74: Mr. A. Stansfield on some Improvements in 



r/F 



-j— to be found for a sufficiently large number of points. 



For rough purposes, these values are obtained by dividing 

 the difference between the observed E.M.F.'s at two adjacent 

 melting-points by the difference in the assumed temperatures, 

 and plotting the result at the mid-point. A more accurate 

 curve may be obtained by a graphic method which it is 

 unnecessary to describe here. 7j™ 



In fig. 6, the thermo-electric power or ^™ curve has 



been obtained in this manner from the curve representing 



the thermo-electromotive force E of couple No. 11. 



Small errors, either in the assumed temperatures of the 



melting-points, or in the observed values of the E.M.F. for 



these temperatures, are thrown into prominence by this 



method of plotting ; errors of a few tenths of a degree in the 



relative observations of the melting-points of tin, bismuth, or 



lead, or of one or two degrees in the melting-point of 



aluminium, make very distinct breaks in the curve, and 



although the observed values of E are all seen to lie closely 



c/E 

 on the E.M.F. curve, the calculated values of ^ show in 



some cases a wide divergence from the smooth mean curve. 

 Thus, for example, the observed melting-point for aluminium 

 must have been several degrees lower than the assumed 

 temperature, unless the curve can be admitted to have a 

 great dip at about 550°. Records of the solidification of the 

 aluminium employed did not show the steady freezing-tem- 

 perature which is characteristic of a pure metal, and the 

 author is inclined to attribute the low reading for aluminium 

 (see Table II., p. 82) to the impurity of the metal rather 

 than to irregularity of the indications of the thermo-couple. 



Theory of the Thermo- Couple. 



The practical value of any pyrometer depends largely on 

 the possibility of expressing, accurately and simply, the 

 relation between the reading of the pyrometer and the 

 temperature to which it is exposed. In the case of the 

 thermo-couple this has been attempted by many experimenters, 

 partly empirically, and partly from theoretical considerations. 



The simple thermo-couple HACB, consisting of two wires 

 A and B of different metals or alloys, has its junctions H 

 and C at temperatures t 2 and t y respectively ; four separate 

 E.M.F.'s are produced : (1) a true contact E.M.F. at H, and 

 (2) another at C, due to the difference in the nature of the 

 two wires (the Peltier E.M.F.), and (3 and 4) an E.M.F. dis- 



