ELECTRICAL ArTIVITY OF ORE BODIES. 327 



interior and tied. The cotton-covered wire used (a b in figure) was passed 

 through this loop, suspended by the other end of the thread. 



A case in which gutta-percha-covered wire trailed on the ground a 

 distance of about 1,000 feet, was made the subject of measurement. A leak 

 was quite perceptible; the insulation offered, however, 

 was about 1 ,000,000 ohms. 



In extending the line from point to point, accord- 

 ing to Reich's very convenient plan, the wire is wrapped 

 on a light wooden reel, but in such a way that the 

 inner end also remains accessible. The outer end being 

 in connection with the measuring apparatus, enough 

 wire is uncoiled to reach the desired hole, and a con- fig. 25.— suspension. 

 nection (contact-bag) between this and the inner end of the wire is then 

 made. In the damp atmosphere the reel soon became saturated with 

 moisture, and, in spite of the insulation of the wire, care had to be taken to 

 insulate the former also. 



Galvanometer. — For tlio mcasuremeut of intensity I was fortunate in secur- 

 ing a magnificent instrument, made for me after the Wiedemann pattern, by 

 Mr. Wm. Grunow, of New York. This instrument is exceedingly conve- 

 nient for the purpose, as by an adjustment of the coils the sensitiveness can 

 be varied over a very wide range. Readings were made with telescope, 



20 

 mirror, and scale. In the adjustment adopted currents as small as :^-^ 



webers could be detected with certainty. 



Measurement of electromotive force. The sluiple method of COnSeCUtlVO Sub- 



stitution for the measurement of electromotive forces! ^— -^y^ • ) — iii- 



somuch as while there were no reasons for abandoning it there were a great 

 many in its favor — was adopted here as on the Comstock. The coils of 

 Grunow's galvanometer could easily be so placed as to enable the observer to 

 measure with sufficient accuracy both the lode current and that due to the 

 latter and the normal electromotive force conjointly, without making any 

 change at the instrument or inserting auxiliary resistances. By means of an 

 inclosed mercury commutator the current in the galvanometer could be 



