﻿Field 
  upon 
  the 
  Electric 
  Discharge 
  through 
  Gases. 
  259 
  

  

  under 
  the 
  cathode, 
  a, 
  is 
  unchanged, 
  but 
  the 
  dark 
  space, 
  b, 
  

   becomes 
  more 
  sharply 
  defined, 
  and 
  a 
  second 
  dark 
  space 
  

   makes 
  its 
  appearance 
  at 
  d. 
  Between 
  these 
  a 
  bluish-violet 
  

   glow, 
  c, 
  is 
  present. 
  

  

  The 
  appearance 
  of 
  the 
  discharge 
  is 
  notably 
  altered 
  if 
  the 
  

   cathode 
  is 
  also 
  earthed 
  (fig. 
  4). 
  The 
  luminous 
  column, 
  BF, 
  

   expands 
  and 
  tills 
  practically 
  the 
  entire 
  tube. 
  In 
  addition 
  to 
  

   the 
  previous 
  distribution 
  of 
  glow 
  and 
  dark 
  spaces 
  in 
  the 
  

   portion 
  BC, 
  a 
  sharply 
  defined 
  and 
  bright 
  pinkish-white 
  

   band 
  of 
  light 
  appears 
  at 
  e, 
  just 
  at 
  the 
  beginning 
  of 
  the 
  blue 
  

   luminous 
  column. 
  This 
  band, 
  e, 
  has 
  the 
  appearance 
  and. 
  

   characteristics 
  of 
  the 
  positive 
  column 
  in 
  a 
  Crookes's 
  tube, 
  

   and 
  is 
  affected 
  by 
  a 
  test-magnet 
  in 
  a 
  similar 
  way 
  to 
  the 
  

  

  Fig. 
  4.. 
  

  

  ry!SSS>*V*».- 
  

  

  

  No 
  magnetic 
  field 
  , 
  screen 
  and 
  cathode 
  eart/ted. 
  

  

  positive 
  column 
  on 
  the 
  right 
  of 
  the 
  cathode. 
  In 
  fact, 
  we 
  

   have 
  a 
  symmetrical 
  and 
  identical 
  distribution 
  on 
  the 
  two 
  

   sides 
  of 
  the 
  cathode 
  — 
  the 
  Crookes's 
  dark 
  space, 
  the 
  negative 
  

   glow, 
  the 
  Faraday 
  dark 
  space, 
  and 
  the 
  positive 
  column. 
  A 
  

   test-magnet 
  bends 
  the 
  luminous 
  column 
  to 
  indicate 
  a 
  positive 
  

   current 
  flowing 
  from 
  B 
  to 
  F. 
  The 
  pitch 
  of 
  the 
  note 
  is 
  

   lower 
  than 
  in 
  the 
  previous 
  case. 
  

  

  We 
  next 
  start 
  a 
  current 
  in 
  the 
  field-magnet, 
  and 
  increase 
  

   it 
  until 
  the 
  typical 
  magnetic 
  ray 
  discharge 
  is 
  obtained. 
  This 
  

   is 
  shown 
  in 
  fig. 
  5. 
  As 
  the 
  field 
  becomes 
  gradually 
  intense, 
  

  

  Fig. 
  5. 
  

  

  Magnetic 
  field, 
  screen 
  and 
  cathode 
  earthed. 
  

  

  the 
  bluish 
  glow, 
  c. 
  retreats 
  from 
  the 
  cathode, 
  and 
  the 
  bright 
  

   pinkish 
  band, 
  e, 
  which 
  we 
  have 
  called 
  tl.e 
  positive 
  column, 
  

   creeps 
  down 
  the 
  tube 
  into 
  the 
  wide 
  portion. 
  When 
  a 
  critical 
  

   point 
  in 
  the 
  field 
  strength 
  is 
  reached, 
  it 
  suddenly 
  expands 
  

   into 
  a 
  long 
  column, 
  BE, 
  while 
  ihe 
  column 
  which 
  formerly 
  

   occupied 
  the 
  whole 
  tube 
  is 
  compressed 
  into 
  the 
  portion 
  EF. 
  

   At 
  the 
  same 
  time 
  the 
  conical 
  core 
  of 
  blue 
  magnetic 
  rays, 
  

  

  