Vol. 7, 1921 
CHEMISTRY: D. P. SMITH 
31 
with occasional regulation of the current, until the cathode wire had at- 
tained saturation, the time required being calculable from the data pre- 
viously obtained. 
After the electrolytic current had been interrupted by withdrawing the 
plug Si the resistance-measuring circuit was closed by throwing the 
switch 5 2 into position 2. The current, derived from the battery B 2 , 
which consisted of four large storage cells in parallel, was now regulated 
by means of the resistance box R 2 until the millammeter M 2 gave the read- 
ing sought. This was from 2 to 4 milliamperes, these values being se- 
lected upon the consideration that for the sake of constancy the current 
should be as small as the precision of the potentiometer would permit. 
£.51 
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$ 65 
60 
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1 — 
"P'42 
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1 
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Z 4 £ 8' 10' IT 14 IG IB' 20' 22' 24 26' 28 
Time in Minutes 
FIG. 2 
Dependence of resistance on current 
By means of the Pohl commutator P the potentiometer was now con- 
nected alternately to the potential leads, p and p f , coming from the ends 
of the cathode wire, and to those of the ten-ohm comparison resistance, 
readings of the fall of potential across the two resistances being taken 
several times in rapid succession, while the time was in each case noted, 
in order that the value for the ten-ohm might be calculated by interpola- 
tion for the exact moment at which that for the cathode wire was observed. 
This made it possible to eliminate the error which would otherwise have 
resulted from the gradual drift in the resistance of the cathode, and hence 
in the measuring current. 
A complete experiment consisted of three such series of observations 
as that which has just been described, obtained at intervals as short as 
possible. The first and last were taken with a measuring current of ap- 
