26 Messrs. Wright and Thompson on the Determination of 
iron plates immersed in a strong solution of ferrous sulphate 
opposed to electro-copper plates in copper-sulphate solution. 
The values obtained show that, whilst with zince-copper, zinc- 
cadmium, cadmium-copper, zinc-silver, and copper-silver cells 
halving the area of the piate on which metal is deposited usually 
causes a greater decrease in the H.M.F. for a given rate of 
current-flow than is caused by halving the area of the other 
plate, the reverse is the case with iron-copper cells. Thus, 
for example, in one experiment the following numbers were 
obtained, and similarly in other cases :— ~ 
| Effect of halving area of the 
Current in micro- 
alperes. 
Tron plate. Copper plate. 
500 008 005 
1000 ‘010 ‘006 
2000 012 ‘008 
5U00 ‘O17 ‘O12 
10,000 "027 021 
With iron-copper-sulphate cells the rate at which the H.M.F, 
of the cell diminishes as the current-density increases is dis- 
tinctly more rapid than with cadmium-copper-sulphate cells, or 
with Daniell cells (zinc-copper-sulphate): thus, for instance: — 
Rate of current-flow 
in microamperes per Average E.M.F. Fall in E.M.F, 
square centimetre. 
0 711 
20 “708 ‘003 
40 698 013 
100 685 026 
200 “671 040 
400 "656 055 
1000 632 ‘079 
2000 “604 107 
It is noticeable that the substitution of dilute sulphuric acid 
for ferrous-sulphate solution in an iron-copper cell uniformly 
tends to lower the H.M.F., also rendering the cell-values less 
permanent and subject to wider ranges of fluctuation than 
before, probably on account of the “‘ local action ”’ of the acid 
on the iron; a similar result is also brought about by the 
substitution of dilute hydrochloric acid for ferrous-chloride 
solution in cells containing iron and other metals with chloride 
solutions. 
