Tables 227-229 



TABLE 227. — Correction for Temperature of Emergent Mercurial Thermometer Thread 



249 



When the temperature of a portion of a thermometer stem with its mercury thread differs much from 

 that of the bulb, a correction is necessary to the observed temperature unless the instrument has been 

 calibrated for the experimental conditions. This stem correction is proportional to n0(T — t), where n 

 is the number of degrees in the exposed stem, /S the apparent coefficient of expansion of mercury in the 

 glass, T the measured temperature, and t the mean temperature of the exposed stem. For temperatures 

 up to 100° C, the value of is for Jena i6 m or Greiner and Friedrich resistance glass, 0.000159, for 

 Jena 59 U I, 0.000164, and when of unknown composition it is best to use a value of about 0.000155. The 

 formula requires a knowledge of the temperature of the emergent stem. This may be approximated in 

 one of three ways: (1) by a " fadenthermometer " (see Buckingham, Bulletin Bureau of Standards, 

 8, p. 239, 1912); (2) by exploring the temperature distribution of the stem and calculating its mean 

 temperature; and (3) by suspending along the side of, or attaching to the stem, a single thermometer. 

 Table 228 is taken from the Smithsonian Meteorological Tables. 



TABLE 228. — Stem Correction for Centigrade Thermometers 



Values of o.oooi55«(r — t). 



TABLE 229. — Reduction of Gas Thermometers to Thermodynamic Scale 



The final standard scale is Kelvin's thermodynamic scale, independent of the properties of any 

 substance, a scale resulting from the use of a gas thermometer using a perfect gas. A discussion of this 

 is given by Buckingham, Bur. Standards Bull., 3, 237, 1907: "The thermodynamic correction of the 

 centigrade constant-pressure scale at the given temperature is very nearly proportional to the constant 

 pressure at which the gas is kept " and " the thermodynamic correction to the centigrade constant- 

 volume scale is approximately proportional to the initial pressure at the ice point." These two rules 

 are very convenient, since from the corrections for any one pressure, one can calculate approximately 

 those for the same gas at any other pressure. 



The highest temperature possible is limited by the container for the gas. Day and Sosman carried 

 a platinum-rhodium gas thermometer up to the melting point of palladium. For most work, however, 

 the region of the gas thermometer should be considered as ending at about 1000° C (1273 K.). 



Note: All corrections in the following table are to be added algebraically. 



273.1° C (ice point) 

 For a discussion of the various values and for the corrections of the various gas thermometers to 

 the thermodynamic scale see Buckingham, BulL Bureau Standards, 3, p. 237, 1907. 

 Scale Corrections for Gas Thermometers. 



Smithsonian Tables 



