Potentials required to Maintain Currents. 83 



the case of very large gas-molecules, such as those of ether- 

 and benzine-vapour *. For the influence of pressure depends — 

 at least to a great extent — on the circumstance that with the 

 increase of pressure the single absorption-lines gradually 

 broaden, to merge finally into one continuous band f , and it 

 is likely that this final result is reached the more easily in 

 proportion as the lines lie nearer together. The real cause 

 of this pressure-effect, however, still remains unexplained. 

 If the formula (1) held absolutely, we should have to seek 

 the cause in a change of the moment of inertia brought about 

 by the impacts of the molecules. This could be caused by the 

 momentary effect of the impacts being that the molecules 

 have various rotation-axes, or that they suffer a temporary 

 deformation. But both these assumptions offer considerable 

 theoretical difficulties. Not less difficult to justify, however, 

 seems to me the assumption of Perrin t, that immediately 

 after impact the molecule has a given energy of rotation (E) , 

 which, however, through some sort of friction or radiation, 

 rapidly decreases, until the remaining rotation-energy 

 satisfies the formula E = n . h . v. 



Uppsala, April 1914. 



XL The Potentials required to Maintain Currents between 

 Coaxial Cylinders. By John S. Townsend, Wykeham 

 Professor of Physics, Oxford §. 



SEVERAL investigations have been made of the potential 

 required to maintain a current between a wire and a 

 large coaxial cylinder when a glow-discharge is produced by 

 a high electric force. A certain definite potential V is 

 required to start the discharge, and the rise of potential 

 V — V required to maintain a given current has been deter- 

 mined experimentally. 



When the current i is small the rise of potential V — V is 

 proportional to j, and in this case a simple formula || con- 

 necting t and V — V may be obtained on the hypothesis that 

 the ions are produced from molecules of the gas by collisions 

 within a short distance of the wire where the electric force 

 is large. 



« v 



* E. v. Bahr, Ann. d. Phys. xxxiii. p. 585 (1010). 



t E. v. Bahr, Verh. d. D.phy*. Ges. xv. p. 710 (1913). 



X J. Perrin, < Die Atorae,' 1914, p. 146. 



§ Communicated by the Author. 



|| ' Electrician/ June 6, 1913. 



G2 



