74 IONISATION OF GASES [CH. vn. 



galvanometer in some cases, or an electrometer 

 arranged so as to show a measured leak in 

 others. 



A saturation value of the current can thus be 

 found, when all the ions present are taking part in 

 the action to the full extent ; the current then 

 reaches a maximum, which cannot be exceeded ; and 

 its amount would furnish an estimate of the number 

 of ions present, if the speed and the charge of each 

 were known. Measurements of the total current Neu, 

 the quantity of electricity conveyed per second, have 

 been made by Lenard 1 and Bighi 2 and Thomson, 3 

 and in various gases by Rutherford, 4 now Professor 

 at Montreal ; by Beattie 6 and de Smolan at Glasgow, 

 by Zeleny 6 of Minnesota, by McClelland 7 on hot 

 gases from flames, by McLennan 8 of Toronto, by 

 Richardson, 9 H. A. Wilson, 10 and Owen 11 on in- 

 candescent filaments. Townsend also has made 

 many experiments on the diffusion speed of ions. 



Professor Zeleny measured the velocity by a safe 

 and direct method of making the particles fly down a 

 tube against a wind, and observing the rate of the 

 current of air which was just able to withstand their 

 progress : these measurements constituting a satis- 

 factory confirmation of Thomson's and Rutherford's 

 more indirectly inferred results. 



1 Wied. Ann., vol. 63, p. 253. 

 z Rend. della R. Accad. dei Linc&i, May, 1896. 

 3 Phil. Mag., November, 1896. 

 *Ibid. t November, 1896, and April, 1897. 



6 Ibid., June, 1897. 6 Ibid., July, 1898. 7 Ibid., July, 1898. 



*Phil. Trans., vol. 195, p. 49, 1899. 

 9 Ibid., vol. 201, 1903. Ibid., vol. 202, 1903. 



11 Phil. Mag., August, 1904. 



