Table 21 6 241 



THE INTERNATIONAL TEMPERATURE SCALE 



Recommended Procedure for Calibration 



i. Oxygen 



The temperature of equilibrium of liquid and gaseous oxygen has been best 

 realized experimentally by the static method, the oxygen vapor-pressure ther- 

 mometer being compared with the thermometer to be standardized in a suitable 

 low temperature bath. 



2. Ice 



The temperature of melting ice is realized experimentally as the temperature 

 at which pure, finely divided ice is in equilibrium with pure, air-saturated water 

 under standard atmospheric pressure. The effect of increased pressure is to 

 lower the freezing point to the extent of 0.007 C per atmosphere. 



3. Steam 



The temperature of condensing water vapor is realized experimentally by the 

 use of a hypsometer so constructed as to avoid superheat of the vapor around 

 the thermometer, or contamination with air or other impurities. If the desired 

 conditions have been attained, the observed temperature should be independent 

 of the rate of heat supply to the boiler, except as this may affect the pressure 

 within the hypsometer, and of the length of time the hypsometer has been in 

 operation. 



4. Sulphur 



For the purpose of standardizing resistance thermometers, the temperature 

 of condensing sulphur vapor is realized by adherence to the following speci- 

 fications relating to boiling apparatus, purity of sulphur, radiation shield, and 

 procedure. 



The boiling-tube is of glass, fused silica, or similar material, and has an 

 internal diameter of not less than 4 nor more than 6 cm. The vapor column 

 must be sufficiently long that the bottom of the radiation shield is not less than 

 6 cm above the free liquid surface and its top is not less than 2 cm below 

 the top of the heat insulating material surrounding the tube. Electric heating is 

 preferable, although gas may be used, but the source of heat and all good con- 

 ducting material in contact with it must terminate at least 4 cm below the free 

 surface of the liquid sulphur. Above the source of heat the tube is surrounded 

 with insulating material. Any device used to close the end of the tube must 

 allow a free opening for equalization of pressure. 



The sulphur should contain not over 0.02 per cent of impurities. Selenium is 

 the impurity most likely to be present in quantities sufficient to affect the tem- 

 perature of the boiling point. 



The radiation shield is cylindrical and open at the lower end, and is provided 

 with a conical portion at the top, to fit closely to the protecting tube of the 

 thermometer. The cylindrical part is 1.5 to 2.5 cm larger in diameter than 

 the protecting tube of the thermometer and at least 1 cm smaller in diameter 

 than the inside of the boiling tube. The cylinder should extend at least 1.5 cm 



Smithsonian Tables 



