﻿442 
  Sir 
  J. 
  J. 
  Thomson 
  on 
  

  

  strip 
  o£ 
  platinum-foil 
  heated 
  to 
  redness, 
  and 
  having 
  on 
  it 
  a 
  

   spot 
  of 
  lime 
  or 
  barium 
  oxide*. 
  

  

  With 
  these 
  cathodes 
  large 
  currents 
  can 
  be 
  sent 
  through 
  

   the 
  tube, 
  and 
  remarkably 
  bright 
  and 
  steady 
  striations 
  obtained 
  

   at 
  lower 
  pressures 
  and 
  with 
  smaller 
  potential-differences 
  than 
  

   with 
  the 
  ordinary 
  type 
  of 
  discharge. 
  The 
  pressure 
  has, 
  

   however, 
  to 
  be 
  low, 
  considerably 
  less 
  than 
  1 
  mm. 
  of 
  mercury, 
  

   to 
  get 
  the 
  full 
  advantages 
  from 
  these 
  cathodes. 
  The 
  first 
  

   point 
  to 
  which 
  attention 
  was 
  directed 
  was 
  the 
  distribution 
  

   of 
  electric 
  force 
  along 
  the 
  line 
  of 
  the 
  discharge. 
  Investiga- 
  

   tions 
  on 
  this 
  point 
  have 
  already 
  been 
  made 
  by 
  Skinner 
  and 
  

   H. 
  A. 
  Wilson, 
  but 
  it 
  seemed 
  to 
  me 
  that 
  the 
  steadiness 
  of 
  the 
  

   striations 
  with 
  the 
  Wehuelt 
  cathode 
  made 
  this 
  method 
  of 
  

   investigation 
  particularly 
  suited 
  for 
  these 
  investigations. 
  

   The 
  first 
  method 
  used 
  to 
  measure 
  the 
  variations 
  in 
  the 
  electric 
  

   force 
  along 
  the 
  discharge 
  was 
  to 
  find 
  the 
  variation 
  in 
  the 
  

   difference 
  of 
  potential 
  between 
  two 
  platinum 
  wires 
  1 
  mm. 
  

   apart 
  as 
  the 
  wires 
  were 
  moved 
  from 
  the 
  cathode 
  to 
  the 
  anode. 
  

   Several 
  devices 
  were 
  used 
  for 
  this 
  purpose: 
  in 
  some 
  the 
  

   platinum 
  wires 
  (surrounded 
  up 
  to 
  about 
  a 
  millimetre 
  from 
  

   their 
  tips 
  with 
  glass 
  rods) 
  were 
  carried 
  on 
  a 
  sort 
  of 
  railroad 
  

   and 
  moved 
  from 
  cathode 
  to 
  anode. 
  The 
  electrodes 
  in 
  the 
  

   discharge-tube 
  in 
  this 
  case 
  were 
  fixed. 
  The 
  measurements 
  

   of 
  the 
  potential-differences 
  made 
  by 
  this 
  method 
  at 
  low 
  

   pressures 
  gave 
  the 
  very 
  remarkable 
  result 
  that 
  just 
  on 
  the 
  

   cathode 
  side 
  of 
  the 
  bright 
  part 
  of 
  a 
  striation 
  the 
  electric 
  

   force 
  was 
  negative 
  (^. 
  e, 
  that 
  the 
  force 
  on 
  a 
  positive 
  charge 
  was 
  

   in 
  the 
  direction 
  from 
  catliode 
  to 
  anode) 
  : 
  on 
  crossing 
  over 
  the 
  

   bright 
  boundary 
  to 
  the 
  anode 
  side 
  the 
  electric 
  force 
  at 
  once 
  

   became 
  positive, 
  and 
  rose 
  to 
  a 
  high 
  value. 
  It 
  soon, 
  however, 
  

   began 
  to 
  diminish, 
  and 
  went 
  on 
  diminishing 
  up 
  to 
  the 
  

   cathode 
  side 
  of 
  the 
  bright 
  part 
  of 
  the 
  next 
  striation 
  on 
  

   the 
  anode 
  side. 
  The 
  distribution 
  of 
  the 
  electric 
  force 
  in 
  

   the 
  striation 
  is 
  represented 
  in 
  fig. 
  1, 
  and 
  the 
  corresponding- 
  

   distribution 
  of 
  positive 
  and 
  negative 
  electricity 
  in 
  fig. 
  2, 
  

   the 
  ordinates 
  representing 
  the 
  distribution 
  of 
  the 
  electri- 
  

   fication 
  both 
  as 
  to 
  magnitude 
  and 
  sign. 
  Thus 
  if 
  these 
  

   measurements 
  of 
  the 
  electric 
  field 
  can 
  be 
  relied 
  on 
  we 
  have 
  

   intense 
  negative 
  electrification 
  at 
  the 
  bright 
  head 
  of 
  a 
  striation 
  

   (by 
  head 
  is 
  meant 
  the 
  side 
  next 
  the 
  cathode) 
  and 
  a 
  weak 
  

   positive 
  electrification 
  through 
  the 
  rest 
  of 
  the 
  field. 
  The 
  

  

  * 
  My 
  assistant, 
  Mr. 
  Everett, 
  has 
  found 
  that 
  these 
  cathodes 
  can 
  be 
  

   very 
  easily 
  made 
  by 
  letting 
  a 
  drop 
  of 
  sealing-wax 
  tall 
  on 
  the 
  foil, 
  and 
  

   then 
  burning 
  away 
  the 
  combustible 
  matter 
  by 
  heating 
  the 
  foil 
  to 
  

   incandescence. 
  Sealing-wax 
  seems 
  to 
  contain 
  large 
  quantities 
  of 
  some 
  

   salt 
  of 
  barium. 
  

  

  