618 Prof. R. A. Lehfeldt on the Electrochemical 
on the side of too much current, sometimes too little. The 
galvanometer-key is kept down by a weight throughout. 
Shortly before the close of the necessary period the 
galvanometer-key is raised, tap of the measure-tube closed, 
so as to leave a slight excess pressure in the apparatus ; the 
current is then stopped, and all temperatures taken again. 
The apparatus is left for a few minutes till the coulometer 
has regained its initial temperature, to within a degree or so ; 
mercury is then run out till the oil -gauge is level, the 
temperature of the water-jacket taken, the barometer read r 
and the mercury weighed. 
Discussion of accuracy attainable. 
1. Electromotive Force. 
The cadmium cells used as a standard were made in the 
laboratory ; the set of four on which most dependence was 
placed being of date August 1907. Of these, three were 
made with mercurous sulphate prepared electrolytically by 
Carharfc's method ; the other with sulphate bought from 
Merck : there was no appreciable difference between the 
two makes, however. The cells were all made with cathodes 
of mercury deposited electrolytically on platinum, and in the 
usual H form. They w T ere kept side by side in a metal water- 
bath, with a thermometer. Comparison of the cells, made 
from time to time, showed hardly any systematic differences. 
Such differences as were noted, averaging three or four 
hundredths of a millivolt, appeared to be due to temperature 
variations — despite the water-bath : this would hardly be the 
case in most laboratories, but the extreme temperature 
fluctuations in a temporary corrugated-iron building in such 
a climate as Johannesburg, constituted the greatest difficulty 
in the way of accurate measurement; the temperature w T ould 
sometimes vary from 8° to 22° C. in the course of the 
morning. The discrepancies between the cells do not really 
matter, how r ever, as no combination of them w r ould differ by 
so much as oqoqo from another. As to the absolute values,, 
though the cells have not been compared with any in 
Europe, experience elsewhere seems to show that the 
cadmium standard is quite reproducible to one part in ten 
thousand. 
The value used is that given in a private communication 
from Mr. F. E. Smith, of the National Physical Laboratory, 
and based on experiments with the ampere balance : — 
E, = l-01830-0-000034 5 (*-17) -0*00000066 (*-17) 2 . 
