Currents to the Study of Ionization by Collision. 471 



In comparing the values ©f a given in these tables with 

 the results obtained by Townsend, it must be remembered 

 that the temperature of the air in these experiments is 

 much higher than the temperature in experiments where 

 the ionization is produced by ultra-violet light, and at a 

 given pressure under these conditions the number of mole- 

 cules per unit volume of the gas is less than it is at the 

 same pressure in Town sen d\s experiments. The number 

 of collisions made by an ion in travelling a distance o£ 

 1 cm. under the electric force is thus reduced by the 

 increased temperature of the gas, and unless this reduction 

 is neutralized in some way by an increased production of 

 ions on account of the higher temperature, the value of a 

 will be less than it is at lower temperatures under the 

 same conditions of electric intensity and gas pressure . 

 The experiments of H. A. Wilson on the thermionic 

 current between a glowing platinum wire and a surrounding 

 platinum cylinder * have led to the conclusion that the 

 number of collisions, resulting in ionization, made by 

 the negative ion, varies inversely as the absolute tem- 

 perature of the gas when its pressure is constant — that 

 is to say, is proportional to the number of molecules present 

 in unit volume of the gas. It follows, therefore, that an 

 increase of temperature of the gas, apart from the alteration 

 of density which it causes, has no direct influence on 

 ionization by collisions, which, in a given electric field, 

 appears to depend solety on the nature of the molecules 

 and their distance apart. This result also follows, on the 

 modern theory of sparking, from the earlier experiments 

 of Snow Harris f and of Cardani J by which they showed 

 that sparking potentials are independent of the temperature 

 of the gas so long as its density is constant. For the 

 purpose of comparing the results of the present experi- 

 ment with those of Townsend, it is thus only necessary 

 to. reduce the pressures to the values they would have if 

 the density of the gas were kept constant and its tem- 

 perature were reduced to that of the laboratory. If the 

 temperature of the laboratory in Townsend's experiments 

 is taken as 17° 0., and the temperature of the gas in the 

 present experiments is t° C, the observed pressure must be 



290 

 multiplied by -^75 to obtain the " corrected pressure."' 



* H. A. Wilson, Phil. Trans A, vol. 202, p. 243 (1903). 

 f W. Snow Harris, Phil. Trar.s. vol. 124. p. 213 (1834). 

 \ P. Cardani, Rend, della R. Ace. dei Lincei, vi. p. 44 (1888 1. 



