24 



tion of ionisation with velocity we find that the ionisation is 

 proportional to 



fcos.-^Z)//? [R-Dsec. $ 

 J J sin 0. r. de. dr. 



o o 



D 

 i.e., to 7?^- D^+ 2RD log. — . 



Thus ijl = 1 



D' D D 



\~ 2 — loo-. — 



R' R "" R 



If DIE = '25 then ill = '25 nearly. In the case of a thin 

 uniform layer we found above that if D f R — '25, theni/ 1 = '4:0. 

 Thus the effect of a screen in cutting down the ionisation 

 effects depends very much on the mode of disposition of the 

 radio-active material below it. 



Part II. 



The apparatus employed was of the usual form, and very 

 similar to that described by Rutherford ("Radio-activity," 

 1905, p. 98). As shown in the figure the material was laid on 

 the high potential plate B, at such a distance from the upper 

 plate A that no a particle could reach it. Thus, every a 

 particle ran to its extreme range ; and, to make more certain 

 of catching all the ions, the upper plate was extended down- 

 wardsi at the sides. 



'~ ^ to Q.E. 



j]iilir 



MR. 



-^10 Battery 



