﻿High-Frequency 
  Discharges 
  in 
  Geissler 
  Tubes. 
  975 
  

  

  the 
  capillary. 
  When 
  the 
  bulb 
  discharge 
  with 
  the 
  silica 
  tube 
  

   was 
  examined 
  the 
  effects 
  already 
  noted 
  generally, 
  in 
  con- 
  

   nexion 
  with 
  fig. 
  4, 
  were 
  clearly 
  illustrated. 
  At 
  higher 
  pres- 
  

   sures 
  the 
  intensity 
  of 
  the 
  negative-band 
  spectrum 
  was 
  imper- 
  

   ceptible, 
  whilst 
  the 
  positive-band 
  spectrum 
  showed 
  clearly, 
  

   and 
  increased 
  in 
  intensity 
  with 
  increasing 
  current. 
  At 
  lower 
  

   pressures, 
  comparable 
  with 
  1 
  mm. 
  of 
  mercury, 
  the 
  change 
  in 
  

   the 
  spectrum 
  of 
  the 
  bulb 
  discharge 
  with 
  heavy 
  currents 
  is 
  

   shown 
  photographically 
  in 
  print 
  2 
  appended 
  (PI. 
  XXI.), 
  for 
  

   which 
  the 
  pressure 
  was 
  1*5 
  mm. 
  By 
  using 
  a 
  long 
  slit, 
  and 
  

   focussing 
  the 
  join 
  of 
  capillary 
  and 
  bulb 
  on 
  this, 
  by 
  means 
  of 
  

   an 
  auxiliary 
  achromatic 
  lens, 
  simultaneous 
  photographs 
  of 
  

   bulb 
  and 
  capillary 
  discharges 
  were 
  obtained. 
  This 
  method 
  

   was 
  frequently 
  used. 
  A 
  result 
  identical 
  with 
  that 
  shown 
  in 
  

   print 
  2 
  was 
  obtained, 
  due 
  to 
  variation 
  of 
  pressure 
  in 
  the 
  

   discharge 
  tube, 
  the 
  current 
  being 
  maintained 
  constant. 
  At 
  

   higher 
  pressures 
  the 
  lower 
  photograph 
  represents 
  the 
  

   spectrum 
  obtained 
  — 
  the 
  negative-glow 
  spectrum 
  being 
  almost 
  

   entirely 
  absent. 
  This 
  spectrum 
  is 
  shown 
  strongly, 
  however, 
  

   in 
  the 
  bulb 
  at 
  low 
  pressure. 
  The 
  evolution 
  of 
  positive-band 
  

   spectrum 
  at 
  the 
  same 
  time 
  becomes 
  much 
  enfeebled 
  in 
  the 
  

   bulb, 
  as 
  shown 
  in 
  the 
  upper 
  photograph, 
  particularly 
  in 
  the 
  

   case 
  of 
  the 
  less 
  refrangible 
  bands. 
  When 
  the 
  pressure 
  in 
  

   the 
  discharge 
  tube 
  reached 
  the 
  critical 
  pressure 
  previously 
  

   alluded 
  to, 
  the 
  character 
  of 
  the 
  spectrum 
  of 
  the 
  bulb 
  no 
  

   longer 
  remained 
  pure. 
  At 
  such 
  low 
  pressures 
  the 
  negative- 
  

   glow 
  spectrum 
  extended 
  into 
  the 
  capillary, 
  whilst 
  the 
  positive- 
  

   band 
  spectrum 
  occupied 
  a 
  prominent 
  place 
  in 
  the 
  bulb 
  

   spectrum. 
  This 
  remark 
  applies 
  especially 
  to 
  the 
  less 
  re- 
  

   frangible 
  bands 
  in 
  the 
  red 
  and 
  orange, 
  and 
  it 
  can 
  be 
  seen 
  

   clearly 
  in 
  print 
  3 
  a. 
  No 
  doubt 
  the 
  bulb 
  owed 
  its 
  rosy 
  

   appearance 
  to 
  the 
  presence 
  of 
  these 
  positive 
  bands 
  under 
  

   such 
  conditions 
  of 
  pressure. 
  The 
  lower 
  photograph 
  3 
  b 
  is 
  

   of 
  the 
  capillary, 
  and 
  it 
  will 
  be 
  noticed 
  that 
  the 
  usual 
  banded 
  

   capillary 
  spectrum 
  is 
  not 
  here 
  shown 
  alone, 
  but 
  it 
  is 
  inter- 
  

   mixed 
  with 
  the 
  negative-band 
  spectrum. 
  (Contrast 
  print 
  1.) 
  

   This 
  change 
  of 
  behaviour 
  of 
  the 
  discharge 
  may 
  be 
  due 
  to 
  

   greater 
  uniformity 
  of 
  electrical 
  conditions 
  in 
  the 
  discharge 
  

   tube 
  at 
  lower 
  pressures, 
  where 
  we 
  would 
  have 
  conditions 
  

   analogous 
  to 
  the 
  wide 
  extension 
  of 
  the 
  Crookes 
  dark 
  space 
  

   in 
  an 
  ordinary 
  discharge 
  tube. 
  From 
  analogy 
  with 
  the 
  

   ordinary 
  induction-coil 
  discharge, 
  it 
  would 
  appear 
  that 
  

   uniformity 
  of 
  electrical 
  intensity 
  would 
  result 
  in 
  the 
  

   appearance 
  of 
  the 
  positive-band 
  spectrum 
  in 
  the 
  bulb. 
  

  

  Using 
  the 
  method 
  of 
  direct 
  coupling, 
  and 
  choking 
  the 
  

   current 
  bv 
  means 
  of 
  the 
  Oudin 
  coil, 
  the 
  writer 
  was 
  enabled 
  

  

  