356 



The apparatus is provided at the top and the bottom with side- 

 tubes which serve for tiie purpose of cleaning and fdling and are 

 sealed off before the measurements. At both ends the tube has a 

 slight enlargement for collecting the mercury. 



When the tube is used for relative measurements, (he formula to 

 be used is 



ni '■ '»i2 = ^1 •■ h 



where i}^ and i]^ are the viscosities of both vapours, ^j and t^ their 

 times of flow. 



A correction has to be applied for gliding by means of the relation 



^ R 

 where G^ and G^ are the mean free paths of the two vapours and 



R is the radius of the capillary. When — is known, a rough approxi- 



G. 

 mation for — is sufficient. 



G^ 

 The kinetic theory gives ï^ = 73 ^Z6^ P^ and p = 7^ (/ F% where f/ 

 is the density, T the square root of the mean velocity square, p 

 the pressure. Eliminating V we iind ï^ = 1^7 j^ö?6^ and therefore 



G. 





Critical velocity. Reynolds' criterion for liquids also holds for 



gases '). For our viscosimeter, := 146 for air (admissible limit 



is 2000) D was .0358 cm.; fZ=.001293; 7^:= .000181; the volume 

 2.246 ccm. ; the time of flow 57 seconds. 



Correction for the capillary action of the mercury drop. 



In consequence of the difference in curvature of the two surfaces 

 of the mercury drop, a correction has to be made in the pressure 

 which may be different for different gases and different temperatures. 

 This correction was determined by Rankine by measuring the time 

 of flow for se\'eral mercury drops of different length. The relation 

 between the mass in of the mercury and the time of fall t may be 

 represented by the equation 



1) RucKES, Ann. Piiys. 25, p. 983, 1908. 



