70 TABLES 37-51.— CONSTANTS FOR TEMPERATURE 



MEASUREMENT 



TABLE 37.— THE INTERNATIONAL TEMPERATURE SCALE OF 1948 20 



The International Temperature Scale that was adopted in 1927 was revised during 1948 

 and is designed to conform as nearly as practicable to the thermodynamic Celsius 21 (Centi- 

 grade) scale as now known. This 1948 International Temperature Scale incorporates certain 

 refinements based on experience to make it more uniform and reproducible than its pre- 

 decessor. The new scale is essentially the same as the one it displaces, but it was improved 

 by changing certain formulas and values for temperatures and constants. 



Only three of the revisions in the definition of the scale result in appreciable changes in 

 the numerical values assigned to measured temperatures. The change in the value for the 

 silver point from 960.5°C to 960.8°C changes temperatures measured with the standard 

 thermocouple. The adoption of a different value for the radiation constant Ci changes all 

 temperatures above the gold point, while the use of the Planck radiation formula instead 

 of the Wien formula affects the very high temperatures. (See Table 40 for the magnitude 

 of the changes due to these two causes for high temperatures.) The 1948 temperature scale, 

 like the 1927 scale, is based upon six fixed points (Table 38) and upon specified formulas 

 for the relations between temperature and the indications of the instruments calibrated at 

 these fixed points. Temperature on the 1948 scale will be designated as °C, or °C (Int. 

 1948) and denoted by the symbol t. 



The means available for interpolation between the fixed points lead to a division of the 

 scale into four parts : 



(a) From 0°C to the freezing points of antimony the temperature t is defined by the 

 formula 



R, = R (\ +At + Bt 2 ) 



where Rt is the resistance, at temperature /, of a standard platinum resistance ther- 

 mometer. 



(b) From the oxygen point (Table 38) to 0°C the temperature t is similarly defined 

 by the formula 



R, = /?.[1 + At + Bf + CO — 100)/ 8 ] 



(c) From the freezing point of antimony to the gold point (Table 38) the temperature t 

 is defined by the formula 



E = a + bt + ct 2 , 



where E is the electromotive force of a standard thermocouple of platinum and platinum- 

 rhodium alloy, when one junction is at 0°C and the other at temperature t. 



Recommendations are given for the construction, calibration, and use of these two types 

 of measuring devices. 



(d) Above the gold point the temperature / is defined by the formula 



/« _ exp[c 3 /(X(<A U -|-7,))] — 1* 

 /a» expk,/(X(f + r.))] — 1 



where /« and /* u are the radiant energies per unit wavelength interval at wavelength X, 



emitted per unit time by unit area of a blackbody at temperature t, and at the gold point 



/ao, respectively. 



c 3 is 1.438 cm degrees. 



To is the temperature of the ice point in °K. 



X is a wavelength of the visible spectrum. 



e i» the base of Naperian logarithms. 



Secondary fixed points. — In addition to the six fundamental and primary fixed points 

 (Table 38), a number of secondary fixed points are available and may be useful for various 

 purposes. Some of the more constant and reproducible of these fixed points and their 

 temperatures on the International Temperature Scale of 1948 are listed in Table 41. The 

 relation between this new temperature scale and the thermodynamic Celsius scale is 

 discussed in this paper also. 



The resulting changes in the 1927 'International Temperature Scale below the gold point 

 (1063°C) to correct it to the 1948 International Temperature Scale are given in Table 39. 



The use of the Planck formula and a wavelength interval within the visible spectrum to 

 determine temperatures presupposes the use of an optical pyrometer. (See Table 77.) 



80 Nat. Bur. Standards Journ. Res., vol. 42, p. 209, 1949. 



"The General Conference, held in October 1948, decided to discontinue the use of the words "Cen- 

 tesimal" and "Centigrade" and to replace them by "Celsius." See also Nat. Bur. Standards Techn. 

 News Bull., vol. 33, p. 110, 1949. 



* See footnote 5a, p. 7. 



SMITHSONIAN PHYSICAL TABLES 



