16 Mr. J. N. Hearder on Electrical Conductivity. 
voltaic purposes; and after various trials I adopted the form 
which I have described in the Philosophical Magazine for May 
1857. The metals were all drawn into wires of the same size, 
and the same lengths were used in each experiment. 
The voltaic batteries which I used were formed upon the prin- 
ciple of Dr. Hare’s calorimotor and coil batteries. One modifi- 
cation consisted of a plate of zinc 6 inches wide and 6 feet in 
length, coiled with a similar plate of copper between its convolu- 
tions so as to maintain a sphere of half an inch between the cop- 
per and zine, the last coil of copper being made entirely to 
enclose the end of the zinc, so that the copper plate was about 
six inches longer than the zinc. Both surfaces of each metal 
were thus opposed to the action of the others. A second form 
consisted of a similar area of zinc and copper cut into plates of 
6 inches square, and fastened alternately in grooves in a wooden 
frame at a distance of half an inch from each other, the two end 
plates being copper. All the zinc plates were united on one side, 
and all the copper plates on the other, thus forming a single 
pair equal in surface and, as ascertained by experiment, equal in 
effect to the coil just described. These batteries were suspended 
over a wooden trough by counterbalancing weights, which ad- 
mitted of their being immersed either wholly or to any depth in 
the acid. 
The exciting fluid consisted, by measure, of sulphuric acid 1 
part, nitric acid 1 part, and water 120. Stout flexible wires 
proceeded from the battery to the thermo-electrometer, and the 
battery was plunged in the acid at each experiment and raised 
again as soon as it was concluded. The results which I shall 
have to detail are rather incomplete in their character, as they 
are merely the remains of some scattered memoranda, a great 
number of which were mislaid owing to the accident which some 
two or three years after deprived me of sight.. I am induced, 
however, to publish such as I have, since I cannot discover in 
my intercourse with electricians that the facts are even now 
generally known. I briefly alluded to these experiments on a 
former occasion (see paper “ On Induction Coil” in the Philo- 
sophical Magazine, May 1857, p. 332, note). 
I shall forbear to enter into the rationale of the phenomena, 
but allow the simple facts to be taken for as much as they are 
worth. 
In my first series of experiments I was met by the curious fact, 
that the order in which the metals were heated by the voltaic 
arrangements which I employed, was the reverse of that which 
took place with the Leyden discharge; that is to say, the best 
conductors were the most heated, and the worst the least, as will 
be seen by the following Table, in which the length of wire 
