296 GEOLOGY OF THE COMSTOOK LODE. 



filled with petroleum. Before each observation this liquid was stirred. The 

 copper wires pass through the commutator B, and thence the one through 

 a double key K to the point h, the other through the galvanometer G-^ to 

 the point a, thus completing the first branch. The smaller resistance r 

 forms the second branch, also terminating at the points a and h. For the 

 convenient insertion of this resistance a number of small holes were bored 

 in a thick piece of wood and filled with mercury. The points a and h are 

 connected with the extreme holes of the series by means of strips of thick 

 copper foil. Finally, the terminals of a zinc sulphate Daniell E pass 

 through the commutator A, thence the one through the key K and directly 

 to 6, the other through a large rheostat R (from 1 to 10,0U0 ohms), and by 

 a thick wire, C, to a, completing the third branch. 

 When the current in G-^ is zero 



T T 



e-zE „ - , or, more simply, ^lE „, 



where e is the electromotive force at r, E at E in the figure; where, further- 

 more, B. is the resistance at i^, ;• at >• in the figure, and where r is negligible 

 in comparison with B. 



Having thus described the general method, it will be pertinent to men- 

 tion a few of the more important details. By means of K two circuits 

 conveying currents due to E and c, respectively, 

 are closed. It is, however, necessary that they 

 should be so closed as to act simultaneously (dif- 

 ferentially) on the galvanometer G-^\ for if the cur- 

 rent due to the electromotive force e were to act alone 

 Fig. •,>i.— Section of key. gerious disturbances might be the result. This can 

 be accomplished by the following simple contrivance in the construction of 

 the key. Fig. 21 gives a section through the line of mercury cups c(?. Fig. 

 '20. Pieces of thick copper wire, bent as shown, are fastened to a thin 

 piece of board, capable of revolving partially about a horizontal axis ^Jarallel 

 to the line c d In this way the pieces m and n can be dipped into the mer- 

 cury cups under their extremities or lifted out of them together. The board 

 is, moreover, provided with a spring so arranged as to keep m and n out of 

 the cups, and the circuit therefore remains open, unless closed by the ob- 



