Two Ions produced in Gases hy Bontgen Radiation. 145 



The numbers show in a marked way that the velocity of 

 the positive ions is less than that of the negative. 



The small deflexion obtained with both plates to earth is 

 very likely due to a diffusion effect in the narrow slits of the 

 apparatus, as is explained in § 7. 



The above results are shown graphically in fig. 10. 



It is seen that the electrification blown out at first increases 

 rapidly with the voltage used, and that it reaches a maximum 

 which is followed by a rapid decrease. 



The following consideration will show in a general way the 

 cause of these changes in this particular case : — 



In fig. 11 AD represents a section between the two slits 

 M and N, of the air which is moving from right to left with 

 a constant velocity V. 



The ions are forced to cress this moving layer by the action 

 of the electric force between the plates. 



Fig. 11. 



-,/t ▼ X Q 



M 



Sr 



9^L 



N 



If u is the velocity of the ions travelling towards the plate 

 P when in a unit electric field, their velocity in a field F is 

 Fu. When this velocity is small compared to that of the 



blast, i. e. so long as -^ < -p~ , all of the ions which cross 



AB moving towards P, and those which cross CD moving 

 from P, will be carried by the blast into the slit M. The 

 nature of the charge carried into M w T ill depend mainly upon 

 the relative number of the two kinds of ions thus entering 

 the area AD ; and as CD is very close to the plate the 

 number coming in from that side is small compared to the 

 number entering at AB, and so the sign and the amount of 

 the charge depend for the most part upon the number cross- 

 ing AB. (Exceptions, due to surface ionization, are mentioned 

 later.) At first the number entering is nearly proportional 

 to F, and the straight part of the curve from A to B in fig. 10 

 is obtained. 



When -^ = -pp, ions that are entering at B take the path 



BC due to the two velocities, and just manage to get into 

 the slit M. As F is increased still more, those ions only 

 Phil. Mag. IS. 5. Yol. 46. No-. 278. July 1898. L 



