534 Royal Society of Edinburgh. 



copper to the platinum through the hot junction, and concluded that, 

 in the thermo-electric series 



Antimony, Iron, { £Eum, } Bismuth ' 



this platinum wire must, at ordinary temperatures, be between iron 

 and copper. He found that the platinum wire retained the same 

 properties after having been heated to redness in a spirit-lamp and 

 cooled again ; but with temperatures above some limit itself con- 

 siderably below that of boiling water, he found that the iron and pla- 

 tinum constituted a more powerful thermo-electric element than the 

 iron and copper ; and he verified that for such temperatures, in the 

 platinum and copper element the current was from the platinum to the 

 copper through the hot junction, and therefore that the copper now 

 lay between the iron and the platinum of the series, or in the position 

 in which other observers have generally found copper to lie with 

 reference to platinum. A second somewhat thinner platinum wire 

 was found to lie invariably on the negative side of copper, for all 

 temperatures above the freezing-point ; but a third, still thinner, 

 possessed the same property as the first, although in a less marked 

 degree, as the superior limit of the range of temperatures for which 

 it was positive towards copper was lower than in the case of the first 

 wire. By making an element of the first and third platinum wire, 

 it was found that the former was positive towards the latter, as was 

 to be expected. 



In conclusion, various objects of experimental research regarding 

 thermo-electric forces and currents are pointed out, and methods of 

 experimenting are suggested. It is pointed out that, failing direct 

 data, the absolute value of the electromotive force in an element of 

 copper and bismuth, with its two junctions kept at the temperatures 

 0° and 100° Cent., may be estimated indirectly from Pouillet's com- 

 parison of the strength of the current it sends through a copper 

 wire 20 metres long and 1 millimetre in diameter, with the strength 

 of a current decomposing water at an observed rate ; by means of 

 determinations by Weber, and of others, of the specific resistance of 

 copper and the electro-chemical equivalent of water, in absolute units. 

 The specific resistances of different specimens of copper having been 

 found to differ considerably from one another, it is impossible, with- 

 out experiments on the individual wire used by M. Pouillet, to deter- 

 mine with much accuracy the absolute resistance of his circuit ; but 

 the author has estimated it on the hypothesis that the specific resist- 

 ance of its substance is 2\ British units. Taking "02 as the electro- 

 chemical equivalent of water in British absolute units, the author has 

 thus found 16300 as the electromotive force of an element of cop- 

 per and bismuth, with the two junctions at 0° and 100° respectively. 

 About 154 of such elements would be required to produce the same 

 electromotive force as a single cell of Daniell's, if all the chemical 

 action in a Daniell's battery were electrically efficient A battery of 

 1000 copper and bismuth elements, with the two sets of junctions at 0° 

 and 100° Cent., employed to work a galvanic engine, if the resistance 



