Pressure- Ganges for the Highest Vacua. 91 



which could be definitely inferred, and would give the part 

 to be subtracted from all readings of the log. dec. in the 

 viscometer gauge as due to the fibre. Or /j, in our formula 

 (34) could be found directly from the readings of the values 

 of the log. dec. as we found it with Crookes's observations, 

 that is to say, by making observations of the log. dec. at 

 different pressures measured by a reliable form of M'Leod 

 gauge. One measurement of L and two measurements of I at 

 suitable pressures would suffice for the determination of fju 

 and 2rtA />o/D, but mean values of these obtained from a 

 number of observations would of course be preferable. The 

 gauge is then ready for the measurement of any pressure in 

 the particular gas under observation ; for although /jl is the 

 same whatever the gas may be, 2a\ p /D varies from one gas 

 to another ; but when once fi has been obtained it is only 

 necessary to take a reading of L and one reading of I at any 

 measured suitable pressure to give 2a\ p /D for the gas 

 under study, and then all other pressures are obtainable by 

 measurements of the log. dec. and a simple calculation. 



One advantage of the viscometer gauge is that when once 

 its constants have been determined for ai^ particular gas, 

 the only operation necessary for obtaining any pressure is 

 that of observing the amplitude of the ??*th, nth, and pth. 

 vibrations, an operation so simple that it need only delay the 

 experimenter a minute or two, and by a short calculation he 

 can satisfy himself in a moment as to the magnitude of the 

 pressure in his gas supply. 



The other form of pressure-gauge, the torsion radiometer, 

 or, as we may call it, the Transpiration Gauge, has already 

 had its essential features indicated in " Thermal Transpiration 

 and Radiometer Motion " as a piston enclosed in a cylinder 

 with a uniform distance between the curved surfaces of both, 

 the ends of the cylinder to be of glass or other transparent 

 material to facilitate irradiation of one end of the piston by 

 means of a constant source of heat such as a platinum spiral 

 carrying a constant electric current or a steam bath ; or the 

 ends might be metal with a steam bath at one end and ice bath 

 at the other. It has been shown in " Thermal Transpiration 

 and Radiometer Motion " that when the temperatures of the 

 two ends of the piston become constant there is a pressure 

 driving the piston from hot to cold with a total force 

 c'/(A"p + W" + 1/p), where the coefficients are all constant 

 for a given apparatus and a given gas, but varying with the 

 gas. It is necessary to attach to the piston some means of 

 enabling it to retain its position against this force, and of 

 measuring the amount of the force; this may be done by 



