140 
PROFESSOR DANIELL ON VOLTAIC COMBINATIONS. 
observations. The arrangement which I adopted was that of hollow cylinders of 
amalgamated zinc with platinum wire, and wire of amalgamated zinc with platinum 
cylinders, all of equal heights ; and the electrolyte in contact with the zinc consisted 
of standard dilute sulphuric acid, separated by a porous tube from strong nitric 
acid in contact with the platinum. A Breguet’s thermometer, adapted to the pur- 
poses of a galvanometer*, was selected as a measure of the effects. The following 
Table exhibits the results : — 
Table I. 
Diameter of Zinc. 
Diameter of Platinum. 
Degrees of Galvanometer. 
inches. 
2f 
wire 
O 
274 Mean of three observations. 
wire 
2f 
255 Mean of three observations. 
wire 
H 
279 
H 
wire 
273 
270 Mean. 
The needle always returned after each experiment to the point from which it 
started. There can be no difficulty in taking these results as sensibly equal ; and it is 
therefore evident that a wire of platinum placed within a cylinder of zinc, established 
a current of exactly the same force, as a wire of zinc placed within a cylinder of pla- 
tinum of equal diameter. Hence we may conclude that the area of the efficient 
section of the electrolyte is the mean of the opposed faces of the metal plates. 
But how shall we account for the contrary results with sulphate of copper as the 
electrolyte, which were carefully made and greatly varied, but which always gave a 
consistent result of about one-half the force when the conducting was very small in 
proportion to the generating surface ? 
By substituting acid sulphate of copper for nitric acid in the arrangement just 
described, and carefully attending to the progress of the experiment, the explanation 
became obvious. 
When the platinum wire was placed in the porous tube in the centre of the zinc 
cylinder, upon first connection with the calorific galvanometer, the needle advanced 
to 83° ; but almost immediately began to return till it reached 28°, at which point it 
remained stationary. When the wire was withdrawn it was found covered with 
copper in a spongy or pulverulent form. It was wiped and replaced, and the needle 
advanced to 85°, but immediately began to retrograde and fell again to 28°. This 
process was repeated many times, and always with the same result. If, while the 
galvanometer was at its highest point, the wire was moved about and the liquid 
kept in a state of agitation, the needle remained steady for a longer time ; but when 
the wire was left at rest, the needle always receded. 
* Philosophical Transactions, 1838, p. 42. This instrument is not absolutely to be depended upon when 
the power is high and the differences to be measured great, on account of the differences in its rates of cooling. 
