﻿Rotation 
  of 
  Electric 
  Discharge. 
  533 
  

  

  With 
  a 
  weak 
  magnetic 
  field 
  the 
  discharge 
  sticks 
  and 
  rotates 
  

   with 
  a 
  jerk, 
  but 
  gradually 
  the 
  rotation 
  becomes 
  steady 
  as 
  the 
  

   magnetic 
  field 
  is 
  intensified. 
  As 
  the 
  pressure 
  is 
  further 
  

   lowered, 
  the 
  " 
  band 
  " 
  gradually 
  broadens 
  and 
  changes 
  in 
  

   colour 
  (fig. 
  5, 
  PL 
  XXL). 
  

  

  7. 
  The 
  dark 
  space 
  is 
  now 
  well 
  defined, 
  increasing 
  in 
  size 
  

   as 
  the 
  pressure 
  becomes 
  less, 
  while 
  the 
  glow 
  surrounding 
  

   the 
  disk 
  is 
  seen 
  to 
  extend, 
  changing 
  in 
  form 
  from 
  oblate 
  

   spheroid 
  through 
  spherical 
  to 
  prolate. 
  And 
  the 
  rotation 
  

   continues 
  so 
  long 
  as 
  a 
  considerable 
  portion 
  of 
  the 
  discharge 
  

   passes 
  very 
  nearly 
  through 
  the 
  same 
  point 
  of 
  the 
  ring. 
  

  

  In 
  air, 
  the 
  rotation 
  at 
  first 
  follows 
  the 
  law 
  investigated 
  

   later, 
  viz., 
  pc> 
  = 
  constant, 
  where 
  p 
  is 
  the 
  pressure 
  and 
  <o 
  the 
  

   corresponding 
  angular 
  velocity 
  (§ 
  22) 
  ; 
  but 
  when 
  the 
  pressure 
  

   is 
  less 
  than 
  a 
  certain 
  amount, 
  depending 
  on 
  E.M.F. 
  of 
  the 
  

   induction-coil, 
  &c. 
  (§ 
  5), 
  the 
  angular 
  velocity 
  increases 
  less 
  

   rapidly 
  than 
  would 
  be 
  consistent 
  with 
  that 
  law. 
  Gradually 
  

   the 
  ring 
  end 
  of 
  the 
  discharge 
  (ring 
  being 
  the 
  anode) 
  extends 
  

   over 
  a 
  finite 
  arc 
  of 
  the 
  ring 
  (fig. 
  6, 
  PL 
  XXI.). 
  In 
  this 
  case 
  

   there 
  is 
  only 
  a 
  twist 
  on 
  the 
  application 
  of 
  the 
  magnetic 
  field. 
  

   The 
  pressure 
  at 
  which 
  this 
  occurs 
  depends 
  on 
  the 
  E.M.F. 
  of 
  

   the 
  induction-coil 
  &c. 
  (§ 
  5), 
  as 
  well 
  as 
  on 
  the 
  magnetic 
  field 
  

   itself. 
  Thus 
  in 
  my 
  experiments 
  with 
  three 
  cells 
  in 
  the 
  

   primary 
  it 
  was 
  about 
  3 
  millimetres 
  ; 
  with 
  four 
  cells 
  4'5 
  

   millimetres 
  ; 
  and 
  with 
  five 
  cells 
  5 
  millimetres 
  (fig. 
  6). 
  

  

  8. 
  Finally 
  the 
  discharge 
  spreads 
  over 
  a 
  considerable 
  

   portion 
  of 
  the 
  ring, 
  gradually 
  becomes 
  striatory 
  and 
  whitish 
  in 
  

   colour, 
  and 
  ultimately 
  fills 
  the 
  whole 
  tube 
  (fig. 
  7, 
  PL 
  XXI.) 
  . 
  

   The 
  only 
  effect 
  of 
  the 
  magnetic 
  field 
  in 
  this 
  case 
  is 
  to 
  

   produce 
  an 
  apparent 
  lengthening 
  of 
  the 
  negative 
  column, 
  

   due 
  to 
  the 
  fact 
  that 
  the 
  ions 
  move 
  in 
  spirals 
  on 
  the 
  application 
  

   of 
  the 
  field 
  (§ 
  17). 
  

  

  9. 
  It 
  will 
  be 
  thus 
  seen 
  that 
  whenever 
  the 
  discharge 
  

   consists 
  of 
  a 
  large 
  number 
  of 
  rays 
  going 
  in 
  all 
  directions 
  

   there 
  is 
  no 
  rotation. 
  

  

  10. 
  Thus 
  rotation 
  is 
  only 
  possible 
  if 
  the 
  bulk 
  of 
  the 
  

   discharge 
  follows 
  a 
  certain 
  path. 
  This 
  path 
  joins 
  the 
  point 
  

   of 
  maximum 
  electric 
  density 
  in 
  the 
  anode 
  to 
  the 
  cathode, 
  

   and 
  is 
  apparently 
  the 
  least 
  path 
  from 
  the 
  cathode 
  to 
  this 
  point. 
  

   Moreover, 
  this 
  is 
  the 
  case 
  when 
  the 
  pressure 
  is 
  between 
  

   certain 
  limits 
  (depending 
  on 
  the 
  E.M.F. 
  of 
  the 
  induction- 
  

   coil 
  and 
  other 
  quantities). 
  The 
  " 
  band 
  " 
  discharge 
  thus 
  

   behaves 
  like 
  a 
  flexible 
  wire 
  carrying 
  current 
  of 
  sufficient 
  

   intensity 
  to 
  produce 
  rotation 
  under 
  a 
  magnetic 
  field 
  of 
  suitable 
  

   intensity. 
  

  

  11. 
  Nearly 
  all 
  the 
  salient 
  points 
  with 
  regard 
  to 
  these 
  

  

  