Two New Pressure- Gauges for the Highest Vacua. 83 



for its discharges, as we have pointed out, are free from the 

 fluctuating effects produced by induction-coils, transformers, 

 and electrical machines. Our present paper is therefore only 

 preliminary to a more exhaustive study of the discharges of 

 electricity through rarefied gases by means of a storage 

 battery often thousand cells, which will give an electromotive 

 force of about twenty thousand volts. 



Harvard University, Cambridge, Mass., U.S.A., 

 December 1st, 189& 



XIY. Two New Pressure- Gauges for the Highest Vacua. 

 By William Sutherland *. 



HITHERTO the M'Leod gauge has been accepted as the 

 most accurate instrument for the measurement of 

 pressures in vacua, although there has been a good deal of 

 misgiving as to the limit up to which its indications are 

 fairly reliable, with an impression that near a millionth of an 

 atmo its measurements become illusory. Baly and Ramsay, 

 in their experiments on rare gases (Phil. Mag. [5] xxxviii.), 

 after working with two M'Leod gauges of very high sen- 

 sitiveness, pronounced the type of gauge worthless for air 

 and C0 2 though reliable for hydrogen ; but the failure of 

 their gauges with air was due to a most interesting and very 

 special phenomenon, to be considered in my next paper, 

 liable to occur only under special circumstances and not 

 affecting the principle of the gauge or its application to 

 air with avoidance of the perturbing conditions. In the 

 case of C0 2 the M'Leod gauge is less reliable than with other 

 gases on account of some action between glass and C0 2 , 

 especially if moisture be present ; but this does not necessitate 

 that the gauge should be worthless for C0 2 , but that there 

 should be a limit to its application, this limit probably 

 depending on the nature of the glass and its treatment. To- 

 wards the end of this paper it will be shown that Crookes's 

 M'Leod gauge begins to fail decidedly at 5/iO 6 atmo, with 

 increasing unreliability at lower pressures, the cause of which 

 will be proved to be probably a small residue of water-vapour 

 dissolved, so to speak, in the glass of the gauge. Thus, if the 

 theoretical reasoning of " Boyle's Law at very Low Pres- 

 sures " f is sound, the principle of the M'Leod gauge ought 

 to be applicable up to any degree of rarefaction, if we can only 

 make it of such a material or so manipulate it as to free it 

 from this trouble with moisture. 



* Communicated by the Author, 

 t Supra, p. 11. 



