418 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
ohms and through a resistance box. The difference in the potential at the terminals 
of this box was balanced in the usual way by the residual electromotive force of the 
two Clark cells. If e,, e,, E be the electromotive forces of the Clarks and the 
Leclanché respectively, and 7 the resistance of the box when there is a balance, 
APO Ore F 
When two Clarks which have approximately the same temperature are compared, 
7 would be equal to only a few ohms, and in that case we may neglect the 7 in the 
denominator, and in many cases take the electromotive force of the Leclanché equal to 
that of the Clark. Care was taken that the Leclanché cell had its normal electro- 
motive force, which was measured whenever any change in its value might have 
produced an appreciable difference in the experiments. 
Large cells can only be used as standards, if they are constructed according to the 
principle of Lord RayueicH’s H cell. In the spring of 1891 a cell was set up having 
the form of fig. 2. It consists of two glass vessels, one being placed inside the other. 
The outer vessel contains the zinc amalgam, the inner vessel the mercury. The vessel 
is covered by a wooden lid, through which the electrodes and a thermometer are 
passed. The upper part of the outer vessel is paraffined so as to prevent the creeping 
of the zinc sulphate over the sides. Very little evaporation takes place, and the lid 
is readily removed so that the cell may be refilled when necessary. It was constructed 
originally so that the liquid could be stirred, but it was found better not to disturb 
the cell. This cell has answered all requirements perfectly, and will be referred to as 
the “standard.” The cell, which is not readily moved owing to its weight, stands in 
a corner of a small cupboard, and was compared at intervals with the C.T.C. between 
November, 1891, and June, 1892. The results are given in Table I. The thermo- 
stat, which was not adapted for temperatures as low as 25° C., had to be readjusted 
occasionally, hence the small variation in temperature shown in the second column. 
The temperature of the standard at different times varied between 11°:2 C. and 
18°°3 C., and the experiments afford a good test as to the limits within which a 
temperature correction could be safely applied. 
