[ 129 



XI. On the Theory of Ionization by Collision. 

 By P.'O.'Pedersen*.' 



1. TN an article in this journal Norman R. Campbell f 

 L. treats of the mathematical theory of.' ionization by 

 collision, making the simplest possible assumptions with 

 regard to the nature ol this ionization. Only ionization 

 due to collisions between electrons and molecules is con- 

 sidered, and his assumptions may briefly be stated as 

 follows : — 



(1) An electron will produce a fresh pair of ions 

 (electron -f- positive ion) when it collides with a 

 neutral molecule, if previously to that collision 

 its velocity v satisfies the following condition : 



-x/ 2 ^.. 



■'5*»-V„ v ' W 



where m is the mass of the electron, — e its charge, 

 and V a certain constant voltage depending on 

 the nature of the molecule. 



(2) Collisions between electrons and molecules take 

 place unelnstically, so that after collision the electron 

 is tree again but has the velocity zero. 



(3) The electrons set free by ionization have also the 

 velocity zero. 



(4) The velocities of the electrons due to the electric 

 field are so great that the thermal velocities of the 

 electrons and the molecules may be disregarded {. 



2. Norman R. Campbell does not give the exact solution. 

 He says § : ci Now it is possible to calculate accurately 

 according to the theory what should be the current between 

 electrodes a distance a apart (fig. 1) in a gas at pressure p, 



* Communicated by the Author. 



t Phil. Mag-. (6) vol. xxiii. p. 400 (1912). 



J With regard to the question of the validity of these assumptions, 

 see J. S. Townsend, 'Electricity in Gases' (Oxford, 1915), and the 

 discussion between Townsend and Norman R. Campbell, Phil. Mac (6) 

 vol. xxiii. pp. 856 & 986 (1912). 



§ Loc. cit. p. 404. 

 Phil. Mag. S. 6. Vol. 40. No. 235. July 1 920. K 



