IQO 



Tables 192-19S. 



CORRECTION FOR TEMPERATURE OF EMERGENT MERCURIAL 

 THERMOMETER THREAD- 



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 pro- 

 portional to «j8(r - 0, where n is the number of degrees in the exposed stem, j8 the apparent 

 coefficient of expansion of mercury in the glass, T the measured temperature, and / the mean 

 temperature of the exposed stem. For temperatures up to ioo° C, the value of |3 is for Jena 

 i6'" or Greiner and Friedrich resistance glass, 0.000159, for Jena 59^", 0.000164, and when of 

 unknown composition it is best to use a value of about 0.000155. The formula requires a knowl- 

 edge of the temperature of the emergent stem. This may be approximated in one of three ways: 

 (i) 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 tempera- 

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

 Table 192 is taken from the Smithsonian Meteorological Tables, Tables 193-195 from Rimbach, 

 Z. f. Instrumentenkunde, lo^ p. 153, 1890, and apply to thermometers of Jena or resistance 

 glass. 



TABLE 192. — Stem Correction for Centigrade Thermometers. 



TABLE 193. — Stem Correction for Thermometer of Jena Glass (0° to 360' C). 



Degree length 0.9 to i.i mm; t = the observed temperature; /' = that of the surrounding air 

 I dm. away; n = the length of the exposed thread. 



Smithsonian Tables 



