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LXIII. Conductivity produced in Hydrogen and Carbonic 

 Acid Gas by the Motion of Negatively Charged Ions. By 

 John S. Townsend, M.A., Wykeham Professor of Physic's, 

 Oxford, and P. J. Kirktjy, ALA., Fellow of New College, 

 Oxford *. 



1. f I X HE experiments described in this paper are a eon- 

 JL tinuation of those which have been published in 

 < Nature/ 9th August, 1900, p. 340, and in the < Philosophical 

 Magazine,' February 1901, p. 198. These papers contain an 

 account o£ experiments which were made at the Cavendish 

 Laboratory, about a year ago, on the conductivity produced 

 in air at low pressures. The current between two parallel 

 plates was found to depend upon the distance between the 

 plates, the pressure, and the electric force in a manner quite 

 different from the current at high pressures. The conduc- 

 tivity was obtained by allowing Rdntgen rays to pass through 

 the gas; and it was found that the accompanying phenomena 

 could be explained by supposing that the negative ions 

 produced by the action of the rays generated others by 

 collisions with neutral molecules, the new negative ions thus 

 generated having the same properties as those produced by 

 the rays. 



The proof of this theory rested, in the first place, on the 

 connexion between the current and electric force for various 

 distances between the plates. The number of ions, a, gener- 

 ated by a single ion in going through a distance of one 

 centimetre was found from the experiments. The values of 

 a thus found experimentally depended on the pressure, p, 

 of the air and the electric force, X, acting between the 

 plates. The relation connecting the three variables a, p, and 



X was found to be of the form ot—pfl — J, which shows that 



a is proportional to p when — is constant. 



Further evidence in support of the theory was thus ob- 

 tained, since from the kinetic theory of gases it is easy to 

 show that a, p, and X should be connected in the above 

 manner. 



The application of this theory led to the conclusion that an 

 ion makes 21 collisions with molecules in going through one 

 centimetre of air at one millimetre pressure. When the force 

 acting on the ion is sufficiently great, two new ions (one 

 positive and one negative) are produced at each collision. 

 For smaller forces new ions are not produced at each collision. 



* Communicated by the Authors. 



