Potentials required to produce Discharges in (rases. 479 



contamination for the iodine vapour during its passage 

 through it. 



In conclusion, we have pleasure in expressing our best 

 thanks to our colleague, Dr. H. Bassett, for much valuable 

 information on points pertaining to the chemistry of the 

 subject. 



Holt Physics Laboratory, 



Liverpool University. 



Feb. 8th, 1911. 



LV. Investigation of the Potentials required to produce Dis- 

 charges in Gases at Low Pressures. By A. B. Meservey, 

 JSew College, Oxford *. 



Sparking Potentials between Concentric Cylinders. 



THE theory of sparking has been completely worked out 

 for uniform fields only, but it would appear that some 

 additional light might be thrown on the processes of ioniza- 

 tion that take place at very low pressures from experiments 

 on sparking in a non-uniform field. These experiments 

 were undertaken with this end in view, and the field between 

 concentric cylinders was selected as the simplest form of a 

 field that is not uniform. Some preliminary experiments f , 

 which were made in this laboratory, with concentric cylin- 

 ders, showed, as might have been expected, that the sparking 

 potential varied with the direction of the field; and the 

 curves obtained in the two cases crossed at a point near the 

 minimum sparking potential. 



Fiar. 1. 



Put* p, Nc Lzod <a«<>£ e, etc , 



Batteries and 

 — - * Voltmeter *- 



The apparatus used in the present experiments is repre- 

 sented in fig. 1. A brass cylinder, about 20 cm. in length 



* Communicated by Prof. J. S. Townsend, F.R.S. 

 t ' The Theory of Ionization of Gases by Collision/ John S. Townsend, 

 P. 54. 



