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LIX. The Collisions of Electrons with Molecules of a Gas. 

 D>/ J. S. Townsend, F.R.S,, Wykeham Professor of 

 Physics, Oxford *. 



IN a paper on Ionization by Collision P. 0. Pedersen f 

 gives a method of calculating ionization potentials from 

 the currents obtained through a gas between parallel plates 

 at various distances apart, when electrons are set free from 

 the negative electrode. In these calculations it is assumed 

 that each collision with a molecule of air has the effect of 

 reducing- the velocity of the electron to zero, and the following 

 formula is obtained by Pedersen for the total number n a of 

 electrons arriving at the positive electrode when n Q electrons 

 start from the negative electrode, 



the additional ions being generated by collisions in which the 

 velocity of the electrons exceeds a certain critical value. 



This formula is applied to the following numbers, which 

 w r ere published several years ago as an example of the expe- 

 riments on which the theory of ionization by collision was 

 founded^, a being the distance between the plates in centi- 

 metres, and q the currents in arbitrary units. The expe- 

 riments were some of those made with air at a pressure of 

 one millimetre, when electrons are set free from the negative 

 electrode by the action of ultra-violet light, the force being 

 350 volts per centimetre: — 



a -2 -4 -6 -8 1-0 11 



q 2-86 8-3 24-2 81 373 2250 



The shortest distance between the plates at which a mea- 

 surement of current was made was "2 centimetre, as it was 

 necessary to allow the electrons starting from the negative 

 electrode to traverse a certain distance in the gas in order 

 that they should attain the final average velocity corre- 

 sponding to the electric force. The currents at the shorter 

 distances '2, *4, and *6 centimetre between the plates are in 

 geometrical proportion, and may be represented by the 

 simple formula /i rl = ji e aa given at the time the experiments 

 were first published, and the only quantity which can be 

 deduced from the ratio of these currents is a the coefficient 



* Communicated bv the Author. 



t P. O. Pedersen, Phil. Mag. July 1920, p. 129. 



t J. S. Towusend, Phil. Mag. (6) vol. vi. p. 598, Nov. 1903. 



Phil. Mag. S. 6. Vol. 40. No. 238. Get. 1920. 2 L 



