PROFESSOR DANIELL ON VOLTAIC COMBINATIONS. 
151 
halving the length of the zinc ; and the resistance R presented by that cell, varying 
directly as the distance, will be 1^ instead of one. 
The formula, therefore, for 20 small cells with 19 zinc rods of the ordinary size, 
and 1 short one, becomes 
instead of 
20 - 2-49 17*51 
19 + 1-33 + 0-541 — 20-871 
= 20-97 cubic inches 
20 - 2-49 _ 17-51 
20 + 0-541 — 20541 
= 2T31 cubic inches. 
when the zincs are all of the full length. 
Thus with a small series of five cells with entire rods 
5 - 2-49 2-51 
5 + 0-541 — 5-541 
with four entire rods and one half rod, 
1T33 cubic inches ; 
5 - 2-49 2-51 
4 + 1-33 + 0-541 — 5-871 
10 69 cubic inches ; 
the differences not being appreciable in the usual mode of measurement. 
In an arrangement containing one or more reversed cells, the formula becomes 
(n — n') E — e . 
(, n + n') R + r 
where n! represents the number of reversed cells. 
My fifth letter contains the results of experiments with the battery under such 
circumstances. Upon comparing these results with those of the formula just given, 
the discrepancies were found to be constant and considerable ; such however as might 
be accounted for by supposing that each reversed cell introduced, in addition to the 
reversed current, an extra resistance. Upon searching for this resistance it was 
proved to exist, and its cause manifested by the following experiments. 
Ten small cells, charged in the usual manner, were arranged in series, including a 
voltameter, and an additional cell with a similar charge ; substituting a copper rod 
in the interior for one of zinc. 
In four minutes seven cubic inches of gas were collected in the voltameter ; the 
action then suddenly declined in intensity, and in the next four minutes only 3^ cubic 
inches of gas were given off. Upon examining the copper rod it was found coated 
with a film of oxide : this was wiped off, and a bright metallic surface was again ex- 
posed, and on once more connecting the battery, seven cubic inches of gas were given 
off, as before, in four minutes. The quantity again declined. This was several times 
repeated, and always with the same general results. It is hence evident, that 
when copper is made the zincode in the series, a layer of oxide is deposited upon it, 
which is not immediately dissolved by the acid, and offers a resistance which will 
vary according to its thickness ; and this again will much depend upon the size of 
the surface. 
