ELECTRO-CHEMICAL EQUIVALENT OF SILVER, 
447 
much care was really necessary in their use to avoid polarisation. The accompanying 
Table IX. shows the variations of E.M.F. of Clark No. 6 on April 28, when very 
rudely treated. The other connexions remaining as usual, the poles of the cell were 
joined through a resistance-box, by means of which the cell could be short circuited 
with any external resistance from 0 to infinity. The numbers entered (such as 4094) 
are proportional to the difference of potential between the poles, being in fact the 
resistance between the points of derivation on the Leclanche circuit. It will be 
seen that in the course of a quarter of an hour the cell recovers, to within a few ten- 
thousandths of its value, from the effects of being short circuited for several minutes 
through such resistances as 1000 ohms. From the electromotive forces during the 
short circuiting it appears that the internal resistance is high, nearly as much as 300 
ohms. 
The manner in which the Clark cells have borne the tests applied to them justifies 
the hope that they may be found generally available as standards of E.M.F. But 
further experience is necessary as to the effect of various modes of preparation, and it 
is to be hoped that this may soon be forthcoming. As used by us, the process is so 
simple that no one need be deterred from setting up cells for himself. 
§ 32. Experiments on Daniell cells gave only a moderately good result. Raoult’s 
form was employed, in which the zinc and copper solutions are placed in separate 
beakers, the connexion being only through a Y-tube charged with zinc sulphate and 
tied over the ends with bladder. One electrode was of pure zinc amalgamated with 
pure mercury, and the other of copper freshly coated electrolytically. The zinc and 
copper solutions were both of sp. gr. IT. 
Table X. 
November 30, 
1883. 
December 3, 
1883. 
December 5, 
1883. 
December 11, 
1883. 
December 12, 
1883. 
Clark No. 1. . . 
Daniell .... 
1-0000 
•7702 
1-0000 
•7710 
LOOOO 
•7705 
1-0000 
•7698 
1-0000 
•7702 
The Daniell cell has of course to be charged freshly on each occasion, and is thus 
far less convenient in use than the Clark’s, which stand for months always ready for 
use. The temperature of the cells at the time of the comparisons tabulated was about 
16° C. 
Through the kindness of the inventor, we have had the opportunity of comparing 
some De La Plue cells with the Clark’s. The cells are of a somewhat modified con¬ 
struction, the atmospheric oxygen being excluded by a layer of paraffine oil. They 
were set up some days before the comparisons, and short-circuited for five minutes in 
order to start the chemical action. 
