Correction of the Gas- Thermo meter. 



67 



In order to make a comparison with Joule and Thomson' 

 it is necessary to take the difference between the nitrogen 

 and hvdrogen scales by subtracting the second formula from 

 the first, which gives the expression 



-^ = ^100-0( + ^ , ^ 18 + ' 00889 ^- 0,0013230 ^ xlO- 6 . . (44) 



This is the formula which is generally quoted for the differ- 

 ence between the nitrogen and hydrogen scales ; but it must 

 be remembered that it refers to an initial pressure of 100 cms. 

 at constant volume, aud that it is obtained as the difference 

 between two comparisons with the mercury thermometer, 

 which may have introduced small constant errors, especially at 

 the higher points of the scale. The differences calculated by 

 formula (44) are compared with those taken from Table V., 

 corrected from air to nitrogen by the factor 103/88, and 

 increased in the proportion 100/76 to reduce to 100 cms. 

 initial pressure, under thy heading n = 2, in the following 

 table, together with later reductions. 



Table VI. — Difference between Scales of Constant-Volume 

 Nitrogen and Hydrogen Thermometers t n — tu at 100 cms. 

 initial pressure. 



1. 



Temp. 

 j Cent. 



Joule-Thomson. 



Chappuis *. 



Lehfeldt, 

 Phil. Mag. 



1898. 



Rose-Innes, 



Phil. Mag. 



1901. 



! n=l-5. 



»='2. 



Formula 

 (44). 



Corrected 



1888. 



o 



-20 



1 --0065 



-•0107 



-•0135 



-0173 







-10 



1 --0030 



-•0048 



- -0u67 



-•0074 





1 



+10 



1 +-0022 



+ •0036 



4 -0057 



+ •0053 



+•006 



+ 0010 



+20 



+ -0040 



+ 0060 



+ •0095 



+•0087 



+ •011 



+•0018 



+30 



+ •0049 



+•0077 



+ 0113 



+ 0105 



+ •014 



+ •0023 



+40 



i +-0055 



+•0087 



+ •0110 



+ •0110 



+ •017 



+•0025 



+50 



+ -0055 



+•0087 



+ •0086 



+•0103 



+ •019 



+ 0025 



+60 



+ •0050 



+•0078 



+ •0049 



+•0090 



+ 019 



+ 0024 



+70 



! +-0045 



+ •0064 



+ -001O 



+ •0069 



+•018 



+•0020 



+80 



j +-0034 



+•0048 



-•0024 



+ -0045 



+ •015 



+ 0015 



+ 90 



+ •0018 



+ •0028 



-0032 



+ •0022 



+ •010 



+ •0008 



It will be observed that formula (44) gives negative values 

 of the difference t n —t h at 80° and 90° C.; but 'these are of 

 the same order as the probable error of observation, which 

 was *003° or '01 mm. Hg. The only observation actually 

 taken at this part of the scale, namely, at 78° C. in the vapour 



* Chappuis' latast observations ('Bureau Intermit. Reports,' 1902) 

 make the difference tn— ^=+005 at 20°, and +-008 at 40° C, which 

 agree better with Joule and Thomson. 



F '2 



