Tables 21 8 and 219 243 



THE INTERNATIONAL TEMPERATURE SCALE 



TABLE 218. — The Standard Thermocouple 



The platinum of the standard couple shall be of such purity that the ratio RJR is 

 initially not less than 1.390 for t = ioo°. The alloy is to consist of 90 per cent platinum with 

 10 per cent rhodium. The completed thermocouple must develop an electromotive force, 

 when one junction is at o° and the other at the freezing point of gold, not less than 10,200 

 nor more than 10,400 international microvolts. The diameter of the wires used for standard 

 thermocouples should lie between the values 0.35 and 0.65 mm. 



The freezing point of antimony, specified for the standardization of the thermocouple, 

 lies within the range of o° to 660° where the international scale is fixed by the indications 

 of the standard resistance thermometer, and the numerical value of this temperature is 

 therefore to be determined with the resistance thermometer. In the appendix the result of 

 such determinations is given as 630.5 , but the temperature of any particular lot of antimony 

 which is to be used for standardizing the thermocouple is to be determined with a standard 

 resistance thermometer. 



The procedure to be followed in using the freezing point of antimony as a fixed tempera- 

 ture is substantially the same as that specified for silver. Antimony has a marked tendency 

 to undercool before freezing. The undercooling will not be excessive if the metal is heated 

 only a few degrees above its melting point and if the liquid metal is stirred. During freez- 

 ing the temperature should remain constant within 0.1° for a period of at least five minutes. 



TABLE 219. — Secondary Calibration Points 



These points and their temperatures on the international scale are listed below. The 

 temperatures correspond to a pressure of one standard atmosphere. The formulas for 

 variation of vapor pressure with temperature are for the range from 680 to 780 mm. 



°C 



Boiling hydrogen t p = hm + 0.0044 (P — 760) — 252.75 



Equilibrium between solid and gaseous carbon dioxide t P = hw -f- 0.1443 



(f P + 273.2) log w (p/760) - 78.5 



Freezing mercury — 38.87 



Transition of sodium sulphate 32.38 



Condensing naphthalene vapor t P — t-w + 0.208 (t P + 273.2) logio (p/760) 217.96 



Freezing tin 231.85 



Condensing benzophenone vapor t P = tieo + 0.194 (tp + 2 73- 2 ) log™ (p/760) .... 305.9 



Freezing cadmium 320.9 



Freezing lead 327.3 



Freezing zinc 41945 



Freezing antimony 630.5 



Freezing copper in a reducing atmosphere 1,083 



Freezing palladium 1,553 



Melting tungsten 3,400 



Isopentane — 159.6 f* Li 2 S0 3 1202. mf 



Carbon dioxide — 111.6 f Nickel 1455- m, f 



Toluene — 95.1 f Cobalt 1490. m. f 



Ethyl acetate — 83.6 f CaAl 2 Si 3 8 1555. m 



Chloroform — 63.5 f Platinum 1755. m 



Carbon tetrachloride — 22.9 f 



* f, freezing, t m, melting. 

 Smithsonian Tables 



