A. L. Day and J. K. Clement— Gas Thermometer. 407 



error as much as '05°. The Reichsanstalt gas thermometer 

 scale, which is bow very generally used as the basis of high 

 temperature measurements, is officially stated to have a prob- 

 able error of 2° or 3° between 300° and 1150°, and of 10° 

 between 1150° and 1600.° 



The gas thermometer problem is one in which theory is often 

 inclined to lose patience with practice. It has been demon- 

 strated over and over again, for example (Barus, Buckingham, 

 loc. cit.), that the constant-pressure system of measurement 

 ought to be more direct and free from error than the constant- 

 volume system, notwithstanding which the major portion of the 

 results which go to make up the real progress of the past fifty 

 years has been obtained through the use of the constant vol- 

 ume principle. Theory has also be'en very apprehensive from 

 time to time of the ultimate outcome of attempting to define 

 temperature in terms of the expansion of a diatomic gas, and 

 yet nitrogen is the only gas which has yet been found practi- 

 cable for long ranges extending to the higher temperatures. 

 It does not react with a platinum bulb and does not diffuse 

 through its wall, and so far (up to 1600°) it has not been found 

 to dissociate. From the laboratory side of gas thermometry, 

 the main difficulty is now, as it has always been, to find a prac- 

 ticable bulb which will hold some expanding gas without loss 

 or change through a long range of temperatures and permit 

 sufficiently accurate measurements of the pressure-volume rela- 

 tion within. After more than three-quarters of a century of 

 the most varied experiences, pure nitrogen in a platin-iridium 

 bulb in which the pressure at constant volume can be measured, 

 is the only arrangement which has not yet encountered some 

 very serious obstacle to the extension of its range or its accu- 

 racy. It was therefore adopted without hesitation for the 

 study here described. 



accuracy of about 'OOl , but this is probably too high on account of the 

 uncertainty in the determination of the expansion coefficient of the ther- 

 mometer bulb. This bulb was of platin-iriclium (90 Pt, 10 Ir) of 1 liter 

 capacity, and the determination of its expansion coefficient had been made 

 by Deville and Mascart, but this determination was rejected in favor of 

 Benoit's value, which was obtained with a different platin-iridium bar. The 

 difference between the two determinations amounts to '007 per cent at 50°, a 

 quantity sufficient to affect the temperature scale at that point by about '004° . 

 The measurements of the same alloy which were made for the present paper 

 (page 440) differ from Benoic's by an amount equivalent to "002° at 50°. 

 There are also differences between the results obtained by Benoit himself for 

 different bars which exceed "003° when interpreted in terms of the tempera- 

 ture. It is perhaps significant that these alloys were prepared before the 

 work of Mylius on the exact determination of the platinum metals had been 

 done. The difference may therefore point to considerable variations of com- 

 position in the alloys employed. At any rate, the uncertainty here is out of 

 all proportion to the accuracy of the remainder of Chappuis's work (see also 

 P. Chappuis, Rapports presentes au Congres International de Physique, vol. 

 i, p. 137, 1900, footnote 1). 



