CONVENIENT STANDARD CELL 
171 
solution of copper sulphate. The limbs were then sealed with 
paraffin, cork and sealing wax, care being taken to admit an air 
bubble to allow for the expansion of the liquid. 
All measurements of the electromotive force were made at 
both 18°±.02 and 25°±.02. The constant temperature bath was 
electrically heated and the temperature electrically controlled 
by means of the familiar contact toluol regulator and telegraph 
relay system. The temperatures were read on a certified ther- 
mometer, graduated in 0.10° and readable to 0.01°. 
A recently certified Clark cell was used as reference cell. It 
was placed in a deep glass beaker and carefully covered with 
cotton. Owing to its high temperature coefficient the beaker and 
the cell were immersed in the water of the bath. The exact 
temperature of the cell was read from a certified thermometer 
imbedded in the Clark cell and the electromotive force of the 
cell corrected for the temperature. 
No readings of the electromotive force were made until after 
the cells had come to temperature' equilibrium. This equilibrium 
was considered as established when readings, taken at half-hour 
intervals, were identical. In no case were readings taken within 
three hours of the time of immersion. 
The measurements of the electromotive force were made by 
means of a 5-dial Wolff potentiometer in connection with a sen- 
sitive galvanometer (type ‘‘H”, L. and N.). 
The temperature coefficients were calculated by means of the 
expression : 
dE Eij— E25 
dt Ei 8 • t 
where and E25 are the electromotive forces measured at 18° 
and 25°, respectively, and t is the temperature interval. No 
attempt was made to derive a more elaborate expression for the 
temperature coefficients. 
The results obtained are recorded in the following tables. 
Tables I to V show the results for five different cells. The ver- 
tical columns represent the successive measurements of different 
dates and the deviations of each from the mean in 0.00001 volt. 
Tables VI and YII give the records of two Clark and two 
Weston cells set up by Guthe and von Ende. These are added 
for comparison. Their measurements were made at 25°, and 
their results show greater deviations than have been found for 
the copper amalgam cells. Table YIII gives a summary of the 
results for the eight copper cells. 
