PROFESSOR DANIELL ON VOLTAIC COMBINATIONS. 
155 
Let us however consider these results with reference to the formula. The expression 
for the arrangement used becomes 
// E — e — e' 
n R + r + r 1 ’’ 
in which e' signifies the contrary electromotive force, introduced by the accumulation 
of the ions on the plates of the voltameter containing the liquid tested, and r' the 
resistance offered by the same. It is clear that we cannot estimate r', which we are 
attempting, unless e' is known or remains constant : now e' is not constant, since 
with nitric acid it vanishes probably altogether, and varies with each of the other 
substances employed. 
If chloride of platinum were not too expensive to allow of its being employed as 
the exterior part of the electrolyte in contact with a platinum, conducting plate, e', 
or the contrary electromotive force would be wholly annihilated, as nothing but 
platinum would be thrown down upon the platinum, and it would constitute the 
most perfect possible arrangement, but would not much, if anything, exceed the effi- 
ciency of nitric acid. 
In the nitric acid battery the electromotive force is nearly double that of the sul- 
phate of copper arrangement, and consequently one cell of that construction is capa- 
ble of effecting the decomposition of dilute sulphuric acid. It is evident that a 
similar series of calculations might be made for this battery and others in which dif- 
ferent electrolytes are employed ; R varying as before with the size and distance of 
the plates. 
I have, however, done enough for the accomplishment of my present purpose ; but 
must not conclude without expressing my obligations to my friend Dr. Miller for 
his able assistance, both in the performance of the experiments and the calculations 
of the results which I have now the pleasure of communicating to you. 
I remain, my dear Faraday, 
Very faithfully yours, 
J. F. Daniell. 
Kings College , 
12 th April , 1842. 
