SUMMARY, 401 



more favorable conditions. The following summary is in Dr. Barus's own 

 words : 



Nature of the problem. — Tlie problcm offercd is not apparently a difficult one, 

 and consists simply in determining the variation of earth-potential at as 

 many points as may be desirable within and in the vicinity of the ore body; 

 or, in other words, in tracing the contour and position of the equipotential 

 surfaces. 



It is practicable, however, to systematize the method of research, a 

 priori. In the first place, Reich's hypothesis that lode-currents, if present, 

 are due to hydro-electric action is quite a safe and natural one. It is known 

 that a number of ores — especially sulphides — possess metallic properties. 

 The presence of two or more of these in the same ore body is not an 

 uncommon occurrence, and electric action at their surfaces of contact 

 may fairly be anticipated. The currents thus generated have a very 

 close analogy to those technically known as "local currents" in batteries, 

 which are due to impurities in the zinc. In the second place, it is obvious 

 that if currents are met with in a region of ore deposits, such currents must 

 be constant, both in intensity and direction, because electrical action has 

 been going on for an indefinite period of time. The equipotentials cor- 

 responding to this flow will, therefore, have fixed and definable positions 

 relatively to the ore body. 



Suppose, now, that from a point remote from the ore body a line has 

 been drawn towards it and prolonged beyond to about the same distance. 

 It is not necessary for the present purposes that this line should actually 

 pierce the deposit; but only that certain of its parts should be sufficiently 

 near ore, and more so than its extreme points, and that it should lie wholly 

 within or entirely upon the surface of the earth. Suppose, moreover, that 

 the ores are so associated as to generate electrical cun-ents. 



If, then, beginning at one end of the line the values of earth-potential 

 are determined at consecutive, approximately equidistant points, it is obvious, 

 inasmuch as the line passes by the seat of an electromotive force, or, in 

 other words, through the field of sensible electrical action, that progress from 

 one extremity of the line to the other must be accompanied by a passage 

 of the corresponding values of earth-potential, through a maximum or min- 

 26 c L 



