﻿Positive 
  Rays, 
  667 
  

  

  Magneto- 
  Cathodic 
  Rays, 
  

  

  Among 
  the 
  rays 
  which 
  sometimes 
  occur 
  near 
  the 
  cathode 
  4 
  

   are 
  some 
  observed 
  by 
  Villard 
  ( 
  Comptes 
  Rendus, 
  cix. 
  p. 
  42, 
  

   1905) 
  and 
  called 
  by 
  him 
  magneto-cathodic 
  rays. 
  These 
  

   rays 
  occur 
  when 
  the 
  discharge-tube 
  is 
  placed 
  in 
  a 
  strong- 
  

   magnetic 
  field 
  ; 
  they 
  are 
  in 
  the 
  direction 
  of 
  the 
  lines 
  of 
  

   magnetic 
  force, 
  and 
  when 
  subject 
  to 
  an 
  external 
  electric 
  field 
  

   they 
  are 
  displaced 
  in 
  a 
  direction 
  at 
  right 
  angles 
  to 
  the 
  electric 
  

   and 
  also 
  to 
  the 
  magnetic 
  force. 
  Ions 
  moving 
  through 
  a 
  

   medium 
  in 
  which 
  they 
  are 
  under 
  the 
  action 
  of 
  electric 
  and 
  

   magnetic 
  forces, 
  and 
  which 
  resists 
  their 
  motion 
  with 
  a 
  force 
  

   proportional 
  to 
  their 
  velocity, 
  would 
  behave 
  exactly 
  in 
  this 
  

   manner. 
  For, 
  just 
  as 
  a 
  stone 
  falling 
  through 
  a 
  resisting 
  

   medium 
  moves 
  at 
  first 
  with 
  nearly 
  uniform 
  acceleration 
  but 
  

   after 
  a 
  time 
  settles 
  down 
  into 
  a 
  state 
  where 
  the 
  velocity 
  is 
  

   constant 
  and 
  equal 
  to 
  what 
  is 
  known 
  as 
  the 
  limiting 
  velocity, 
  

   so 
  ions, 
  when 
  exposed 
  to 
  electric 
  and 
  magnetic 
  forces 
  and 
  to 
  

   a 
  resistance 
  proportional 
  to 
  their 
  velocity, 
  will 
  after 
  a 
  time 
  

   settle 
  down 
  to 
  a 
  state 
  where 
  the 
  velocity 
  is 
  uniform. 
  The 
  

   time 
  required 
  to 
  reach 
  this 
  state 
  is 
  inversely 
  proportional 
  to 
  

   the 
  resistance 
  when 
  the 
  velocity 
  is 
  unity 
  and 
  directly 
  pro- 
  

   portional 
  to 
  the 
  mass 
  of 
  the 
  ion. 
  I 
  have 
  shown 
  in 
  my 
  

   4 
  Conduction 
  of 
  Electricity 
  through 
  Gases 
  ' 
  (2nd 
  edition, 
  

   p. 
  106) 
  that 
  when 
  the 
  ions 
  have 
  reached 
  this 
  state, 
  and 
  the 
  

   magnetic 
  force 
  H 
  is 
  so 
  large 
  that 
  Hu 
  is 
  large 
  compared 
  with 
  

   unity, 
  u 
  being 
  the 
  velocity 
  acquired 
  by 
  an 
  ion 
  when 
  unit 
  

   force 
  acts 
  upon 
  it, 
  and 
  inversely 
  proportional 
  to 
  the 
  pres- 
  

   sure, 
  then 
  the 
  ions 
  move 
  nearly 
  along 
  the 
  lines 
  of 
  magnetic 
  

   force, 
  but 
  have 
  a 
  small 
  component 
  in 
  the 
  direction 
  at 
  right 
  

   anoles 
  to 
  both 
  the 
  electric 
  and 
  magnetic 
  forces. 
  In 
  these 
  

   respects 
  they 
  resemble 
  Villard's 
  magneto-cathodic 
  rays 
  ; 
  the 
  

   ions, 
  however, 
  would 
  carry 
  electric 
  charges, 
  while 
  Villard 
  

   could 
  detect 
  no 
  charge 
  when 
  the 
  magneto-cathodic 
  rays 
  

   entered 
  a 
  Faraday 
  cylinder. 
  It 
  is 
  perhaps 
  possible 
  that 
  the 
  

   absence 
  of 
  charge 
  may 
  have 
  been 
  due 
  to 
  the 
  rays 
  making 
  the 
  

   gas 
  round 
  the 
  Faraday 
  cylinder 
  so 
  good 
  a 
  conductor 
  of 
  

   electricity, 
  that 
  no 
  appreciable 
  charge 
  could 
  accumulate 
  in 
  

   the 
  cylinder. 
  

  

  On 
  the 
  Method 
  by 
  which 
  the 
  Retrograde 
  Rays 
  acquire 
  

   their 
  Velocity. 
  

  

  We 
  have 
  seen 
  that 
  the 
  velocity 
  of 
  those 
  positive 
  rays 
  

   which 
  move 
  away 
  from 
  the 
  cathode, 
  and 
  which 
  are, 
  

   when 
  under 
  observation, 
  free 
  from 
  the 
  influence 
  of 
  the 
  

   electric 
  field 
  in 
  the 
  tube, 
  is 
  practically 
  the 
  same 
  as 
  that 
  of 
  

  

  