﻿Field 
  upon 
  the 
  Electric 
  Discharge 
  through 
  Gases. 
  261 
  

  

  coaxial 
  magnetic 
  field 
  produces 
  an 
  enormous 
  drop 
  in 
  the 
  

   potential 
  difference 
  ; 
  and 
  also 
  that 
  the 
  magnetic 
  field 
  causes 
  

   the 
  discharge 
  to 
  concentrate 
  in 
  one 
  or 
  more 
  bright 
  streams 
  

   instead 
  of 
  discharging 
  from 
  the 
  whole 
  cathode. 
  This 
  con- 
  

   centration 
  is, 
  of 
  course, 
  a 
  feature 
  of 
  the 
  discharge 
  from 
  

   a 
  hot 
  lime 
  cathode. 
  Gouy 
  and 
  Bloch 
  * 
  have 
  both 
  noted 
  

   this 
  effect 
  of 
  a 
  magnetic 
  field 
  on 
  the 
  potential. 
  The 
  former 
  

   has 
  advanced 
  a 
  rather 
  complicated 
  theory 
  to 
  explain 
  this 
  

   action, 
  but 
  it 
  seems 
  more 
  reasonable 
  to 
  accept 
  the 
  opinion 
  

   of 
  Bloch, 
  who 
  bases 
  it 
  on 
  the 
  principle 
  of 
  disruptive 
  discharge 
  

   in 
  a 
  magnetic 
  field. 
  This 
  lowering 
  of 
  the 
  potential 
  and 
  

   consequent 
  increase 
  of 
  the 
  current 
  is 
  also 
  obtained 
  by 
  

   earthing 
  the 
  cathode 
  and 
  the 
  gauze 
  screen, 
  as 
  is 
  shown 
  

   by 
  the 
  marked 
  increase 
  in 
  the 
  length 
  of 
  the 
  discharge 
  in 
  the 
  

   tube 
  (see 
  figs. 
  3 
  and 
  4) 
  ; 
  after 
  discovering 
  this 
  effect 
  of 
  

   connecting 
  the 
  cathode 
  to 
  the 
  screen 
  and 
  to 
  earth, 
  we 
  found 
  

   that 
  Almy 
  f 
  had 
  already 
  discovered 
  that 
  this 
  effect 
  can 
  be 
  

   obtained 
  without 
  the 
  magnetic 
  field 
  if 
  the 
  cathode 
  is 
  con- 
  

   nected 
  to 
  a 
  tinfoil 
  covering 
  placed 
  on 
  the 
  tube 
  near 
  the 
  

   cathode. 
  

  

  If 
  the 
  action 
  of 
  the 
  magnetic 
  field 
  is 
  to 
  cause 
  a 
  stream 
  of 
  

   electrons 
  to 
  move 
  in 
  a 
  helical 
  path 
  rather 
  than 
  to 
  create 
  

   neutral 
  doublets, 
  it 
  is 
  important 
  to 
  make 
  this 
  path 
  evident. 
  

   Gouy 
  J 
  has 
  recently 
  shown 
  the 
  spiral 
  form 
  of 
  these 
  ravs 
  

   by 
  an 
  ingenious 
  experiment. 
  He 
  allows 
  a 
  pencil 
  of 
  the 
  

   magnetic 
  rays 
  to 
  fall 
  on 
  an 
  inclined 
  crystal 
  and 
  observes 
  

   the 
  trajectory 
  of 
  the 
  path 
  of 
  the 
  reflected 
  light. 
  We 
  have 
  

   found 
  it 
  easy 
  to 
  show 
  that 
  the 
  path 
  is 
  a 
  helix. 
  To 
  accomplish 
  

   this, 
  an 
  extremely 
  small 
  spot 
  of 
  sealing-wax 
  was 
  deposited 
  

   on 
  the 
  cathode. 
  The 
  cathode 
  ray 
  emitted 
  from 
  this 
  spot 
  

   was 
  a 
  fine 
  line 
  of 
  brilliant 
  blue 
  light. 
  Under 
  the 
  action 
  of 
  

   an 
  intense 
  magnetic 
  field, 
  this 
  ray 
  was 
  twisted 
  into 
  a 
  sharply 
  

   defined 
  helix 
  of 
  small 
  pitch. 
  

  

  A 
  study 
  of 
  the 
  appearance 
  of 
  the 
  discharge, 
  as 
  shown 
  in 
  

   figs. 
  4 
  and 
  5, 
  when 
  the 
  screen 
  and 
  cathode 
  are 
  earthed 
  and 
  

   with 
  or 
  without 
  a 
  magnetic 
  field, 
  is 
  convincing 
  as 
  to 
  the 
  

   identical 
  arrangement 
  of 
  glow 
  and 
  dark 
  spaces 
  on 
  the 
  two 
  

   sides 
  of 
  the 
  cathode. 
  We 
  have 
  first, 
  on 
  the 
  left 
  side, 
  the 
  

   glow 
  on 
  the 
  cathode 
  accompanied 
  by 
  long 
  streams 
  of 
  cathode 
  

   ravs 
  from 
  the 
  hot 
  lime 
  deposit. 
  Then, 
  in 
  order, 
  are 
  a 
  dark 
  

   space 
  similar 
  to 
  the 
  Crookes's 
  dark 
  space; 
  a 
  bluish 
  luminous 
  

   glow 
  similar 
  to 
  the 
  negative 
  glow 
  ; 
  next, 
  a 
  second 
  dark 
  

   space 
  corresponding 
  to 
  the 
  Faraday 
  dark 
  space 
  ; 
  and 
  finally, 
  

  

  * 
  See 
  articles 
  by 
  Gouy 
  and 
  by 
  Bloch, 
  C. 
  R. 
  1910 
  and 
  1911, 
  passim. 
  

  

  f 
  Almv, 
  /. 
  c. 
  

  

  X 
  Gouy, 
  C. 
  R. 
  vol. 
  clii. 
  p. 
  363 
  ; 
  Lc 
  Radium, 
  vol. 
  viii. 
  p. 
  129 
  (1011). 
  

  

  