On Secondary Radiation. 



231 



protected. The radium is placed at 11 so as to send a pencil 

 of rays through a hole in a thick lead screen, other lead screens 

 being placed to protect the tube T from direct radiation. 



C£LLS 



A plate of the substance under examination is placed at P, 

 and the secondary radiation from it enters the tube T and 

 produces ionization, which is measured in the usual way by 

 the electrometer. The end of the tube T is covered with a 

 single sheet of tinfoil. The distance from the radium to the 

 plate P was in most of the experiments about 26 cms., and 

 from P to the nearest point of T was usually about 9 cms. 

 The distances are given in every case where they are of 

 importance. 



In every experiment the ionization observed in T is cor- 

 rected for whatever small conductivity existed between the 

 tube T and its inner terminal when the plate P is not in 

 position: this small conductivity being due partly to the 

 normal ionization inside T, and partly to insufficient screening 

 of direct radiation from the radium, and also to secondary 

 radiation from the air traversed by the primary rays. 



Fifty milligrams of radium bromide were used, enclosed 

 in a vessel which stopped the a radiation. 



