﻿214 
  Welliseh 
  and 
  Woodrow 
  — 
  Columnar 
  Ionization. 
  

  

  Art. 
  XXIV. 
  — 
  Experiments 
  on 
  Columnar 
  Ionization 
  / 
  by 
  

   E. 
  M. 
  Wellisck, 
  Assistant 
  Professor 
  of 
  Physics, 
  Yale 
  

   University, 
  and 
  J. 
  W. 
  Woodrow, 
  Ph.D., 
  Yale 
  University. 
  

  

  Introduction. 
  

  

  1. 
  In 
  their 
  experiments 
  on 
  the 
  distribution 
  of 
  the 
  active 
  

   deposit 
  of 
  radium 
  in 
  an 
  electric 
  field, 
  Welliseh 
  and 
  Bronson"^ 
  

   found 
  that 
  the 
  fraction 
  of 
  the 
  total 
  amount 
  of 
  active 
  deposit 
  

   that 
  settled 
  on 
  the 
  cathode 
  increased 
  with 
  the 
  potential-differ- 
  

   ence 
  in 
  a 
  manner 
  quite 
  similar 
  to 
  the 
  increase 
  of 
  the 
  electric 
  

   current 
  which 
  passed 
  through 
  the 
  gas 
  during 
  the 
  process 
  of 
  

   activation. 
  The 
  curve 
  connecting 
  the 
  cathode 
  activity 
  and 
  the 
  

   potential-difference 
  exhibited 
  the 
  characteristic 
  ' 
  lack 
  of 
  satura- 
  

   tion 
  ' 
  which 
  had 
  previously 
  been 
  investigated 
  by 
  Bragg, 
  

   Moulin, 
  and 
  others 
  in 
  the 
  case 
  of 
  the 
  electric 
  current 
  due 
  to 
  

   alpha-ray 
  ionization. 
  This 
  experimental 
  result 
  suggested 
  the 
  

   probability 
  that 
  the 
  electric 
  current 
  would 
  attain 
  its 
  saturation 
  

   value 
  only 
  when 
  all 
  the 
  active 
  deposit 
  settled 
  on 
  the 
  cathode 
  ; 
  

   or 
  more 
  generally, 
  that 
  the 
  cathode 
  activity 
  was 
  a 
  measure 
  of 
  

   the 
  degree 
  of 
  the 
  saturation 
  of 
  the. 
  electric 
  current. 
  

  

  On 
  investigating 
  experimentally 
  the 
  activity 
  distribution 
  

   when 
  the 
  radium 
  emanation 
  was 
  present 
  in 
  air 
  at 
  a 
  pressure 
  of 
  

   260 
  mm 
  , 
  it 
  was 
  found 
  that 
  for 
  potentials 
  above 
  80 
  volts 
  the 
  

   cathode 
  activity 
  did 
  not 
  perceptibly 
  increase 
  ; 
  but 
  the 
  curves 
  

   connecting 
  the 
  percentage 
  of 
  cathode 
  activity 
  and 
  the 
  ioniza- 
  

   tion 
  with 
  the 
  potential-difference 
  had 
  the 
  characteristic 
  hori- 
  

   zontal 
  portion 
  which 
  suggests 
  that 
  saturation 
  has 
  been 
  attained. 
  

   On 
  the 
  other 
  hand, 
  the 
  measurements 
  showed 
  that 
  there 
  was 
  

   still 
  about 
  17 
  per 
  cent 
  of 
  the 
  active 
  deposit 
  which 
  failed 
  to 
  

   reach 
  the 
  cathode. 
  

  

  These 
  experimental 
  results 
  appeared 
  to 
  indicate 
  that 
  the 
  

   saturation 
  obtained 
  for 
  alpha-particle 
  ionization 
  at 
  pressures 
  

   below 
  about 
  one-half 
  of 
  an 
  atmosphere 
  was 
  only 
  apparent 
  : 
  the 
  

   results 
  were 
  most 
  suitably 
  explained 
  on 
  the 
  supposition 
  

   that 
  part 
  of 
  the 
  electric 
  current 
  observed 
  at 
  one 
  atmos- 
  

   phere 
  was 
  due 
  to 
  the 
  ionization 
  by 
  collision 
  with 
  molecules 
  

   which, 
  though 
  electrically 
  neutral, 
  had 
  been 
  brought 
  into 
  an 
  

   unstable 
  condition 
  by 
  the 
  action 
  of 
  the 
  alpha 
  particle. 
  These 
  

   ' 
  neutrons' 
  would 
  be 
  in 
  the 
  most 
  favorable 
  position 
  for 
  ioniza- 
  

   tion 
  when 
  the 
  electric 
  field 
  was 
  parallel 
  to 
  the 
  alpha-ray 
  

   column 
  and 
  also 
  when 
  the 
  pressure 
  was 
  not 
  too 
  low. 
  

  

  On 
  this 
  view, 
  the 
  characteristic 
  upward 
  slope 
  of 
  the 
  curve 
  

   connecting 
  the 
  electric 
  current 
  with 
  the 
  field 
  for 
  alpha 
  particle 
  

  

  * 
  Welliseh 
  and 
  Bronson, 
  Phil. 
  Mag. 
  (6), 
  xxiii, 
  p. 
  714, 
  May, 
  1912. 
  

  

  