ELECTRICAL ACTIVITY OF ORE BODIES. 



353 



within the scope of a g-ood electrometer. Such ;in instruiuont, properly 

 protected against the moisture of the underground air, would have been 

 most serviceable for the purpose. Unfortunately, one could not be obtained 

 in time for the work. The following methods were therefore resorted to: 



In the first place the greater part of the data were checked by the 

 method already described. This, it will be remembered, was chosen because 

 of its simplicity and the comparative ease with which any fault in the con- 

 nections could be ascertained. 



The potential of the same holes was now measured by a method 

 in which the electromotive force is expressed in terms of the increment of 

 the reciprocal of intensity of current, and the corresponding increment of 

 the resistance of the circuit, to which the former is due. In order to vary 

 the resistance at pleasure a rheostat was introduced. If the resistancess w^ 

 and Wo correspond to the intensities /, and L, respectively. 



where e is the electromotive force to be measured. 



Finally, the whole of the experiments formerly made on the GOO-foot 

 level were again repeated by a zero 

 method. Here great care had to bo 

 taken to effect the complete insulation 

 of all parts. This was accom))lished 

 in the nuinner previously indicated, 

 by suspending the terminal wires, as 

 well as all the connections, from 

 threads. The accompanying diagram, 

 Fig. 31, will show how this was done. 

 A and B are clamp screws, suspended 

 from the threads a and d, respectively, 

 E (rheostat) is the lai-ge, r the small 

 resistance. A' a double key, Ca com- fh;. ■.u.— Disposition of iipparatns. 



mutator, G the galvanoscope. For a zero current in the latter (the effects 



