THERMOMETRICAL TABLES. XIX 



TABLE 6. 



Table 6. Conversion of differences Centigrade to differences Fahrenheit. 



The table gives for every tenth of a degree from o° to 9.9 C. the corre- 

 sponding lengths of the Fahrenheit scale. 



Example: 



To find the equivalent difference in Fahrenheit degrees for a difference 



of 4°72 Centigrade. 

 From the table, 4°70 C. = 846 F. 



From the table by moving the decimal point for 0.2, 0.02 = 0.04 



^C. ^^F. 



TABLES 7, 8. 



Tables 7,8. Correction for the temperature of the emergent mercurial column 

 of thermometers. 



When the temperature of the thermometer stem containing a portion 

 of the mercury column is materially different from that of the bulb, a correc- 

 tion needs to be applied to the observed reading unless the instrument has 

 been previously graduated for the condition of use. This correction fre- 

 quently becomes necessary in physical experiments where the bulb only, or 

 else the bulb with a portion of the stem, is immersed in a bath whose tem- 

 perature is to be determined. In meteorological observations the correction 

 may become appreciable in wet-bulb, dew-point, and solar-radiation ther- 

 mometers, when the temperature of the bulb is considerably above or below 

 the air temperature. 



If ^ ' be the average temperature of the emergent mercury column, / the 

 observed reading of the thermometer, n the length of the mercury in the 

 emergent stem in scale degrees, and a the apparent expansion of mercury 

 in glass for i°, the correction is given by the expression 



an {t - t'),ov - an {f - t) 



which latter may be the more convenient form when t ' is greater than t. 



The value of a varies with the composition of the glass of which the 

 thermometer stem is composed. For glass of unknown composition the best 

 average value for centigrade temperatures appears to be 0.000155, while for 

 stems of Jena i6^^\ or similar glasses, or Jena 59^^\ the values 0.00016 

 for the former and 0.000165 for the latter maybe preferred. (Letter from 

 U.S. Bureau of Standards dated January 5, 191 8.) 



The use of the formula given above presupposes that the mean tempera- 

 ture of the emergent column has been determined. This temperature may 

 be approximately obtained in one of three ways, (i) By a " fadenthermo- 

 meter" (Buckingham, Bulletin, Bureau of Standards, 8,239, 1911, Scientific 

 Paper 170); (2) by exploring the temperature distribution along the stem 

 and calculating the mean temperature; (3) by suspending along the side of, 

 or attaching to the stem, a single thermometer. If properly placed this 



