74 



TABLE 46.— SOME OLD THERMOELECTRIC TEMPERATURE SCALES* 



Comparisons 



Prior to the adoption of the 1927 International Temperature Scale, the Pt-Ptl0% Rh 

 thermocouple was almost universally used for scales 450° to 1100 C 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 (419.0°C), Sb (630.6°C), and Cu (1084.1°C), and a quadratic equation, E = a + vt + 

 ct 2 , 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 1083.0°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 was realized by using a standard 

 reference table, giving the average t-emi 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 emf and the emf from the 

 reference table against the obs. emf 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 Sd point (320.9°C), Sb (630°C), Au (1063°C), and Pd (1557°C). No 

 comparison 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 (1083.0°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), the Ag point (960. 5°C), 

 and the Au point (1063.0°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 660° and 1063° 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 : 



REFERENCE TABLES FOR THERMOCOUPLES » 



The emf developed by thermocouples of the same materials, even very carefully made, 

 differ slightly for the same temperature. It has been found convenient to compare the 

 emf of a couple being calibrated with that of a standard thermocouple of the same materials. 

 If the differences in emf's between the standard and the calibrated couple be plotted against 

 the temperature, the temperature for an observed emf can be read very accurately. 

 Reference tables for three types of thermocouples follow. 



* These values are now superseded by* the introduction of the 1948 International Temperature Scale 

 and are given for reference only. 



23 Taken from Nat. Bur. Standards Res. Papers RP 1080, RP 767, and RP 530. 



SMITHSONIAN PHYSICAL TABLES 



