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  LY* 
  The 
  Quality 
  of 
  the 
  Secondary 
  Ionization 
  due 
  to 
  ft 
  Rays. 
  

   ByW. 
  H. 
  Bjragg, 
  M.A., 
  F.R.S., 
  Elder 
  Professor 
  of 
  

   Mathematics 
  and 
  Physics 
  in 
  the 
  University 
  of 
  Adelaide, 
  

   and 
  J. 
  P. 
  Y. 
  Madsen", 
  B.Sc, 
  B.E., 
  Lecturer 
  on 
  Electrical 
  

   Engineering 
  *. 
  

  

  THE 
  so-called 
  secondary 
  radiation 
  due 
  to 
  ft 
  rays 
  has 
  been 
  

   studied 
  by 
  many 
  observers. 
  Quite 
  recently 
  McClelland 
  

   and 
  H. 
  W. 
  Schmidt 
  have 
  contributed 
  important 
  sets 
  of 
  papers 
  

   upon 
  the 
  subject. 
  

  

  It 
  is 
  usual 
  to 
  estimate 
  the 
  amount 
  or 
  the 
  energy 
  of 
  the 
  

   primary 
  and 
  secondary 
  streams 
  by 
  measuring 
  the 
  ionization 
  

   produced 
  within 
  an 
  ordinary 
  ionization-chamber 
  placed 
  in 
  

   the 
  path 
  of 
  the 
  rays, 
  the 
  principal 
  reason 
  being 
  that 
  such 
  

   measurements 
  can 
  be 
  made 
  with 
  convenience 
  and 
  accuracy. 
  

  

  But 
  it 
  is 
  only 
  under 
  very 
  special 
  conditions 
  that 
  this 
  

   procedure 
  is 
  legitimate, 
  because 
  the 
  ft 
  particle 
  produces 
  more 
  

   ions 
  in 
  traversing 
  a 
  given 
  space 
  when 
  its 
  velocity 
  is 
  small 
  

   than 
  when 
  it 
  is 
  great 
  ; 
  in 
  other 
  words, 
  the 
  ionization 
  increases 
  

   as 
  the 
  speed 
  diminishes. 
  If 
  two 
  streams 
  of 
  ft 
  radiation 
  are 
  

   to 
  be 
  compared 
  by 
  this 
  method, 
  it 
  is 
  one 
  of 
  the 
  essential 
  

   conditions 
  of 
  success 
  that 
  the 
  velocities 
  of 
  the 
  two 
  streams 
  

   shall 
  be 
  the 
  same, 
  or 
  similarly 
  distributed. 
  This 
  would 
  not 
  

   be 
  necessary 
  if 
  the 
  ionization-chamber 
  were 
  made 
  so 
  large 
  

   that 
  even 
  the 
  fastest 
  ft 
  particles 
  spent 
  their 
  ionizing 
  powers 
  

   within 
  it, 
  and 
  if 
  the 
  whole 
  ionization 
  produced 
  by 
  a 
  

   ft 
  particle 
  might 
  be 
  taken 
  as 
  a 
  measure 
  of 
  its 
  energy. 
  But 
  

   it 
  is 
  usual 
  to 
  employ 
  a 
  small 
  chamber, 
  placed 
  comparatively 
  

   near 
  to 
  the 
  source 
  of 
  the 
  rays 
  ; 
  and 
  in 
  this 
  way 
  an 
  unfair 
  

   advantage 
  is 
  given 
  to 
  the 
  particles 
  of 
  smaller 
  velocity. 
  

  

  The 
  experiments 
  to 
  be 
  described 
  in 
  this 
  paper 
  show 
  that 
  

   the 
  secondary 
  radiations 
  returned 
  by 
  different 
  substances 
  

   struck 
  by 
  the 
  same 
  primary 
  stream 
  are 
  not 
  uniform 
  in 
  quality, 
  

   but 
  vary 
  from 
  substance 
  to 
  substance. 
  When 
  the 
  atomic 
  

   weight 
  of 
  the 
  substance 
  is 
  small, 
  the 
  radiation 
  is 
  relatively 
  

   soft 
  ; 
  it 
  contains 
  a 
  larger 
  proportion 
  of 
  less 
  penetrating 
  rays. 
  

   If, 
  therefore, 
  the 
  constants 
  of 
  secondary 
  ionization 
  are 
  

   measured 
  in 
  the 
  usual 
  way, 
  they 
  are 
  exaggerated 
  in 
  the 
  case 
  

   of 
  substances 
  of 
  low 
  atomic 
  weight, 
  as 
  compared 
  with 
  those 
  

   the 
  atoms 
  of 
  which 
  are 
  heavier. 
  

  

  For 
  the 
  purpose 
  of 
  observing 
  the 
  secondary 
  ionization 
  due 
  

  

  * 
  From 
  the 
  Transactions 
  of 
  the 
  Koyal 
  Society 
  of 
  South 
  Australia, 
  

   vol. 
  xxxi. 
  1907. 
  Communicated 
  by 
  the 
  Authors. 
  

  

  