334 



Prof. W. H. Bragg on the Ionization of 



Tii conclusion, I have discussed very briefly the form of 

 the function f(v), and such conclusions as it seems legitimate 

 to draw from the results so far obtained. Amongst these is 

 the following: — The ionization per molecule (ks, where s 

 is the stopping power) is closely connected to the molecular 

 volume. 



§i. 



The method of this research has already been described 

 briefly in the preliminary paper (loc. cit.). For the sake of 

 clearness, however, and in order to facilitate a discussion of 

 the validity and the experimental details of the method, it 

 will be well to insert a short description here also. 



A small platinum plate is coated with a, very thin layer 

 of radium bromide, and placed below a horizontal ionization- 

 chamber of 3 mm. width, at a distance which can be altered 

 at will. (See Plate IX.) A set of narrow vertical tubes is 

 placed over the radium, and stops all a particles which move in 

 any direction which is not almost vertical. Thus the particles 

 •cross the narrow chamber at right angles to its greater dimen- 

 sions, and all spend 3 mm. of their paths in the air within it. 

 The resulting ionization being plotted against the distance 

 from the radium to the 

 middle of the chamber, 

 we obtain an "ionization 

 curve," as in fig. 1, where 

 •ordinates represent dis- 

 tances and abscissae repre- 

 sent ionization currents. 

 Each reading of current 

 is the difference between 

 two others, one measured 

 when a very thin copper 

 screen is placed over the 

 radium, and one when it 

 is not. 



In this curve the portion 

 AE is due to the ft rays 

 only, and represents the 

 effect of such ft radiation 

 us is intercepted by the 

 screen : the chamber is 

 out of range of a rays. 

 Let EA be produced to 

 meet the axis of x in D. q d 

 The portion ABP repre- 

 sents part of the effect of the « 



particles from RaC. 



