﻿904 
  Dr. 
  F. 
  Horton 
  on 
  Low 
  Potential 
  

  

  The 
  effect 
  was 
  not 
  very 
  fully 
  investigated, 
  for 
  it 
  seemed 
  to 
  

   be 
  readily 
  explained 
  on 
  the 
  theory 
  of 
  ionization 
  by 
  collision'. 
  

   Let 
  us 
  first 
  imagine 
  the 
  experiment 
  to 
  be 
  performed 
  in 
  a 
  

   perfect 
  vacuum 
  where 
  there 
  are 
  no 
  gas 
  molecules 
  to 
  interfere 
  

   with 
  the 
  motion 
  of 
  the 
  electrons. 
  In 
  the 
  absence 
  of 
  the 
  

   magnetic 
  field, 
  the 
  electrons 
  from 
  the 
  cathode 
  go 
  normally 
  

   outwards 
  towards 
  the 
  anodes. 
  When, 
  however, 
  the 
  magnetic 
  

   force 
  acts, 
  the 
  electrons 
  are 
  deflected 
  from 
  their 
  initial 
  paths, 
  

   those 
  from 
  one 
  side 
  of 
  the 
  heated 
  strip 
  being 
  bent 
  upwards 
  

   and 
  those 
  from 
  the 
  other 
  side 
  downwards. 
  The 
  curvature 
  of 
  

   the 
  paths 
  of 
  the 
  electrons 
  can 
  be 
  increased 
  by 
  increasing 
  the 
  

   magnetic 
  field 
  until 
  they 
  are 
  so 
  curled 
  up 
  that 
  none 
  reach 
  

   the 
  anodes, 
  and 
  thus 
  when 
  the 
  magnetic 
  field 
  reaches 
  a 
  

   certain 
  value 
  (the 
  other 
  conditions 
  being 
  constant) 
  there 
  is 
  a 
  

   more 
  or 
  less 
  sudden 
  drop 
  in 
  the 
  current 
  across 
  the 
  tube. 
  

   The 
  same 
  result 
  may 
  also 
  be 
  obtained 
  by 
  keeping 
  the 
  applied 
  

   magnetic 
  force 
  constant 
  and 
  gradually 
  reducing 
  the 
  potential 
  

   difference 
  — 
  this 
  being 
  the 
  procedure 
  adopted 
  by 
  Sir 
  J. 
  J. 
  

   Thomson 
  in 
  his 
  determinations 
  of 
  the 
  specific 
  charge 
  of 
  

   the 
  ions 
  produced 
  by 
  ultra-violet 
  light 
  and 
  by 
  an 
  incandescent 
  

   carbon 
  filament*. 
  If, 
  however, 
  gas 
  is 
  present 
  in 
  the 
  dis- 
  

   charge-tube, 
  the 
  electrons 
  from 
  the 
  glowing 
  cathode 
  collide 
  

   with 
  the 
  gaseous 
  molecules, 
  and 
  if 
  they 
  collide 
  with 
  sufficient 
  

   energy, 
  new 
  ions 
  will 
  be 
  produced 
  by 
  these 
  collisions. 
  If 
  

   the 
  gas 
  pressure 
  in 
  the 
  apparatus 
  is 
  so 
  low 
  that 
  the 
  mean 
  

   free 
  path 
  of 
  the 
  electrons 
  is 
  about 
  equal 
  to 
  the 
  distance 
  

   between 
  the 
  electrodes, 
  the 
  number 
  of 
  collisions 
  will 
  be 
  

   comparatively 
  few, 
  but 
  when 
  the 
  magnetic 
  field 
  is 
  created 
  

   the 
  paths 
  of 
  the 
  electrons 
  are 
  lengthened, 
  and 
  the 
  number 
  of 
  

   collisions 
  that 
  each 
  of 
  the 
  initial 
  electrons 
  experiences 
  is 
  

   consequently 
  increased. 
  If 
  the 
  applied 
  potential 
  difference 
  

   is 
  great 
  enough 
  to 
  give 
  the 
  electrons 
  from 
  the 
  cathode 
  

   sufficient 
  velocity 
  to 
  ionize 
  the 
  molecules 
  with 
  which 
  they 
  

   collide, 
  an 
  increased 
  ionization 
  is 
  produced 
  when 
  the 
  magnetic 
  

   field 
  is 
  established, 
  and 
  consequently 
  an 
  increase 
  occurs 
  in 
  

   the 
  current 
  through 
  the 
  tube. 
  If, 
  however* 
  the 
  gas 
  pressure 
  

   is 
  exceedingly 
  low, 
  so 
  that 
  the 
  mean 
  free 
  path 
  of 
  the 
  

   electrons 
  is 
  much 
  larger 
  than 
  the 
  distance 
  between 
  the 
  

   electrodes, 
  the 
  curved 
  paths 
  produced 
  by 
  the 
  magnetic 
  field 
  

   may 
  not 
  lead 
  to 
  many 
  more 
  collisions, 
  and 
  the 
  creation 
  of 
  

   the 
  magnetic 
  field 
  will 
  therefore 
  have 
  no 
  effect 
  upon 
  the 
  

   current. 
  This 
  accounts 
  for 
  the 
  absence 
  of 
  effect 
  recorded 
  in 
  

   the 
  above 
  table 
  with 
  a 
  gas 
  pressure 
  of 
  *002 
  mm. 
  and 
  a 
  

   potential 
  difference 
  of 
  220 
  volts. 
  It 
  will 
  be 
  seen 
  that 
  at 
  

   this 
  same 
  pressure 
  but 
  with 
  —40 
  volt<, 
  the 
  magnetic 
  field 
  

  

  * 
  Phil. 
  Mag. 
  [5] 
  vol. 
  xlviii. 
  p. 
  547 
  (1899). 
  

  

  