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  LX. 
  Low 
  Potential 
  Discharges 
  in 
  High 
  Vacua. 
  

   Gentlemen, 
  — 
  

  

  IN 
  a 
  paper 
  published 
  by 
  Strutt 
  in 
  the 
  'Proceedings 
  ' 
  of 
  the 
  

   Royal 
  Society 
  (Hon. 
  R. 
  J. 
  Strutt, 
  Proc. 
  Roy. 
  Soc. 
  

   vol. 
  lxxxix. 
  p. 
  68, 
  1913) 
  an 
  interesting 
  seizes 
  of 
  experiments 
  

   is 
  described 
  on 
  the 
  phenomena 
  of 
  low-potential 
  discharges 
  in 
  

   high 
  vacua. 
  The 
  experiments 
  show 
  definitely 
  that 
  one 
  simple 
  

   method 
  of 
  treating 
  some 
  of 
  the 
  observed 
  effects, 
  is 
  to 
  consider 
  

   the 
  case 
  of 
  a 
  discharge 
  between 
  two 
  coaxial 
  cylinders 
  when 
  a 
  

   magnetic 
  force 
  is 
  acting 
  in 
  a 
  direction 
  parallel 
  to 
  the 
  axis 
  of 
  

   the 
  cylinders. 
  

  

  When 
  the 
  gas 
  is 
  at 
  low 
  pressure 
  and 
  the 
  magnetic 
  field 
  is 
  

   zero, 
  a 
  potential 
  difference 
  of 
  the 
  order 
  of 
  200,000 
  volts 
  is 
  

   required 
  to 
  produce 
  a 
  discharge, 
  but 
  when 
  the 
  magnetic 
  field 
  

   is 
  created 
  the 
  discharge 
  passes 
  when 
  a 
  potential 
  difference 
  of 
  

   300 
  or 
  400 
  volts 
  is 
  established 
  between 
  the 
  cylinders. 
  In 
  

   the 
  latter 
  case 
  the 
  electrons 
  travel 
  spiral 
  paths. 
  

  

  Strutt 
  also 
  gives 
  a 
  theory 
  of 
  these 
  phenomena 
  in 
  which 
  he 
  

   adopts 
  the 
  view 
  that 
  in 
  an 
  ordinary 
  vacuum-tube, 
  without 
  

   magnetic 
  force, 
  electrons 
  starting 
  from 
  the 
  cathode 
  are 
  not 
  

   able 
  to 
  ionize 
  the 
  gas 
  until 
  they 
  have 
  travelled 
  a 
  certain 
  

   distance 
  represented 
  by 
  the 
  Crookes 
  dark 
  space. 
  

  

  This 
  hypothesis 
  is 
  not 
  in 
  accordance 
  with 
  the 
  results 
  of 
  

   simple 
  experiments 
  on 
  the 
  currents 
  obtained 
  between 
  parallel 
  

   plates. 
  

  

  The 
  normal 
  fall 
  of 
  potential 
  across 
  the 
  Crookes 
  dark 
  space 
  

   in 
  air 
  is 
  about 
  340 
  volts, 
  and 
  measurements 
  of 
  the 
  effects 
  

   obtained 
  by 
  collisions 
  show 
  that 
  an 
  electron 
  ionizes 
  a 
  mole- 
  

   cule 
  when 
  it 
  collides 
  with 
  a 
  velocity 
  acquired 
  under 
  a 
  

   potential 
  fall 
  of 
  25 
  volts. 
  The 
  electric 
  force 
  is 
  very 
  high 
  

   near 
  the 
  cathode 
  in 
  a 
  discharge-tube, 
  so 
  that 
  if 
  an 
  electron 
  

   started 
  with 
  zero 
  velocity, 
  it 
  would 
  acquire 
  sufficient 
  velocity 
  

   to 
  ionize 
  a 
  molecule 
  before 
  it 
  had 
  traversed 
  one 
  twentieth 
  

   part 
  of 
  the 
  dark 
  space. 
  There 
  can 
  thus 
  be 
  very 
  little 
  doubt 
  

   that 
  these 
  electrons 
  ionize 
  many 
  of 
  the 
  molecules 
  in 
  the 
  dark 
  

   space. 
  

  

  In 
  order 
  to 
  explain 
  the 
  effect 
  of 
  the 
  magnetic 
  force 
  the 
  

   electron, 
  after 
  starting 
  from 
  the 
  cathode, 
  is 
  supposed 
  to 
  

   acquire 
  the 
  property 
  of 
  ionizing 
  a 
  molecule 
  after 
  it 
  has 
  

   travelled 
  some 
  distance 
  in 
  its 
  spiral 
  path, 
  although 
  it 
  would 
  

   not 
  have 
  acquired 
  this 
  property 
  after 
  travelling 
  a 
  short 
  

   straight 
  path 
  under 
  the 
  action 
  of 
  the 
  same 
  potential 
  fall 
  when 
  

   the 
  magnetic 
  force 
  is 
  zero. 
  A 
  distinction 
  is 
  thus 
  drawn 
  

   between 
  the 
  properties 
  of 
  electrons 
  that 
  have 
  traversed 
  long 
  

  

  