316 



OEOLOdY OF THE COMSTOOK LODE. 



tliroug-h any right section of the wire in the unit of time. Now 

 it is upon / that the deflection of a magnetic needle surrounded 

 by a coil of wire, the plane of the windings being vertical and 

 parallel to the needle, cai. par., depends; whence it follows, even 

 if the same arrangement of coil and needle were used throughout, 

 that the deflection just mentioned would contain an incidental 

 element; in other words, that it depends upon the means which 

 have been adopted to eft'ect the connection between the equipo- 

 tentials^^ and q. 



Returning to the problem in hand, it will be found that the 

 mere measurement of deflections would be of but little avail. An 

 efl'ort must be made to determine the values of p and q at the 

 points tapped by the ends of a wire. These quantities, more- 

 over, are particularly significant, insomuch as the potential at 

 any given point in the vicinity of the ore body depends princi- 

 pall}^ upon the character and distribution of the electrically active 

 ore-matter, and of the rock surrounding it, or wholly on con- 

 ditions fixed by nature. Hence, instead of seeking for the ore 

 body itself, an attempt will be made to add to the few clews 

 available to the prospector by investigating some characteristic 

 variation of the potential at consecutive, similarly disposed points, 

 as indicating jjroximity to it. But what has been said of p) and 

 q applies equally well to p) — g, which latter quantity is, moreover, 

 easily measurable, either directly (electrometrically, or by cer- 

 tain galvanometric methods) or indirectly, by the determination 

 of the magnitude of deflection of the needle described above, 

 under known conditions, p—q is technically called electromotive 

 force. 



To an observer the equipotentials are accessible for measure- 

 ment either on the surface or in those places where drifts pene- 

 trate them. Let a, v, r. Fig. 22, be a line lying either upon or 

 within the surface of the earth. Suppose the electromotive forces 

 be measured between a })oint a, and consecutive points /J, y, 6 

 . . . ju, V, a, . . . . o, r, v, . . . taken at convenient, approximately 

 equal, distances apart. The points ju, y, H . . . are supposed to 



