﻿Currents 
  through 
  Air 
  Films. 
  353 
  

  

  seems 
  inconsistent 
  with 
  an 
  explanation 
  based 
  on 
  the 
  assumption 
  

   of 
  metallic 
  contact. 
  

  

  Another 
  possible 
  explanation 
  would 
  be 
  the 
  presence 
  of 
  dust 
  

   or 
  ions 
  in 
  the 
  air 
  contained 
  in 
  the 
  copper 
  box. 
  Every 
  effort 
  

   was 
  made 
  to 
  free 
  the 
  air 
  from 
  dust, 
  but, 
  no 
  doubt, 
  some 
  still 
  

   remained. 
  That 
  the 
  current 
  is 
  not 
  due 
  to 
  dust 
  seems 
  to 
  be 
  

   proved 
  by 
  the 
  following 
  experiment. 
  When 
  the 
  applied 
  

   potential 
  was 
  100 
  volts, 
  the 
  pressure 
  was 
  reduced 
  to 
  about 
  

   2 
  millimetres 
  of 
  mercury 
  and 
  kept 
  at 
  that 
  for 
  about 
  

   20 
  minutes, 
  with 
  the 
  result 
  that 
  the 
  current 
  stopped 
  com- 
  

   pletely, 
  or 
  was 
  so 
  small 
  as 
  not 
  appreciably 
  to 
  affect 
  the 
  

   electrometer. 
  The 
  pressure 
  was 
  then 
  increased 
  very 
  gra- 
  

   dually 
  by 
  allowing 
  air 
  to 
  go 
  slowly 
  through 
  the 
  drying- 
  

   apparatus 
  and 
  the 
  glass-wool; 
  and 
  it 
  was 
  not 
  until 
  the 
  

   pressure 
  became 
  nearly 
  half 
  an 
  atmosphere 
  that 
  an 
  appre- 
  

   ciable 
  current 
  was 
  observed. 
  It 
  then 
  increased 
  rapidly 
  with 
  

   increase 
  of 
  pressure, 
  and 
  finally, 
  at 
  atmospheric 
  pressure, 
  

   became 
  nearly 
  what 
  it 
  was 
  before 
  the 
  box 
  was 
  exhausted. 
  

   In 
  all 
  cases 
  A 
  and 
  B 
  were 
  kept 
  earthed 
  unless 
  when 
  a 
  

   measurement 
  of 
  current 
  was 
  being 
  made, 
  the 
  measurement 
  

   lasting 
  for 
  about 
  half 
  a 
  minute. 
  

  

  J£ 
  the 
  conductivity 
  were 
  due 
  to 
  dust, 
  there 
  does 
  not 
  seem 
  

   to 
  be 
  any 
  reason 
  why 
  there 
  should 
  be 
  such 
  an 
  enormous 
  

   lag 
  between 
  the 
  increase 
  of 
  pressure 
  and 
  the 
  increase 
  of 
  

   potential. 
  

  

  If 
  the 
  conductivity 
  were 
  due 
  to 
  the 
  presence 
  of 
  ions 
  

   throughout 
  the 
  air 
  in 
  the 
  copper 
  box, 
  it 
  ought 
  to 
  be 
  possible 
  

   to 
  diminish 
  it 
  by 
  applying 
  a 
  high 
  voltage 
  to 
  one 
  of 
  the 
  bars 
  

   so 
  as 
  to 
  remove 
  them. 
  But 
  if 
  this 
  is 
  done 
  the 
  current 
  

   afterwards 
  becomes 
  much 
  greater, 
  provided 
  the 
  high 
  voltage 
  

   is 
  not 
  applied 
  too 
  long. 
  

  

  Everything, 
  therefore, 
  points 
  to 
  the 
  formation 
  of 
  ions 
  in 
  

   the 
  surfaces 
  of 
  the 
  film 
  of 
  air 
  in 
  contact 
  with 
  the 
  ends 
  of 
  the 
  

   bars, 
  which 
  become 
  available 
  as 
  carriers 
  when 
  the 
  opposing 
  

   surfaces 
  are 
  close 
  enough. 
  These 
  ions 
  can 
  be 
  removed 
  by 
  

   removing 
  the 
  air; 
  and 
  when 
  the 
  air 
  is 
  introduced 
  again 
  they 
  

   are 
  again 
  formed, 
  but 
  not 
  instantaneously. 
  Their 
  formation 
  

   requires 
  time. 
  It 
  is 
  probable 
  that 
  such 
  layers 
  of 
  ions 
  exist 
  

   at 
  the 
  surfaces 
  of 
  all 
  bodies 
  in 
  contact 
  with 
  the 
  atmosphere, 
  

   and 
  that 
  they 
  play 
  a 
  prominent 
  part 
  in 
  coherer 
  action 
  and 
  in 
  

   all 
  cases 
  of 
  the 
  discharge 
  of 
  bodies 
  by 
  contact. 
  

  

  