determined by Hydrogen and Helium Gas Titer mometers. 45 



absolute. As this value depended on an empirical law correlating 

 temperature and resistance, which might break down at such an excep- 

 tional temperature, and was in any case deduced by a large extrapola- 

 tion, it became necessary to have recourse to the gas thermometer. 



In the present investigation the advantage claimed for the constant 

 pressure gas thermometer over the constant volume thermometer is 

 absent. The effect of high temperature combined with large increase 

 of pressure does not occur in these experiments, where only very low 

 temperatures and a maximum range of pressure of less than one atmo- 

 sphere were encountered. At the same time, before dispensing with the 

 effect of pressure upon the capacity of the reservoir of the thermometer, 

 it was carefully estimated and found that it could not affect the volume 

 of the reservoir by as much as 1 /60,000th part. This being determined, 

 a particular advantage results from the use of the constant volume 

 form, because in its case it is unnecessary to know the actual volumes of 

 the reservoir, and of the " outside " space. It is only necessary to know 

 the ratio of these two volumes, and as this ratio appears only in the 

 small terms of the calculation, it is not a serious factor in the estimation 

 of such low temperatures. 



Two constant volume thermometers (called No. I and No. II) were 

 employed, in each of which the volume of the reservoir was about 

 40 c.c, and the ratio of the outside space to the volume of the reservoir 

 was 1/50 and 1/115 respectively. A figure of the apparatus is given 

 herewith, where A is the thermometric bulb covered with a vacuum 

 vessel to hold the liquid hydrogen, and be exhausted when necessary ; 

 B is the manometric arrangement for adjusting the mercury at C to 

 constant volume, and D is the barometer. The readings were made 

 on a fixed scale by means of a telescope with cross-wires and level 

 attached. A similar telescope was permanently fixed on the mark to 

 which the volume had to be adjusted. As the observations had to be 

 made quickly, it was found convenient to use both telescopes on the 

 same massive stand and to read the barometer placed alongside 

 simultaneously. 



The formula of reduction used was that given by Chappuis in the 

 ' Travaux et Memoires du Bureau International des Poids et Mesures,' 

 torn. vi } p. 53, namely, 



+ r^) H ° = ( V -^ +JM + M ^ t »> •■■ 



where V is volume of reservoir at 0° C, 



T, temperature of reservoir, measured from 0° C, 



v, volume of " outside " space at the temperature of the room, 



t, temperature of the room, 



a, coefficient of expansion of the thermometric gas, 



