320 GEOLOGY OF THE COMSTOCK LODE. 



Steel plugs are therefore not greatly inferior to those of copper or zinc 

 in cases where a few hundredths of a volt are believed to be of minor im- 

 portance; whereas, on the other hand, their use for the purpose in view is 

 attended with much convenience. It was found, however, that great care 

 had to be taken in keeping them bright, as otherwise the electrical difference 

 between the gads themselves was apt to rise to many times the value given 

 above. It was also necessary to maintain a thorough contact between the 

 ends of the metallic circuit and the gads. 



Great difficulty was encountered in avoiding leaks in the copper wire 

 connecting the plugs with the galvanometer. At first wire covered with a 

 double thickness of cotton and waxed was employed, but proved to be 

 wholly inadequate. Even gutta-percha wire scarcely offered as complete 

 an insulation as was desired, in the hot and damp atmosphere of the Com- 

 STOCK, when laid in long lines without special precautions. After testing a 

 number of devices, it was finally found sufficient to suspend the wire from 

 silk or waxed cotton threads, care being taken to prevent it from anywhere 

 touching either rock or timbers. This plan of swinging the line was adhered 

 to throughout, in spite of the loss of time frequently occasioned thereby. In 

 shoi't, the rule was finally adopted of arranging all the connections just as 

 though the experiments contemplated were to be made with frictional elec- 

 tricity. 



The galvanometer used in these experiments was an ordinary instrument 

 with an astatic needle, capable of measuring intensities as small as 0.0001 

 in Weber's electromagnetic scale (mg. mm. sec.) with certainty. Readings 

 were made directly, the needle swinging over a graduated arc. 



For the measurement of electromotive forces a method of compensation 

 was first employed. But in the course of the investigation it was found 

 absolutely necessary to abandon all complications and to reduce the method 

 of research to the utmost simplicity. This will be evident to the reader 

 when he remembers that the heat of the mines is such as to cause pi'ofuse 

 persjiiration, and thus seriously interfere with manipulation; that it was 

 desirable to make the first observations near or on the vein — hence in the 

 busiest part of the mine — so that expeditious operation was extremely 



