:Vl± Bumstead and Mc(l<>u<j<in — Emission of Electrons by 



Let a be the current of 8-electrons leaving the electrode due 

 to the pencil of a-rays which strike it; this is present in- all 



the curves. Let b be the current due to a soft radiation from 

 the polonium which is unaffected by the field ; this is present 

 in III but absent in I and II. Let s be the current due to a 

 secondary radiation (consisting of electrons) when there is no 

 obstacle between the polonium and the electrode (Curve III) ; 

 this will vary with the electric field. The secondary 

 radiation which produces s in clue to nearly all the 

 a -particles liberated by the polonium, through the complete 

 solid angle, 47r, but the secondary rays from the plug 

 which carries the polonium and from the deeper parts of the 

 cylinder are cut down by the limited aperture. When the 

 polonium is covered, the secondary rays are due to the a- par- 

 ticles which emerge through a solid angle approximately equal 

 to 27r; their effect will be equal to m s wbere m < 1. (Curve II.) 

 Finally when the thin foil is interposed below the cylinder, the 

 secondary rays from the lower side of the foil will be due to 

 the a-rays which get out of the cylinder and pass through the 

 foil ; the solid angle in this case is about - l47r, but the 

 beam of secondary rays is not limited by any diaphragm. We 

 may write n s, (?i<jn) for the 8-ray current produced in this 

 case (Curve I). 



If, then, y„ y„ and y s are respectively the ordinates of Curves 

 I, II and III, we have, 



y^ = a + b+s 



y s =a -\-ms (1) 



y l = a +ns 



where a, b, m, n are constants and s A^aries with the potential 

 applied to the case. From these we obtain 



. . , 1— m. , 



(y-yJ=*+-— Uv-yJ; 



by plotting (y 3 — y 2 ) against (y 2 — yO from the curves in fig. 4, 

 and drawing a straight line to represent the points as well as 

 possible, we obtain, 



1 — m 



6=0-20; =0-62 



' m — n 



From an extension to 1700 volts of Curves I and II which was 



made later, we get as an approximate value of — , 0'15. This 



makes m = 0'65 ; w = - 10. 



With these values we may calculate * for different values of 

 the potential on the case, from the curves, by the equations 



y,-.y. y 3 -y.~ 5 



m — n ' 1 — m 



