494 



We have now for the first time made measurements with a helium 

 thermometer in which a heated-wire manometer aceordino; to Knldskn 

 serves as manometer (oomp. § 7 Comin. Snppl. 34). With this thermo- 

 meter it will be possible to go to still lower ten)i)eratiires than 

 heietofore. In these measurements the correction for the thermal 

 molecular pressure became even now of paramount importance. We 

 succeeded in calculating a formula for this correction, albeit with 

 the aid of a iiypothesis regarding the etfective molecnlar free path 

 which leads to a semi-empirical relation. 



The new constant in this foi'mula which is a characteristic constant, 

 for helium could be chosen such, that for all our measurements 

 wilh the theruiomeler with hot-wire-manometer a satisfactoiy agreement 

 was obtained with the thermometer with mercury-manometer. 



2. Survey of the difficulties inherent in the detenninafions of the 

 loioest temperatures. All measurements of temperature in the helium- 

 region will ultimately have to be reduced to readings on a helium- 

 thermometer supposed to be filled with helium in the Avogadro- 

 condilion. Hence the importance of knowing, how to arrive at 

 accurate determinations with a helium-thermometer, even at very 

 low pi'essures. For measuring those temperatures, at which the 

 vapour-pressure of helium approaches a very small value, no other 

 helium-thermometers but those with gas at very low pressure can 

 be used, as the pressure in the thermouieter must in any case remain 

 below the vapour-pressure corresponding to the temperature to be 

 measured. 



Various circumstances thus render it difficult to raise the accuracy' 

 to the level which would be permitted by the high degree of constancy 

 of the temperature of the helium-bath which can be attained when 

 the experiment is not unduly prolonged. 



It will be necessary to take care that the following conditions 

 are fulfilled. 



1. Tire dead space, or rather that part of the dead space the 

 temperature of which is uncertain, must be made as small as possible. 



2. The adjustment of the equilibrium must take place in a 

 sufficiently small time in order to prevent the temperatui'e-changes 

 of the helium-bath affecting the measurements. 



3. The deviations of the equation of state for the thermometric 

 gas from the AvoG.\DRO-condition must not come too much into account. 



4. The correction for the thermal molecular pressure must not 

 become too large and this pressure must not reach a region, for 

 which the correction is less ai'curately known. 



