250 



Table 230 

 PRACTICAL THERMOELECTRIC SCALES 



(Comparisons) 

 (Adapted from Roeser, Bur. Standards Res. Paper 99, 1929, which see for details of use.) 



Prior to the adoption of the International Temperature Scale, the Pt-Ptio% Rh ther- 

 mocouple was almost universally used for scales 450 to noo°C, and defining equations 

 were quadratic or cubic depending upon the number of calibration points. 



The scale based on the work of Holborn and Day was calibrated at the freezing point of 

 Zn (419. o°C), Sb (630. 6°C) and Cu (io84.i°C) and a quadratic equation, E = a + vt + cP, 

 for interpolation. This was almost universally used from 1900-1909. Work of Waidner, 

 Burgess, 1909, and Day, Sosman, 1910-1912, necessitated a readjustment. In 1912 the 

 Bureau of Standards redefined its scale, assigning values determined with the resistance 

 thermometer to the Zn and Sb points, while the freezing point of Cu was taken as 1083. o°C. 

 This 1912 scale, used from 1912-1916, will be called the Zn,Sb,Cu temperature scale. 



A scale proposed by Sosman and revised by Adams (Journ. Amer. Chem. Soc, 36, 65, 

 1914) was realized by using a standard reference table, giving the average /.e.m.f. relation 

 for thermocouple used by Day and Sosman. A deviation curve, determined by any other 

 couple by calibration at several points would be plotted relating the difference between 

 observed e.m.f. and the e.m.f. from the reference table against the obs. e.m.f. of the couple. 

 This scale, although very convenient, is not completely defined and no comparison is made 

 here. 



In 1916, the Physikalische-Technische Reichsanstalt adopted a scale with the couple 

 calibrated at the Cd point (320.9°C), Sb (630°C), An (io63°C) and Pd (i557°C). No com- 

 parison will be made here. 



A scale adopted by the Bureau of Standards in 1916 was defined by calibration at the 

 Zn and Al points with a Cu point (1083. o°C). This was used from 1916-1926 and is here 

 designated the Zn,Al,Cu scale. 



The scale adopted by the P.T.R. and the Bureau of Standards in 1924 was calibrated 

 at Zn and Sb points (determined by resistance thermometer) and the Ag point (96o.5°C) 

 and the Au point (1063. o°C). It will be designated the Zn,Sb,Ag,Au scale. 



The 1927 7th Annual Conference of Weights and Measures (31 nations) unanimously 

 adopted what is between 66o° and io63°C the Zn,Sb,Ag,Cu scale with the Zn point omitted. 

 The table below shows a comparison of the various scales. The following values for the 

 freezing points were used : 



Zn 4i947°C Al 659.23^ Au io63.o°C 



Sb 630.52^ Ag 96o.5°C Cu (reducing atm. e ) io83.o°C 



Table 231 gives the corresponding difference of temperature. 



Comparison of t°C — e.m.f. relations with International Temperature Scale. (Comparisons 

 with two thermo-couples are given.) 



Smithsonian Tables 



