328 GEOLOGY OF THE COMSTOOK LODE. 



reversed and the deflection thus doubled. All intensities (i and /) were 

 determined as a mean of five consecutive commutations — not that it was 

 desirable or necessary to increase the accuracy by such a process, but 

 because it appeared essential not to hurry the measurements and to test the 

 constancy of the current as appearing in the five data obtained Errors from 

 condensation of moisture on the commutator were avoided by excluding the 

 latter entirely from time to time, the measurements being made by simply 

 connecting the wires with clamp-screws.^ 



As a matter of especial importance it will be necessary to consider a 

 scheme of operations by which discrepancies due to extraneous causes can 

 be eliminated as completely as possible. In the experiments the following 

 order of observations was adopted and rigidly adhered to throughout: 



1. Measurement of the apparent intensity of the lode current (i'). 



2. The same, with the terminals exchanged (i"). 



3. Measurement of the current produced by the normal element and 

 lode conjointly (/). 



4. With the battery left in place the circuit is broken at the temporary 

 contact; no deflection nmst ensue {E supposed to be acting with the lode 

 electromotive force). 



If a mean of the intensities derived from the first and second opera- 

 tions [i = J(i" + ^')] be taken, the intensity of the current («) due to the 

 lode only will be obtained. That due to diff'erences in the amalgamated 

 zincs is thus eliminated. In b}' far the greater number of experiments three 

 exchanges were made, so that the first and third positions of the terminals 

 were identical. Analogously, then, 



=<^"+'^"> 



The fourth operation in this scheme insures the perfect insulation of 

 the circuit between the T. C. and the galvanometer. The part between 

 the latter and P. C. — the two being always placed in close proximity, this 



'The commutator used was made of wood boiled in linseed oil, and supported on three conical 

 feet of wood boiled in wax aud resin. The holes, moreover, were coated with a thick layer of w.ix 

 (see page :!54). Whole seta of observations had to be discarded on account of the iusutiicieut insiilatiuu 

 of an earlier appaiatus. 



