Ionization produced by fi Sf 7 Rays at High Pressures. 173 



consisting of a bottom one of brass and two others of alu- 

 minium (3*3 mm. thick) and lead (3*2 mm. thick), the latter 

 two being reversible. This compound plate, supported and 

 insulated from the vessel by means of a plate of ebonite, 

 was connected to a battery which gave a voltage of 1800. 



Fig-/!. 



fi/tDIUM 



TOLARTH 



The 7 rays which pass through the bottom of the vessel 

 and compound plate remain approximately constant. The 

 lead absorbed some of the soft 7 rays scattered by the base 

 of the vessel. The upper plate, either of Al or Pb, was sup- 

 ported by means of a brass tube to an electrode at the top of 

 the vessel. The distance apart of the two plates could be 

 varied. For certain experiments the upper plate was thin 

 aluminium -foil (0*00024 cm. thick) stretched over a brass 

 ring, which was supported by knitting-needles to the 

 electrode. 



