486 



Mr Hughes. On the MohtUties of the Ions 



In view of Leuard's results, it appeared advisable to measure 

 the mobilities of these ions. An absohite determination would 

 have been more difficult than a comparative one, and for the 

 purpose of the research hardly as conclusive as a direct comparison 

 with X-ray ions. The method adopted was a moditicatiou of 

 Zeleny's* or Eutherfordsf. 



If ionised air be drawn through the ionisation cylinder (fig. 1) 

 and the quantities of electricity received in unit time by the 

 " far " electrode F be plotted against the potentials on the 

 cylinder, then a curve Avill be obtained cutting the abscissa at 

 the smallest potential required to drive all the ions into the 

 " near "' electrode T. The form of this curve will depend upon 

 the distribution of the ions in the stream of air. One would 

 expect a fairly uniform distribution on consideration of the way 

 in which the air streams up against the tiuorite window. 



/■■u^/. 



'^^^y 



\^ 



Fig. 1. 



In the comparative experiments the air was ionised by X-rays 

 in as nearly as possible the same place as when ionised by ultra- 

 violet light. A beam of X-rays 2 cms. wide was directed at that 

 portion of the apparatus just under the fiuorite window, all other 

 parts of the apparatus being shielded by thick lead screens. The 

 X-ray bulb was placed at a distance of 25 cms. from the fiuorite 

 window. An idea of the effect of the light may be obtained from 

 the fact that the positive leak in the apparatus was of the same 

 order when the ionisation was produced by the light as when 

 produced by the X-ray bulb working vigorously. (The X-rays 

 had to pass through the glass tube, the walls of which were 1 nnn. 

 thick.) The negative leak when the ionisation was produced by 

 the light was about twenty times as large as the positive. 



* Zeleny, Phil. Trans. A. 195, p. 193, 1900. 

 t Eutherford, Phil. Mag. Vol, 47, p. 109, 1899. 



