ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
449 
§ 34. In order to obtain the balance of electromotive forces two distinct methods 
have been followed. In the earlier determinations there was no electromotive force 
in the derived branch except that of the standard cell, and the adjustment was effected 
bj variation of a comparatively high auxiliary resistance from a box, placed in multiple 
arc with the [4], The readings were taken by reversal of the galvanometer con¬ 
nexions at a mercury commutator, and the small outstanding galvanometer displacement 
was allowed for with the aid of observations of the effect of a known change in the 
auxiliary resistance. In this way could be determined the auxiliary resistance, and 
lorn it (by addition of conductivities) the effective resistance between the points of 
derivation necessary for a balance with the actual current. The value of the current 
at the moment in question is deduced from the curves representing the two sets of 
current-weighings (§ 24). In the course of half an hour several almost independent 
determinations of the electromotive force could be completed. 
This method is the simplest, and could usually be made to work satisfactorily. It 
is, however, open to the objection that if the current changes rapidly we must either 
allow for a considerable galvanometer displacement or else alter the auxiliary resist¬ 
ance. But the latter change reacts upon the principal current, and renders the 
current weighing curves discontinuous, thereby increasing the difficulty of specifying 
the value of the current at the moment of observation. 
§ 35. In the second method the resistance between the points of derivation is the 
[4] simply, and compensation is made in the galvanometer branch by the introduction 
of a graduated E.M.E. (Plate 17 fig. 1). The arrangement is in fact almost the same as 
in the comparison of two cells by the method of difference (§ 28), one of the cells being 
replaced by the resistance [4] traversed by the main current. As the apparatus for 
these comparisons was always ready for use, this method was, under the circumstances 
of the case, really more convenient than the other, and was employed in the later 
determinations. The procedure will be best understood from an example. 
On March 29, 1884, determinations of silver and of electromotive force were made 
simultaneously, so that the same set of current weighings might serve for both pur¬ 
poses. Accordingly the main current traversed the three voltameters, the current 
weighing apparatus and the resistance [4]. In the derived branch (Plate 17, fig. 1) 
were the standard cell No. 4 Clark, the galvanometer with its commutator, and 
coils from a resistance box, through which passed the current from the two Leclanche 
cells (§ 28). If the compensation between the Clark and the difference of potentials 
at the terminals of the [4] were incomplete the balance could be restored by the intro¬ 
duction of a graduated part of the E.M.F. of the Leclanche’s, the value of which, in 
terms of the Clark, is found by a subsequent experiment, in which the [4] is excluded. 
It will be understood that the Leclanche’s worked in a perfectly constant manner, 
the whole resistance in circuit being always made up to 10.000 ohms (in addition to 
that of the cells themselves). If E be the E.M.E. of the Clark, p the resistance 
(traversed by the current of the Leclanche’s) which must be used to get a balance 
MDCCCLXXXIV. 3 M 
