﻿1875.] 
  Electrical 
  Discharges 
  through 
  Rarefied 
  Gases. 
  459 
  

  

  assistant, 
  Mr. 
  P. 
  Ward, 
  to 
  whose 
  intelligence 
  and 
  skill 
  I 
  am 
  much 
  indebted 
  

   throughout 
  this 
  investigation, 
  intended 
  for 
  fine 
  adjustment. 
  Wherever 
  the 
  

   resistance 
  be 
  introduced 
  the 
  following 
  law 
  appears 
  to 
  be 
  established 
  by 
  a 
  

   great 
  number 
  and 
  variety 
  of 
  experiments, 
  viz. 
  that, 
  the 
  striae 
  being 
  previously 
  

   stationary, 
  an 
  increase 
  of 
  resistance 
  produces 
  a 
  forward 
  flow, 
  a 
  decrease 
  

   of 
  resistance 
  a 
  backward 
  flow. 
  I 
  have 
  generally 
  found 
  that 
  a 
  variation 
  

   of 
  3 
  or 
  4 
  ohms, 
  or, 
  under 
  favourable 
  conditions, 
  of 
  1 
  or 
  2 
  ohms, 
  in 
  the 
  

   primary 
  current 
  is 
  sufficient 
  to 
  produce 
  this 
  effect. 
  But 
  as 
  an 
  altera- 
  

   tion 
  in 
  the 
  current 
  not 
  only 
  affects 
  the 
  discharge 
  directly, 
  but 
  also 
  

   reacts 
  upon 
  the 
  break, 
  the 
  effect 
  is 
  liable 
  to 
  be 
  masked 
  by 
  these 
  indirect 
  

   f 
  causes. 
  The 
  latter, 
  so 
  far 
  as 
  they 
  are 
  dependent 
  upon 
  a 
  sudden 
  altera- 
  

  

  tion 
  of 
  the 
  resistance, 
  may 
  be 
  diminished 
  by 
  the 
  use 
  of 
  the 
  rheostat 
  ; 
  but 
  

   when 
  the 
  striae 
  are 
  sufficiently 
  sensitive 
  to 
  admit 
  the 
  use 
  of 
  this 
  delicate 
  

   adjustment, 
  some 
  precautions 
  are 
  necessary 
  to 
  insure 
  perfect 
  uniformity 
  

   of 
  current, 
  so 
  as 
  to 
  avoid 
  disturbances 
  due 
  to 
  uneven 
  contact 
  in 
  the 
  

   rheostat 
  itself. 
  

  

  When 
  the 
  strise 
  are 
  flowing 
  they 
  preserve 
  their 
  mutual 
  distances, 
  and 
  

   do 
  not 
  undergo 
  increase 
  or 
  decrease 
  in 
  their 
  numbers. 
  Usually 
  one 
  or 
  

   two 
  remain 
  permanently 
  attached 
  to 
  the 
  positive 
  electrode 
  ; 
  and 
  as 
  the 
  

   moving 
  column 
  advances 
  or 
  recedes, 
  the 
  foremost 
  stria 
  diminishes 
  in 
  

   brilliancy 
  until, 
  after 
  travelling 
  over 
  a 
  distance 
  less 
  than 
  the 
  intervals 
  

   between 
  the 
  two 
  stria), 
  it 
  is 
  lost 
  in 
  darkness. 
  The 
  reverse 
  takes 
  place 
  at 
  

   i 
  the 
  rear 
  of 
  the 
  column. 
  As 
  the 
  last 
  stria 
  leaves 
  its 
  position, 
  a 
  new 
  one, 
  

  

  at 
  first 
  faint 
  and 
  shadowy, 
  makes 
  its 
  appearance 
  behind, 
  at 
  a 
  distance 
  

   equal 
  to 
  the 
  common 
  interval 
  of 
  all 
  the 
  others 
  : 
  this 
  new 
  one 
  increases 
  

   in 
  brilliancy 
  until, 
  when 
  it 
  has 
  reached 
  the 
  position 
  originally 
  occupied 
  

   by 
  the 
  last 
  stria 
  when 
  the 
  column 
  was 
  at 
  rest, 
  it 
  becomes 
  as 
  bright 
  as 
  the 
  

   others. 
  The 
  flow 
  may 
  vary 
  very 
  much 
  in 
  velocity 
  ; 
  it 
  may 
  be 
  so 
  slow 
  that 
  

   the 
  appearances 
  and 
  disappearances 
  of 
  the 
  terminal 
  striae 
  may 
  be 
  watched 
  

   in 
  all 
  their 
  phases, 
  or 
  it 
  may 
  be 
  so 
  rapid 
  that 
  the 
  separate 
  striae 
  are 
  no 
  

   longer 
  distinguishable, 
  and 
  the 
  tube 
  appears 
  as 
  if 
  illuminated 
  with 
  a 
  con- 
  

   tinuous 
  discharge. 
  In 
  most 
  cases 
  the 
  true 
  character 
  of 
  the 
  discharge 
  

   and 
  the 
  direction 
  of 
  the 
  flow 
  may 
  be 
  readily 
  distinguished 
  by 
  the 
  aid 
  of 
  

   a 
  revolving 
  mirror. 
  In 
  some 
  tubes, 
  especially 
  in 
  those 
  whose 
  length 
  is 
  

   great 
  compared 
  with 
  their 
  diameter, 
  the 
  whole 
  column 
  does 
  not 
  present 
  

   the 
  same 
  phase 
  of 
  flow 
  ; 
  one 
  portion 
  may 
  be 
  at 
  rest 
  while 
  another 
  is 
  

   flowing, 
  or 
  even 
  two 
  conterminous 
  portions 
  may 
  flow 
  in 
  opposite 
  directions. 
  

   This 
  is 
  seen 
  also 
  in 
  very 
  wide 
  tubes, 
  in 
  which 
  the 
  striae 
  appear 
  generally 
  

   more 
  mobile 
  than 
  in 
  narrow 
  ones. 
  But 
  in 
  all 
  cases 
  these 
  nodes 
  or 
  

   junction-points 
  of 
  the 
  flow 
  retain 
  their 
  positions 
  under 
  similar 
  conditions 
  

   of 
  pressure 
  and 
  current 
  ; 
  and 
  it 
  therefore 
  seems 
  that, 
  under 
  similar 
  con- 
  

   ditions, 
  the 
  column 
  in 
  a 
  given 
  tube 
  always 
  breaks 
  up 
  into 
  similar 
  flow- 
  

   segments. 
  

  

  These 
  nodes 
  will 
  often 
  disappear 
  under 
  the 
  action 
  of 
  a 
  magnetic 
  pole. 
  

   Thus 
  if 
  the 
  first 
  segment, 
  measured 
  from 
  the 
  positive 
  terminal, 
  be 
  sta- 
  

  

  