53 
ments with single pairs of elements. (See tables appended 
at the end). The combinations which I have examined 
comprise more than are practically required, and are mostly 
useful for the purpose of showing what forms of battery 
will not be available. These experiments also point out 
that Ohm's law, expressed in the equation F = — , is not 
to be relied on in calculating the practical working of bat- 
teries, unless it be seen that another condition be complied 
with, that is, that the facility of chemical action be fully 
equivalent to the electrolitic action required to take place, 
(maintaining the constancy of the electro-motive force). 
Thus, as regards the action of solution of potash, as the 
electrolite in contact with the positive of metal, we have a 
very high electro-motive force, a moderate resistance, but 
a totally inefficient arrangement; because the solvent pro- 
perties of the potash on the positive metal are not capable 
of removing the oxide from the surface of the metal with 
sufficient rapidity to keep it in an efficient condition. In 
like manner in the other combinations, the action of the 
electrolite on some of the metals which are not very soluble 
in sulphuric acid, is not sufficient to maintain the electric 
action. 
A very slight consideration of the tables may show that 
both for economy and efficiency we should not use any other 
metal than zinc, when an active combination is required ; 
and even when a feeble but prolonged current is required, 
we cannot, with economy, substitute any metal for zinc, 
because we cannot, with iron, (the only other metal at all 
likely,) substitute a process equivalent to the amalgamation 
of the zinc, for the purpose of avoiding local action. 
The combination which we should first notice is zinc and 
copper, as in Wollaston's battery ; this, though apparently 
economical as employing a cheap negative metal, is one 
the least serviceable ; for even when the zinc is amalgamated, 
