1822.] produced by Electricittj. 6 



time in making the connections, all interfere with their perfect 

 accuracy. 



The most remarkable general result that I obtained by these 

 researches, and which 1 shall mention first, as it influences all 

 the others, was, that the conducting power of metallic bodies varied 

 with the temperature^ and was lotver in some inverse ratio as the 

 temperature was higher. 



Thus a wire of platinum of -^ .^„-, and three inches in length, 

 when kept cool by oil, discharged the electricity of two bat- 

 teries, or of 20 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 ap- 

 plied to it from some other source, the effect was the same. 

 Thus a wire of platinum, of such length and diameter as to dis- 

 charge 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 abund- 

 ance 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 ceased as soon as the source of heat was withdrawn.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 apphed 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 converse 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 

 immediately 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 absolute temperature of the whole wire is diminished by 

 heating a part of it, and, vice versa, 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 -^pJ— discharged the whole of the electri- 

 city of 65 pair of plates of zinc and double copper made active 

 by a mixture of about one part of nitric acid of commerce, and 

 15 parts of water. Sixinches of copper wire of the same diame- 

 ter discharged the electricity of 56 pairs of the same combina- 

 tion, six inches of tin of the same diameter carried off that of 12 

 only, the same quantity of wire of platuium that of 11, and of 

 iron that of 9. Six inches of wire of lead of -i^-i^ seemed equal 

 in their conducting powers to the same length of copper wire of 



