24O TABLE 21 5 (continued) 



THE INTERNATIONAL TEMPERATURE SCALE 



Standard atmospheric pressure is defined as the pressure due to a column of 

 mercury 760 mm high, having a mass of 13.5951 g/cm 3 , subject to a gravita- 

 tional acceleration of 980.665 cm/sec. 2 and is equal to 1,013,250 dynes/cm 2 . 



It is an essential feature of a practical scale of temperature that definite 

 numerical values shall be assigned to such fixed points as are chosen. It should 

 be noted, however, that the last decimal place given for each of the values in 

 the table is significant only as regards the degree of reproducibility of that fixed 

 point on the International Temperature Scale. It is not to be understood that 

 the values are necessarily known on the Thermodynamic Centigrade Scale to 

 the corresponding degree of accuracy. 



The means available for interpolation lead to a division of the scale into 

 four parts. 



(a) From the ice point to 66o° C the temperature t is deduced from the 

 resistance R t of a standard platinum resistance thermometer by means of the 

 formula 



R t = R (i+A t +Bt°-) 



The constants R , A, and B of this formula are to be determined by calibration 

 at the ice, steam, and sulphur points, respectively. 



The purity and physical condition of the platinum of which the thermometer 

 is made should be such that the ratio R t /Ro shall not be less than 1.390 for 

 t=ioo° and 2.645 f° r ^ = 444-6°. 



(b) From — 190 ° to the ice point, the temperature / is deduced from the 

 resistance R t of a standard platinum resistance thermometer by means of the 

 formula 



R t = R [i+At + Bt 2 + C(t-ioo)t s ] 



The constants R , A, and B are to be determined as specified above, and the 

 additional constant C is determined by calibration at the oxygen point. 



The standard thermometer for use below o° C must, in addition, have a ratio 

 Rt/Ro less than 0.250 for t— — 183°. 



(c) From 660° C to the gold point, the temperature t is deduced from the 

 electromotive force e of a standard platinum v. platinum-rhodium thermo- 

 couple, one junction of which is kept at a constant temperature of 0° C while 

 the other is at the temperature t defined by the formula 



e = a +bt + ct 2 



The constants a, b, and c are to be determined by calibration at the freezing 

 point of antimony, and at the silver and gold points. 



(d) Above the gold point the temperature t is determined by means of the 

 ratio of the intensity J 2 of monochromatic visible radiation of wave length 

 A cm, emitted by a black body at the temperature t 2 , to the intensity A of radia- 

 tion of the same wave length emitted by a black body at the gold point, by 

 means of the formula 



, h c 2 



A a L 1,336 (/+273)J 



The constant c 2 is taken as 1.432 cm degrees. The equation is valid if A(£ + 273) 

 is less than 0.3 cm degrees. 



Smithsonian Tables 



