CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



FRICTION IN GASES AT LOW PRESSURES. 

 By J. L. Hogg. 



Presented by John Trowbridge June 29, 1909 ; received June 29, 1909. 



Under the title " Friction and Force due to Transpiration as de- 

 pendent on Pressure in Gases," there was published x some time ago 

 an account of some experiments made to determine the relation be- 

 tween the friction of a gas and the pressure in it, and also the relation 

 between the force exerted by a lamp on a mica vane, blackened on the 

 face which is turned towards the lamp, and the mean pressure in the 

 gas in which the vane is placed. The three-fold purpose of the inves- 

 tigation was pointed out there, viz. : 



First, to investigate the relation between friction and pressure where 

 the pressures were so small that "slip " is appreciable; second, to de- 

 termine the relation of transpiration force in the special form of appa- 

 ratus described there ; 2 and, third, to make use, if possible, of these two 

 relations to test the validity of the McLeod gauge measurements of 

 pressure, and, if these measurements should prove unreliable, to make 

 use of one of the relations named above to measure gas pressure. 



There has been much delay in carrying out the investigation with 

 the apparatus improved in the manner indicated in the closing para- 

 graphs of that paper, but now some results have been obtained in so 

 far as the friction problem is concerned. 



As was pointed out in the paper mentioned, the investigation was 

 defective in two respects. It was found that, in spite of the care which 

 was taken to exclude mercury vapor from the apparatus, some of this 

 vapor was undoubtedly present. This no doubt was due to the fact 

 that the whole apparatus had to be maintained at a high temperature 

 for long periods to insure drying, and thus the presence of the least 

 speck of liquid mercury would cause, when evaporation took place, the 

 diffusion of comparatively large quantities of the vapor through the 



1 Proc. Am. Acad., 42, 6 (1906). 2 See p. 129 of that paper. 



