44 Messrs. Wright and Thompson on the Determination oj 
fitted with uniformly sized plates (8 square centim. total surface 
in each case) at an equal distance asunder, so that the resistance 
of the cell should be sensibly the same in each case. In the 
first cell the plates were of zinc (amalgamated) and platinum, 
and in the second of copper and platinum respectively; the 
two were arranged in series with a couple of Leclanche cells, 
so that the platinum plates were necessarily the — electrodes; 
a large variable resistance being included in the circuit, the 
current could be regulated at pleasure. A current of some 
fifty microamperes being sent through for three days, the 
readings became steady when all the dissolved air around the 
platinum plates was eliminated; the current was then varied 
from time to time, and a series of readings of the potential- 
difference between each pair of plates taken. By interpola- 
tion as before, the following figures were then deduced from 
the average values. 
Current in 
micro- 
amperes. 
Micro- 
amperes per 
square 
centim. 
Difference of potential. 
e 2 -e v 
+ zinc. 
+ copper. 
20 
40 
80 
2-5 
50 
100 
— •552 
-•558 
-•498 
+ •449 
+ •448 
+ •521 
+1-001 
+1-006 
+ 1-019 
The value of e 2 — e x consequently increases with the current- 
density as before. The numerical values observed in this expe- 
riment are somewhat lower than those found in the experi- 
ments above described, as might be expected, since the largest 
current-density employed in this case, being only 10 micro- 
amperes per square centim., is considerably below the smallest 
cited in the previous observations, in the last of which a mini- 
mum current of '00002 C.Gr.S. units (or 200 microamperes) 
was employed with plate-surfaces of 1*5 square centim., giving 
a density of 133'3 microamperes per square centim., in which 
case the value of e 2 — e x was T040: whilst in the former expe- 
riments a minimum current of -000005 C.G.S. unit (50 micro- 
amperes) was employed with a plate-surface of *50 square 
centim., giving a density of 100 microamperes per square 
centim., when values of from 1*054 to 1*065 were observed. 
145. Some experiments were also made with analogous 
pairs of cells in which the + electrodes were made of metals 
not attacked by the nascent products arising from the electro- 
lysis of sulphates, e. g. gold and platinum. In these instances 
it was found that platinum behaved in reference to gold just 
