Pressure-Gauges for the Highest Vacua. 93 



contemplated in " Thermal Transpiration and Radiometer 

 Motion/'' For the highest accuracy it would be necessary 

 to take steps to keep the whole drum at a constant mean 

 temperature. 



A special advantage of this form of gauge is that after a 

 certain degree of rarefaction is reached A!'p + B w may be 

 neglected, and the deflecting force and angle of torsion 

 become proportional to the pressure, so that the experimenter 

 can ascertain at a glance what pressure he is dealing with. 



To show a defect to which both new forms of gauge are 

 subject, and yet how they can support one another and bring- 

 to light an interesting phenomenon, we will discuss Crookes's 

 observations on hydrogen just as we have done those on air. 

 In " Thermal Transpiration and Badiometer Motion " it was 

 shown that the deflecting force for hydrogen is not expressed 

 by the ordinary formula until changed to 



Now the term A!'p in the ordinary formula originates from 

 a term involving the product of the mean free path near the 

 solid and the mean free path far from the solid : and the 

 mean free path near the solid was shown to be altered by 

 condensation of the gas, but in such a manner that it is 

 always proportional to the free path in the gas far from the 

 solid ; thus molecular attraction of the solid for the gas alters 

 slightly the numerical value of A", but does not introduce 

 any factor 1/(1 — ap); therefore the introduction of this 

 factor is probably due to some other surface action ; if it is 

 really connected with some other alteration of the free path 

 of a molecule of the gas near the solid surface we ought to 

 find the slipping of the gas affected ; for theoretically the 

 amount of slipping is proportional to the mean free path near 

 the solid, and at the same time, as the free path far from the 

 surface is unaffected, the viscosity of the gas should be prac- 

 tically unaltered by the surface action. Accordingly some 

 interest attaches to the study of slipping in Crookes's experi- 

 ments with hydrogen. The constant upper limit for the log. 

 dec. in H 2 is '049 9, and in the case of air we took //,, the part 

 of the log. dec. due to viscosity of fibre, as '0040; thus in (34) 

 we have L and /uu, the values of I being given in the next 

 table at the pressures given as found by the M'Leod gauge 

 and Boyle's law in terms of 1/10 6 atmo as unit ; in the third 

 row will be found (L— /*)/(£ — //,) —1, and in the fourth 



Phil. Mag. S. 5. Vol. 43. No. 261. Feb. 1897. I 



