IONS PKODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 



251 



It is noticeable from the figure that this curve consists of portions of straight lines 

 intersecting at points which have as abscissae 0, eZ/& 2 X, d/^X. (for the case k a > X^), 

 and which correspond respectively, as far as time considerations are concerned, to the 

 momentary Rontgen-ray discharge, the withdrawal of all the negative ions, and the 

 receipt of all the positive ions by the plate A. It is evident that, if a curve with the 

 above characteristic features .could be obtained experimentally, we could deduce from 

 the positions of the points the values of djk^. and dfkyX and thus obtain the 

 mobilities of the ions for the gas or vapour under consideration. This procedure 

 would not, however, be advisable in practice. The curve in fig. 1 corresponds only to 



I 



TIME 



Fig. 1. 



ideal conditions ; as has been mentioned above, the ionisation must be uniform 

 throughout the interval between the two plates, such uniformity being only approxi- 

 mately realised in practice ; moreover, LANGEVIN has shown that, as a result of the 

 recombination and diffusion of the ions, the effects of which have been neglected in 

 the theoretical treatment, the curve as realised experimentally does not consist of 

 separate straight lines, but of separate curves which form nicks at their points of 

 intersection, the positions of which, however, have not been displaced. 



If the ionisation consist of a uniform distribution between the plates, together with 

 a layer of intense secondary ionisation close to the plate A, the theoretical curve 

 expressing the relation between Q and t can readily be obtained by adding the 

 ordinates due to each part of the ionisation. This curve is shown by the thick lines 

 in fig. 2, the thin lines denoting the curve due to the uniform ionisation as before, and 

 the dotted lines the curve due to the ionisation localised near the plate A. 



As a result of recombination and diffusion the nicks will be rounded off ; moreover, 

 the part QR of the resultant curve is scarcely realisable in practice, especially if there 

 be little difference in the values of the two mobilities. In fig. 3 is given the type 



2 K 2 



