PEIliCE AND WILLSON. THERMAL CONDUCTIVITIES. 



45 



of the purest obtainable copper of equal cross section. Our finest wire 

 thermal elements, less than one tentli of a millimeter in diameter, were 

 so skilfully made by Mr. Sven Nelson, of Cambridge, that the joint was 

 hardly perceptible. Our German silver and copper ribbon thermal ele- 

 ments, about one eighth of a millimeter thick, were made by Mr. T. W. 

 Gleeson of Boston. These last were first soldered with the help of a 

 holder constructed for the purpose, and the joint was then rolled or 

 scraped until it was as nearly as might be of the same thickness as the 

 adjacent nietal. 



For wire thermal elements we had large quantities of three kinds of 

 platinoid, approximately 0.74, 0.30, and 0.097 mm. in diameter. The 

 first two specimens were obtained about ten years ago from Messrs. 

 Elliott Brothers, and have been thoroughly seasoned. Each is thermo- 

 electrically pretty definite, though the two are quite different in their 

 properties. The electromotive force, in microvolts, of [)latinoid and 

 electrolytically deposited copper elements made of these wires may be 

 tabulated as follows for low temperatures. 



Besides platinoid we have used with copper for wire thermal elements 

 two kinds of German silver wire respectively about 0.1 mm. and about 



