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  Quality 
  of 
  the 
  Secondary 
  Ionization 
  due 
  to 
  ft 
  Bays. 
  693 
  

  

  to 
  ft 
  rays 
  we 
  have 
  adopted 
  the 
  form 
  of 
  apparatus 
  shown 
  

  

  in 
  the 
  figure. 
  The 
  plate 
  

   Fig. 
  1. 
  at 
  A 
  scatters 
  in 
  all 
  direc- 
  

  

  tions 
  the 
  radiation 
  which 
  

   it 
  receives 
  from 
  the 
  ra- 
  

   dium 
  at 
  R. 
  Nearly 
  all 
  

   the 
  returned 
  particles 
  

   strike 
  the 
  hemispherical 
  

   wall 
  BB' 
  of 
  the 
  ioniza- 
  

   tion-chamber, 
  the 
  larger 
  

   hemisphere 
  CC 
  being- 
  

   connected 
  to 
  the 
  electro- 
  

   meter. 
  The 
  usual 
  shield- 
  

   ing 
  devices 
  are 
  used, 
  but 
  not 
  shown 
  in 
  the 
  figure. 
  This 
  

   arrangement 
  gathers 
  in 
  the 
  effects 
  of 
  all 
  the 
  secondary 
  rays, 
  

   so 
  that 
  a 
  measurable 
  effect 
  is 
  obtained, 
  even 
  when 
  only 
  a 
  

   small 
  quantity 
  of 
  radium 
  is 
  used. 
  

  

  When 
  the 
  cup 
  or 
  dome 
  BB' 
  is 
  made 
  of 
  very 
  thin 
  Al 
  foil, 
  

   stretched 
  on 
  a 
  frame 
  of 
  a 
  few 
  fine 
  wires, 
  the 
  effects 
  obtained 
  

   by 
  placing 
  plates 
  of 
  different 
  metals 
  at 
  A 
  approximate 
  to 
  

   those 
  given 
  by 
  McClelland 
  — 
  that 
  is 
  to 
  say, 
  the 
  curve 
  which 
  

   shows 
  the 
  relation 
  between 
  secondary 
  ionization 
  and 
  atomic 
  

   weight 
  is 
  of 
  the 
  same 
  general 
  form 
  as 
  McClelland's, 
  though 
  

   somewhat 
  flatter. 
  But 
  when 
  the 
  dome 
  is 
  thickened 
  by 
  the 
  

   addition 
  of 
  layers 
  of 
  tinfoil, 
  there 
  is 
  a 
  considerable 
  alteration. 
  

   The 
  addition 
  of 
  tinfoil 
  of 
  a 
  tenth 
  of 
  a 
  millimetre 
  in 
  thick- 
  

   ness 
  is 
  sufficient 
  to 
  make 
  the 
  curve 
  almost 
  linear, 
  and 
  the 
  

   ionization 
  is 
  then 
  nearly 
  proportional 
  to 
  the 
  atomic 
  weight 
  

   of 
  the 
  radiator. 
  

  

  The 
  figures 
  in 
  the 
  following 
  table 
  show 
  the 
  results 
  of 
  an 
  

   experiment 
  of 
  this 
  kind. 
  They 
  give 
  the 
  ionization 
  currents, 
  

   on 
  an 
  arbitrary 
  scale, 
  for 
  different 
  radiators 
  and 
  different 
  

   thicknesses 
  of 
  the 
  dome. 
  The 
  smaller 
  figures 
  are. 
  only 
  

  

  >p 
  

  

  roxnn? 
  

  

  ite 
  : 
  — 
  

  

  

  

  

  

  

  

  

  

  

  

  Pb. 
  

  

  Ag. 
  

  

  Zn. 
  

  

  Fe. 
  

  

  S. 
  

  

  Al. 
  

  

  C. 
  

  

  1. 
  

  

  Thin 
  Al 
  leaf 
  

  

  .. 
  1300 
  

  

  1010 
  

  

  798 
  

  

  679 
  

  

  484 
  

  

  391 
  

  

  166 
  

  

  2. 
  

  

  Sn 
  foil 
  

  

  •0065 
  mm. 
  . 
  

  

  .. 
  1124 
  

  

  829 
  

  

  627 
  

  

  519 
  

  

  352 
  

  

  276 
  

  

  141 
  

  

  3. 
  

  

  ., 
  

  

  •043 
  ,,- 
  

  

  .. 
  414 
  

  

  272 
  

  

  189 
  

  

  145 
  

  

  85 
  

  

  57 
  

  

  21 
  

  

  4. 
  

  

  ,, 
  

  

  •080 
  „ 
  

  

  .. 
  210 
  

  

  129 
  

  

  87 
  

  

  65 
  

  

  35 
  

  

  21 
  

  

  8 
  

  

  5. 
  

  

  ,, 
  

  

  •116 
  „ 
  

  

  .. 
  125 
  

  

  71 
  

  

  45 
  

  

  32 
  

  

  19 
  

  

  12 
  

  

  5 
  

  

  0. 
  

  

  , 
  

  

  •153 
  „ 
  

  

  .. 
  76 
  

  

  42 
  

  

  29 
  

  

  21 
  

  

  13 
  

  

  7 
  

  

  5 
  

  

  Assuming 
  that 
  the 
  returned 
  radiation 
  is 
  really 
  due 
  to 
  a 
  

   scattering 
  of 
  the 
  primary 
  beam, 
  there 
  are 
  two 
  ways 
  of 
  

   interpreting 
  these 
  results. 
  The 
  ft 
  radiation 
  of 
  radium 
  is 
  

  

  