First Immersion of the Plates—Their Number. 125 
freely transfers the electricity. Double coppers are, however, 
effective, mainly because they virtually double the acting sur- 
face of the zinc, or nearly so; for in a trough with single cop- 
per plates and the usual construction of cells, that surface of 
zine which is not opposed to a copper surface is thrown almost 
entirely out of voltaic action, yet the acid continues to act 
upon it and the metal is dissolved, producing very little more 
than local effect (947. 996.). But when by doubling the cop- 
per, that metal is opposed to the second surface of the zinc 
plate, then a great part of the action upon the latter is con- 
verted into transferable force, and thus the power of the 
trough as to quantity of electricity is highly exalted. 
1150. First immersion of the plates.—The great effect pro- 
duced at the first immersion of the plates, (apart from their 
being new or used (1146.),) I have attributed elsewhere to the 
unchanged condition of the acid in contact with the zinc plate 
(1003. 1037.): as the acid becomes neutralized, its exciting 
‘power is proportionably diminished. Hare’s form of trough 
secures much advantage of this kind, by mingling the liquid, 
and bringing what may be considered as a fresh surface of 
acid against the plates every time it is used immediately after 
a rest. 
1151. Number of plates*.—The most advantageous num- 
ber of plates in a battery used for chemical decomposition, 
depends almost entirely upon the resistance to be overcome at 
the place of action; but whatever that resistance may be, 
there is a certain number which is more ceconomical than 
either a greater or a less. Ten pairs of four-inch plates in a 
porcelain trough of the ordinary construction, acting in the 
volta-electrometer (1126.) upon dilute sulphuric acid of spec. 
gray. 1°314, gave an average consumption of 15:4 equivalents 
per plate, or 154 equivalents on the whole. Twenty pairs of 
the same plates, with the same acid, gave only a consump- 
tion of 5°5 per plate, or 110 equivalents upon the whole. 
When forty pairs of the same plates were used, the consump- 
tion was 3°54 equivalents per plate, or 141°6 upon the whole 
battery. Thus the consumption of zinc arranged as twenty 
plates was more advantageous than if arranged either as ten 
or as forty. 
1)52. Again, ten pairs of my four-inch plates (1129.) lost 
6°76 each, or the whole ten 67°6 equivalents of zinc, in effect- 
ing decomposition; whilst twenty pairs of the same plates, 
excited by the same acid, lost 3°7 equivalents each, or on the 
whole 74 equivalents. In other comparative experiments of 
numbers, ten pairs of the three-inch plates (1125.) lost 3°725, 
* Gay-Lussac and Thenard, Recherches Physico-chimiques, tom. i. p. 29. 
