272 PROCEEDINGS OP THE AMERICAN ACADEMY. 



the junctions of a thermo-electric element. Suppose, for example, 

 that a plate of iron is coated on its two broad faces with a layer 

 of copper, and that copper wires lead from these copper sheets to a 

 galvanometer. If one copper side is exposed to water at t°, and the 

 other copper side to water at t'°, heat will flow through the whole 

 plate, and the difference in temperature of the two surfaces where 

 the iron joins the copper sheets will give rise to a thermo-electric cur- 

 rent, which may be made to give an accurate measure of the difference 

 of temperature of the two faces of the iron plate. No attempt need 

 be made to determine the difference in temperature of the outer sur- 

 faces of tlie copper sheets. This is an outline of the most distinctive 

 feature of the method now to be described. The chief obstacle has 

 been, and still is, the difficulty of getting a sufficiently even tem2:)era- 

 ture over the whole of each surface. 



Figure 1 shows with tolerable accuracy a vertical section through 

 the conducting disk and the apparatus most closely connected with it, 

 the parts being represented at one third their actual size. Certain 

 unessential changes have been made in the drawing, in order to illus- 

 trate as many features as possible in this one figure. 



(Sis the conducting disk of iron, or, rather, "open-hearth" boiler- 

 plate steel. The following information concerning its quality and 

 composition was furnished bj' INIr. Parker, of Parker, Field, and 

 Mitchell, Cambridgeport, who made the disk : — 



Tensile strength 62,300 lbs. per sq. in. 



Elastic limit 34,800 '' « 



Phosphorus .0035. 



Carbon .001. 



The analysis was made by the Central Iron and Steel Works of 

 Harrisburg. The metal is very soft and tough, as the composition 

 would indicate. 



The mean thickness of the disk, which was carefully measured at 

 many points, is 0.295 cm., the smallest thickness found being 0.290 

 cm., and the largest, 0.299 cm. The diameter of the disk is 9.80 cm. 



The parts c and c' are the coatings of copper deposited electro- 

 lytically upon the steel. The author hesitated at first to apply 

 copper in this way, fearing that galvanoplastic copper might be 

 seriou.cily different in thermo-electric quality from ordinary copper 

 wire, which must be attached to the coatings ; but all his own experi- 

 ence and that of Professor B. O. Peirce, who has had occasion to 



