﻿Low 
  Potential 
  Discharges 
  in 
  High 
  Vacua. 
  731 
  

  

  paths, 
  and 
  those 
  that 
  have 
  traversed 
  short 
  paths 
  with 
  the 
  same 
  

   mean 
  velocity. 
  

  

  There 
  does 
  not 
  seem 
  to 
  be 
  any 
  adequate 
  reason 
  why 
  these 
  

   hypotheses 
  should 
  be 
  adopted, 
  and 
  the 
  experiments 
  may 
  be 
  

   explained 
  more 
  simply 
  by 
  considering 
  the 
  processes 
  which 
  

   contribute 
  to 
  the 
  formation 
  of 
  ions 
  in 
  the 
  discharges 
  that 
  

   occur 
  when 
  a 
  potential 
  difference 
  of 
  the 
  order 
  of 
  400 
  volts 
  

   is 
  established 
  between 
  the 
  electrodes. 
  

  

  In 
  general, 
  the 
  formation 
  of 
  ions 
  in 
  discharges 
  is 
  due 
  to 
  

   different 
  effects 
  which 
  vary 
  in 
  importance 
  according 
  to 
  the 
  

   pressure 
  of 
  the 
  gas. 
  For 
  simplicity, 
  the 
  discharges 
  in 
  air 
  

   between 
  parallel 
  plates 
  one 
  centimetre 
  apart 
  may 
  be 
  con- 
  

   sidered. 
  At 
  high 
  pressures 
  exceeding 
  '5 
  millimetre 
  the 
  

   principal 
  effects 
  may 
  be 
  attributed 
  to 
  the 
  action 
  of 
  the 
  

   positive 
  and 
  negative 
  ions 
  on 
  the 
  molecules 
  of 
  the 
  gas. 
  

  

  If 
  ol 
  represents 
  the 
  number 
  of 
  ions 
  generated 
  by 
  a 
  negative 
  

   ion, 
  per 
  centimetre 
  of 
  its 
  path, 
  under 
  a 
  given 
  force, 
  /3 
  the 
  

   corresponding 
  number 
  for 
  positive 
  ions, 
  at 
  high 
  pressures, 
  a 
  

   and 
  ft 
  increase 
  when 
  the 
  force 
  is 
  increased 
  or 
  the 
  pressure 
  

   diminished. 
  Hence 
  as 
  the 
  pressure 
  diminishes 
  the 
  potential 
  

   required 
  to 
  produce 
  a 
  discharge 
  also 
  diminishes. 
  

  

  At 
  a 
  certain 
  pressure, 
  depending 
  on 
  the 
  force, 
  a 
  attains 
  a 
  

   maximum 
  value. 
  If 
  the 
  force 
  is 
  340 
  volts 
  per 
  centimetre, 
  a 
  

   has 
  a 
  maximum 
  value 
  when 
  the 
  pressure 
  is 
  *95 
  millimetre, 
  

   and 
  the 
  velocity 
  with 
  which 
  the 
  electron 
  collides 
  with 
  mole- 
  

   cules 
  is 
  sufficiently 
  great 
  to 
  produce 
  ionization 
  in 
  a 
  large 
  

   number 
  of 
  cases. 
  A 
  further 
  reduction 
  in 
  the 
  pressure 
  

   increases 
  the 
  velocity 
  of 
  the 
  electron 
  and 
  a 
  larger 
  proportion 
  

   of 
  the 
  molecules 
  with 
  which 
  it 
  collides 
  are 
  ionized, 
  but 
  the 
  

   total 
  number 
  of 
  collisions 
  is 
  reduced, 
  so 
  that 
  a 
  diminishes. 
  

   At 
  pressures 
  less 
  than 
  *1 
  millimetre 
  the 
  value 
  of 
  a 
  is 
  prac- 
  

   tically 
  proportional 
  to 
  the 
  pressure 
  and 
  independent 
  of 
  the 
  

   force, 
  provided 
  the 
  force 
  exceeds 
  300 
  volts 
  per 
  centimetre. 
  

   Thus 
  a 
  becomes 
  very 
  small 
  at 
  low 
  pressures 
  owing 
  to 
  the 
  fact 
  

   that 
  the 
  number 
  of 
  collisions 
  the 
  electron 
  makes 
  with 
  mole- 
  

   cules 
  in 
  traversing 
  the 
  distance 
  between 
  the 
  electrodes 
  is 
  very 
  

   small. 
  

  

  The 
  effect 
  of 
  the 
  positive 
  ions 
  does 
  not 
  diminish 
  when 
  the 
  

   pressure 
  is 
  less 
  than 
  *95 
  millimetre. 
  The 
  value 
  of 
  /3 
  increases 
  

   but 
  not 
  so 
  rapidly 
  as 
  to 
  supply 
  the 
  deficiency 
  caused 
  by 
  the 
  

   reduction 
  in 
  a, 
  so 
  that 
  at 
  '01 
  millimetre 
  the 
  potential 
  required 
  

   to 
  produce 
  the 
  discharge 
  becomes 
  very 
  large 
  and 
  ions 
  are 
  

   generated 
  by 
  other 
  processes, 
  in 
  addition 
  to 
  the 
  effects 
  pro- 
  

   duced 
  by 
  collisions 
  with 
  molecules 
  of 
  the 
  gas. 
  

  

  In 
  Strutt's 
  experiments 
  in 
  high 
  vacua 
  the 
  electrons 
  produce 
  

   very 
  little 
  effect 
  in 
  traversing 
  the 
  distance 
  R 
  — 
  r 
  between 
  the 
  

  

  