﻿Discharges 
  in 
  Higli 
  Vacua. 
  905 
  

  

  slightly 
  decreases 
  the 
  current, 
  showing 
  that 
  some 
  o£ 
  the 
  

   electrons 
  from 
  the 
  cathode 
  do 
  not 
  reach 
  the 
  anode 
  when 
  the 
  

   magnetic 
  "field 
  is 
  established. 
  At 
  a 
  pressure 
  of: 
  '015 
  mm. 
  

   with 
  this 
  voltage 
  ( 
  — 
  40 
  volts) 
  the 
  magnetic 
  field 
  has 
  no 
  

   effect 
  ; 
  evidently 
  the 
  decrease 
  due 
  to 
  some 
  of 
  the 
  electrons 
  

   from 
  the 
  cathode 
  not 
  reaching 
  the 
  anode 
  is 
  now 
  just 
  balanced 
  

   by 
  the 
  extra 
  ionization 
  produced 
  by 
  collisions 
  in 
  the 
  magnetic 
  

   field. 
  This 
  extra 
  ionization 
  is 
  great 
  enough 
  at 
  the 
  higher 
  

   pressure 
  of 
  *191 
  mm. 
  to 
  cause 
  an 
  increase 
  in 
  the 
  current 
  

   between 
  the 
  electrodes. 
  At 
  a 
  pressure 
  of 
  1*39 
  mm. 
  the 
  

   mean 
  free 
  path 
  of 
  the 
  electrons 
  is 
  so 
  small 
  that 
  they 
  do 
  not 
  

   acquire 
  sufficient 
  velocity 
  under 
  the 
  potential 
  difference 
  of 
  

   40 
  volts 
  to 
  ionize 
  the 
  molecules 
  with 
  which 
  they 
  collide, 
  but 
  

   it 
  will 
  be 
  seen 
  from 
  the 
  table 
  that 
  with 
  a 
  potential 
  difference 
  

   of 
  220 
  volts 
  this 
  phenomenon 
  again 
  occurs. 
  

  

  We 
  thus 
  see 
  that 
  in 
  order 
  to 
  obtain 
  an 
  increased 
  current 
  

   on 
  establishing 
  a 
  magnetic 
  field 
  in 
  this 
  experiment, 
  it 
  is 
  

   necessary 
  to 
  have 
  the 
  conditions 
  of 
  gas 
  pressure 
  and 
  potential 
  

   difference 
  suitable 
  for 
  the 
  production 
  of 
  ionization 
  by 
  

   collision 
  ; 
  also 
  the 
  magnetic 
  field 
  must 
  not 
  be 
  too 
  strong, 
  

   otherwise 
  no 
  effect, 
  or 
  even 
  a 
  diminution 
  in 
  the 
  current, 
  may 
  

   be 
  produced. 
  The 
  result 
  obtained 
  will 
  also 
  depend 
  upon 
  the 
  

   distance 
  between 
  the 
  electrodes. 
  

  

  An 
  arrangement 
  of 
  coaxial 
  cylinders 
  with 
  a 
  magnetic 
  force 
  

   parallel 
  to 
  the 
  axis, 
  as 
  used 
  by 
  Professor 
  Strutt, 
  gives 
  a 
  more 
  

   simple 
  method 
  of 
  investigating 
  this 
  phenomenon, 
  for 
  com- 
  

   plications 
  are 
  introduced 
  when 
  small 
  parallel 
  plates 
  are 
  

   used, 
  but 
  one 
  would 
  expect 
  the 
  discharge 
  in 
  Professor 
  

   Strutt's 
  experiment 
  to 
  vary 
  with 
  the 
  gas 
  pressure, 
  the 
  

   potential 
  difference, 
  the 
  magnetic 
  force, 
  and 
  the 
  distance 
  

   between 
  the 
  electrodes 
  in 
  the 
  same 
  general 
  manner 
  as 
  the 
  

   current 
  in 
  the 
  experiments 
  described 
  above. 
  

  

  The 
  theory 
  adopted 
  by 
  Professor 
  Strutt 
  to 
  explain 
  his 
  

   results 
  has 
  been 
  criticised 
  by 
  Professor 
  Townsend 
  in 
  the 
  

   October 
  number 
  of 
  the 
  Philosophical 
  Magazine. 
  It 
  will 
  be 
  

   seen 
  that 
  the 
  explanation 
  which 
  the 
  writer 
  found 
  accounted 
  

   for 
  the 
  results 
  of 
  his 
  experiments 
  is 
  similar 
  to 
  that 
  given 
  by 
  

   Professor 
  Townsend, 
  and 
  depends 
  upon 
  the 
  fact 
  that 
  the 
  

   lengthened 
  path 
  in 
  a 
  magnetic 
  field 
  gives 
  an 
  electron 
  a 
  

   greater 
  opportunity 
  of 
  producing 
  ionization 
  by 
  collision, 
  

   because 
  it 
  meets 
  with 
  more 
  molecules. 
  

  

  The 
  Cavendish 
  Laboratory. 
  

   Cambridge. 
  

  

  