E. 31. Wellisch — Mobilities of Ions in Air. 583 



Art. XLII. — The Mobilities of Ions in Air ; by E. M. 



Wellisch. 

 1. Introduction. 



It is a well-known result of experiment that over a wide 

 range of pressures the mobility (k) of the ion varies inversely 

 as the pressure (p) of the gas through which the ion is mov- 

 ing. For the positive ion in air the law pk = constant has 

 been verified down to a pressure of l mm ; Todd* made several 

 determinations at still lower pressures and found that below 

 l mm the value of the product pk showed a marked increase 

 over its normal value. The case of the negative ion in air has 

 been studied by a large number of experimenters who all 

 agree in stating that the product pk increases when the pres- 

 sure is reduced below about 10 cms . Kovarikf detected an 

 increase when the pressure was as high as 20 cms . This result 

 indicates a simplification in the nature of the ion as the pres- 

 sure is reduced. Prof. Townsend drew attention to the neces- 

 sity of thoroughly drying the gas employed in the experiments ; 

 it was then found that in a dry gas at low pressures the veloci- 

 ties of the negative ion reached very large values indicating 

 that at least for part of its career it was in the electronic state. 

 As a result of numerous experiments undertaken in conjunc- 

 tion with his students Prof. Townsend arrived at the conclu- 

 sion that the velocity (v) of the negative ion should be 

 expressed in general as a function of the electric field (X) and 



the pressure (p) in the form v—f[—\ indicating that the 



nature of the ion depends in general on the field and the pres- 

 sure. At the higher pressures v was directly proportional to 



— for moderate field strengths ; this indicates that the nega- 

 tive ion remains unaltered in character over this range of pres- 

 sures. But although the results obtained by numerous experi- 

 menters were in accord as far as the general nature of the 

 variation in the value of pk was concerned, a close examination 

 of the actual values obtained revealed such discrepancies as to 

 raise at least the presumption that some residual disturbing 

 factor had not been eliminated or even detected. 



The experiments which are described here were undertaken 

 to throw further light on this phenomenon ; the present paper 

 must be regarded as merely a brief account of the results 

 obtained in this connection as much of the data needs further 



*Todd, Phil. Mag., ser. 6, xxii, p. 791, 1911. 

 fKovarik, Phys. Eev., xxx, p. 415, 1910. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 233. -May, 1915. 



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