properties of easily absorbed Rontgen Radiation. 575 



of a series of metallic plates mounted on the magnetically 

 controlled carriage T, so that any one can be brought under the 

 action of the cathode stream. The velocity of the cathode rays 

 can be varied by altering the potential applied to the tube. 



The Rontgen rays produced are allowed to stream out through 

 the aluminium window W and are measured by the ionization 

 they produce in the ionization chamber I. 



Absorbing screens can be introduced between iV and W. 



The following are some of the more important results. 



1. There is a cathode ray velocity below which no Rontgen 

 radiation can be detected. 



2. Using cathode rays produced by a potential of 2500 volts 

 and over, the anticathodes arrange themselves in the following 

 descending order as radiators : aluminium, platinum, silver, lead, 

 tin, nickel, cadmium, iron, antimony, copper, zinc. Antimony has 

 a tendency, at greater potentials, to take a higher position in this 

 scale. 



Aluminium at these low potentials is about five times as 

 efficient an anticathode as copper. 



There is strong evidence in favour of the view that aluminium 

 and platinum give out characteristic secondary radiations. The 



mass absorption coefficient ( — ) in aluminium is about 600. 



3. There is reason to believe that this secondary radiation 

 can be excited by a primary radiation somewhat less penetrating 

 than itself. 



4. The emission of the secondary radiation from aluminium 

 and platinum is accompanied by corpuscular emission. 



The investigation is being continued to higher potentials. 



