﻿Gaseous 
  Ionization 
  and 
  Pressure. 
  889 
  

  

  ionization 
  current 
  reaches 
  a 
  maximum 
  at 
  a 
  certain 
  pressure, 
  

   and 
  afterwards 
  diminishes 
  with 
  a 
  further 
  increase 
  in 
  the 
  

   pres-ure. 
  For 
  example 
  he 
  quotes 
  a 
  result 
  which 
  indicated, 
  

   in 
  the 
  case 
  of 
  air, 
  a 
  maximum 
  current 
  at 
  70 
  atmospheres, 
  

   when 
  the 
  potential-difference 
  between 
  two 
  concentric 
  cylin- 
  

   drical 
  electrodes 
  separated 
  by 
  6 
  mms. 
  was 
  18 
  volts 
  ; 
  with 
  a 
  

   potential- 
  difference 
  of 
  1000 
  volts, 
  the 
  maximum 
  current 
  

   occurred 
  at 
  150 
  atmospheres. 
  Xow 
  we 
  find 
  (p. 
  883) 
  that 
  for 
  

   the 
  intensity 
  of 
  ionization 
  we 
  used 
  — 
  and 
  it 
  was 
  small 
  — 
  air 
  at 
  

   15 
  atmospheres 
  requires 
  a 
  field 
  of 
  2500 
  volts 
  per 
  cm. 
  \o 
  

   produce 
  a 
  saturation 
  current. 
  Thus 
  the 
  air 
  in 
  Erikson's 
  

   experiments, 
  unless 
  his 
  degree 
  of 
  ionization 
  was 
  exceptionally 
  

   small, 
  was 
  unsaturated 
  for 
  pressures 
  over 
  about 
  15 
  atmospheres. 
  

   His 
  published 
  abstract 
  is 
  silent 
  on 
  the 
  point, 
  but 
  his 
  results 
  

   are 
  such 
  as 
  would 
  be 
  expected 
  if 
  the 
  velocity 
  of 
  the 
  ions 
  

   diminished 
  as 
  the 
  pressure 
  increased, 
  and 
  in 
  consequence 
  a 
  

   larger 
  and 
  larger 
  proportion 
  of 
  the 
  ions 
  present 
  recombined 
  

   and 
  never 
  reached 
  the 
  electrodes. 
  

  

  Summary. 
  

  

  1. 
  The 
  ionization-pressure 
  curves 
  have 
  been 
  determined 
  

   over 
  a 
  range 
  of 
  pressures 
  from 
  1 
  to 
  17 
  atmospheres 
  for 
  

   hydrogen, 
  air, 
  and 
  carbon 
  dioxide, 
  when 
  contained 
  in 
  an 
  

   aluminium 
  vessel 
  and 
  exposed 
  to 
  hard 
  7 
  rays 
  from 
  radium. 
  

  

  2. 
  The 
  amount 
  of 
  ionization 
  in 
  hydrogen 
  is 
  approximately 
  

   proportional 
  to 
  the 
  pressure 
  ; 
  in 
  air 
  it 
  increases 
  less 
  rapidly 
  

   than 
  the 
  pressure 
  ; 
  in 
  carbon 
  dioxide 
  still 
  less 
  rapidly 
  than 
  

   the 
  pressure, 
  especially 
  at 
  the 
  higher 
  pressures. 
  

  

  3. 
  "We 
  conclude 
  from 
  a 
  discussion 
  of 
  these 
  results 
  that 
  the 
  

   ionization 
  arising 
  from 
  

  

  (a) 
  the 
  7 
  rays, 
  which 
  is 
  small 
  in 
  amount, 
  varies 
  as 
  the 
  

  

  pressure 
  ; 
  

  

  (b) 
  the 
  secondary 
  /5 
  rays 
  (excited 
  by 
  the 
  7 
  rays) 
  from 
  

  

  the 
  aluminium 
  electrodes, 
  varies 
  as 
  the 
  pressure 
  ; 
  

   while 
  that 
  excited 
  in 
  the 
  gas 
  itself 
  produces 
  

   ionization 
  small 
  in 
  amount 
  and 
  proportional 
  to 
  

   the 
  square 
  of 
  the 
  pressure 
  ; 
  

  

  (c) 
  a 
  soft 
  tertiary 
  radiation, 
  increases 
  less 
  rapidlv 
  than 
  

  

  the 
  pressure. 
  

  

  4. 
  It 
  is 
  interred 
  from 
  these 
  results 
  that 
  when 
  j3 
  rays 
  

   (velocity 
  about 
  half 
  velocity 
  of 
  light) 
  are 
  absorbed 
  they 
  excite 
  

   soft 
  secondary 
  ft 
  radiation. 
  

  

  We 
  wish 
  to 
  thank 
  Professor 
  Thomson 
  for 
  the 
  interest 
  he 
  

   took 
  in 
  these 
  experiments. 
  

   The 
  Cavendish 
  Laboratory, 
  

   Cambridge. 
  

  

  