Resistance of Vacuum. 17 



when the pressure of the gas is very slight the resistance is 

 inversely proportional to the surface of the negative electrode- 

 Two tubes of equal width, and with equal electrodes, between 

 the extremities of which the distances were 12 and 146 mil- 

 lims. respectively, were filled with hydrogen and connected, 

 side by side, with the poles of the induction-apparatus. The 

 current divided itself between the two tubes ; and it appeared 

 that the two portions continually approached nearer to equality 

 as the hydrogen was rarefied. When the pressure had fallen 

 to 1'85 millira., the ratio between the intensities was 1*65; as 

 soon as it had fallen to 055, the two intensities became per- 

 fectly equal, although the distance between the extremities of 

 the electrodes in one of the tubes was twelve times as great as 

 that in the other. Therefore the resistance designated by r x 

 in the foregoing decreased with the pressure of the gas, and 

 at a pressure of 0'55 millim. i\l could be neglected compara- 

 tively with r. 



On the other hand, other experiments proved that the resist- 

 ance r really increases as the gas becomes more rarefied. Two 

 equal ellipsoidal glass vessels were furnished with equal elec- 

 trodes of aluminium, then filled with hydrogen at a pressure of 

 1-05 millim., and connected, side by side, with the poles of an 

 induction-apparatus. The two vessels being equal, the current 

 ought to have been shared equally between them ; but it was 

 found that, in consequence of a slight difference between the 

 electrodes, the ratio between the respective resistances of the 

 two vessels was 1*17. The pressure of the gas was afterwards 

 diminished by one half in the vessel which had the less resist- 

 ance ; and now it was ascertained that the resistance was 

 thereby so much increased that it became 2"34 times as great 

 as in the vessel in which the pressure was not changed. Con- 

 sequently, if the two vessels had had exactly the same resist- 

 ance before the reduction of the pressure in one of them, the 

 resistance would have been raised to 2'34 x 1'17 = 2 # 74. If in 

 both vessels the pressure was 05 millim. and was afterwards 

 reduced in one to 0'25 millim., the resistance was thereby 

 augmented in that vessel from 1 to 2*59. These experiments 

 were modified in different ways and with different current- 

 intensities; but the result remained nearly the same. 



The gas-pressure in these last experiments being only 1*05 

 millim. or less, according to the foregoing r x l can be neglected 

 in comparison with r. These last observations consequently 

 prove, as we have had cause to admit for other reasons before- 

 mentioned, that the resistance r increases as the gas is rarefied. 

 Much importance attaches to Hittorf 's observation that the 

 resistance r depends on the material of which the negative 



Phil. Mag. S. 5. Vol. 13. No. 78. Jan. 1882. C 



