478 Prof. Bumstead and Mr. McGougan on Emission of 



Let a be the current of 6-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 radia- 

 tion 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 is due to nearly 

 all the a-particles liberated by the polonium, through the 

 complete solid angle 4-77 ; but the secondary rays from the 

 ping 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-particles which emerge through a solid angle approxi- 

 mately equal to 27r; their effect will be equal to ms where 

 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 0'14 ir, but the beam of secondary rays is not 

 limited by any diaphragm. We may write ns (n<m) for 

 the S-ray current produced in this case (Curve I.). 



If, then, ?/!, ^' 2 , and y z are respectively the ordinates of 

 Curves I., IT., and III., we have, 



y z = a + b + S ") 



?j 2 =a +msj* (1) 



?/x= a +ns J 



where «, b, m, ??, are constants and s varies with the potential 

 applied to the case. From these we obtain 



by plotting (j/3— #2) against (y 2 —yi) from the curves in fig. 4, 

 and drawing a straight line to represent the points as well 

 as possible ,we obtain, 



b = 0-20 ; l ^ T>l = 0-62. 

 m — n 



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



was made later, we set as an approximate value of — , 0*15. 



m 



This makes 



m = 0-65; n = O'lO. 



