depth being about 

 63 per cent of the length. (8) Thickness of an ore body was effective only 

 up to a certain point; a saturation effect was soon reached. (9) Detecta- 

 bility increased distinctly with dip. 



Considerable experimental work on model ore bodies has been done by 

 Lundberg and Sundberg.^* Fig. 10-42 shows equipotential lines as traced 

 by the ordinary method (phones), in contrast with the in-phase equipo- 

 tential lines located with a compensator. Out-of-phase equipotential lines 

 appear to be of equal diagnostic value, as shown by GalP in some model 

 experiments (where, however, the out-of-phase potentials were introduced 

 artificially by feeding an out-of-phase current into the arrangement at 

 right angles to the main current). 



4. Distortions of equipotential lines due to power leads. Generator leads 

 often cause induction currents which are 90° out of phase with respect to 

 the currents produced by contact. These induction currents deflect the 

 equipotential lines from their regular position, and elliptical polarization 

 occurs. Since it is not possible to apply a correction for the effect of the 

 cable (in the ordinary procedure of mapping with amplifier and phones), 

 the leads are generally so laid out that the interference is at a minimum. 



»i Proc. World Eng. Congr. (Tokio, 1929), Part 5, Paper No. 436, pp. 143-281. 

 »» Beitr. angew. Geophys., 1(3), 298-361 (1931). 

 » J. Sci. Instr., 8(10), 311 (Oct., 1931). 



