410 On the magnetic Phenomena 
or of twenty double plates; but when suffered to be heated by 
exposure in the air, it barely discharged one battery. 
Whether the heat was occasioned by the electricity, or applied 
to it from some other source, the effect was the same. Thus, a 
wire of platinum, of such length and diameter as to discharge a 
combination without being considerably heated, when the flame 
of a spirit lamp was applied to it so as to make a part of it red 
hot, lost its power of discharging the whole electricity of the 
battery, as was shown by the disengagement of abundance of gas 
in the secondary circuit ; which disengagement ceased as soon as 
the source of heat was withdrawn. 
There are several modes of exhibiting this fact, so as to pro- 
duce effects which, till they are’ witnessed, must almost appear 
impossible. Thus, let a fine wire of platinum of four or five 
inches in length be placed in a Voltaic circuit, so that the elec- 
tricity passing through it may heat the whole of it to redness, 
and let the flame of a spirit lamp be applied to any part of it, so 
as to heat that part to whiteness, the rest of the wire will instantly 
become cooled below the point of visible ignition. For the con- 
verse of the experiment, let a piece of ice or a stream of cold 
air be applied to a part of the wire; the other parts will imme- 
diately become much hotter ; and from a red, will rise to a white 
heat. The quantity of electricity that can pass through that 
part of the wire submitted to the changes of temperature, is so 
much smaller when it is hot than when it is cold, that the abso- 
lute temperature of the whole wire is diminished by heating a 
part of it, and, vice versd, increased by cooling a part of it. 
In comparing the conducting powers of different metals, I 
found much greater differences than I had expected. ‘Thus, six 
inches of silver wire of 31, discharged the whole of the electri- 
city of sixty-five pair of plates of zinc and double copper made 
active by a mixture of about one part of nitric acid of com- 
merce, and fifteen parts of water. Six inches of copper wire 
of the same diameter discharged the electricity of fifty-six pairs 
of the same combination, six inches of tin of the same diameter 
carried off that of twelve only, the same quantity of wire of pla- 
tinum that of eleven, and of iron that of nine. Six inches of 
wire of lead of =4-5 seemed equal in their conducting powers to 
the same length of copper wire of 3,5. All the wires were kept 
as cool as possible by immersion in a basin of water *. 
I made a number of experiments of the same kind, but the 
results were never precisely alike, though they sometimes ap- 
* Water is so bad a conductor, that in experiments of this kind its effects 
may be neglected altogether, and these effects were equal in all the experi- 
ments. 
proached 
