224 



uncertainty arising out of this cause at present. In those experiments 

 a little more time will have to be allowed for the settinj^ in of the 

 equilibrium pressure. 



'^ 5. Deienninniions ivitli the hot-wire manometer. 



With regard to the choice of the shape and dimensions of the 

 x'apour-pressui'e tube R, used in the measurements with the absolute 

 manometer, the question arises whether sufficient account has been 

 taken of two causes of disturliance which may influence the results 

 obtained. The tube being long cannot in itself be considered a 

 sufiicient precaution to exclude radiation from above, by which the 

 temperature-equilibrium of the evaporating surfiice is rendered un- 

 certain ; furthermore it has to be ascertained, whether the surface 

 of the liquid bath may possibly be at a lower temperature than the 

 bath at the spot, where the temperature is measured ^). 



In view of the degree of accuracy of the measurements with our 

 absolute manometer the errors arising out of these causes of disturb- 

 ance may be neglected. 



With our hot-wire manometer the accuracy which could be attained 

 in the region of less low pressures, at which measurements were 

 also ijitended, was considerably higher. In designing the vapour 

 pressure apparatus intended for the measurements with this manometer 

 the possible influence of the aforesaid causes of error was therefore 

 carefully avoided. 



The hot-wire manometer which we used is represented in fig. 3. 

 It consists of a WoLLASTON-wire (0.005 mm. in diameter and about 

 10 cm. long), stretched in a frame of platinum Avire abc which 

 forms at the same time the one electrode of the current. The second 

 electrode e is insulated from abc h.\ means of glass. The Wollaston- 

 wire is fastened between /> and (/. 



The dimensions are so chosen, that a pressure of 1 barye can be 

 measured with great accuracy. This is the case when the diameter 

 of the wire is small compared with the mean distance described by 

 the molecules since their last collision : for as long as this condition 

 is satisfied, the loss of heat by the wire may be taken approximately 

 proportional to the density (or the pressure) of the gas. Owing to a 

 deviation from proportionality, which is due to the heal flowing 



1) Gomp. Kamerlingh Onnes and Braak, Comm. W. 107a. Even after 6 liours 

 there was no sign wliatsoever of a distillalion of carbon dioxide at the bottom of 

 U to the neiglibourhood of the level of the liquid in the batli. 



