﻿from 
  an 
  Electrified 
  Point. 
  55 
  

  

  ions 
  have 
  two 
  opportunities 
  of 
  producing 
  corpuscles, 
  (1) 
  by 
  

   impact 
  with 
  the 
  electrode, 
  (2) 
  by 
  impact 
  with 
  the 
  molecules 
  

   of 
  the 
  gas 
  ; 
  while 
  when 
  the 
  point 
  is 
  the 
  anode 
  only 
  the 
  

   second 
  of 
  these 
  is 
  available. 
  This 
  would 
  have 
  the 
  effect 
  of 
  

   making 
  Y 
  for 
  the 
  positive 
  point 
  greater 
  than 
  for 
  the 
  

   negative, 
  and 
  thus 
  making 
  the 
  minimum 
  potential 
  required 
  

   for 
  point 
  discharge 
  greater/' 
  

  

  The 
  curves 
  shown 
  in 
  Pi. 
  I. 
  figs. 
  2 
  and 
  3 
  will 
  support, 
  for 
  

   the 
  most 
  part, 
  the 
  interpretation 
  placed 
  on 
  such 
  phenomena 
  

   by 
  Prof. 
  Thomson. 
  The 
  discharge 
  potentials 
  given 
  in 
  the 
  

   tables 
  refer 
  to 
  dark 
  discharge 
  undoubtedly. 
  Upon 
  repeating 
  

   the 
  measurements, 
  with 
  the 
  apparatus 
  outside 
  the 
  box 
  in 
  a 
  

   fairly 
  dry 
  atmosphere, 
  almost 
  identical 
  results 
  were 
  obtained. 
  

   In 
  this 
  case 
  I 
  was 
  not 
  able 
  to 
  perceive 
  any 
  luminosity 
  at 
  

   the 
  surfaces. 
  

  

  The 
  potentials 
  at 
  which 
  discharge 
  occurred 
  could 
  readily 
  

   be 
  determined 
  by 
  the 
  behaviour 
  of 
  the 
  voltmeter-needle. 
  

   On 
  decreasing 
  the 
  resistance 
  in 
  the 
  liquid 
  rheostat 
  a 
  potential 
  

   would 
  be 
  reached 
  when 
  the 
  needle 
  came 
  to 
  rest, 
  wavered 
  

   and 
  sometimes 
  fell 
  back 
  ; 
  on 
  still 
  further 
  decreasing 
  the 
  

   resistance 
  slightly, 
  the 
  voltmeter 
  indicated 
  no 
  further 
  rise 
  in 
  

   potential. 
  If 
  the 
  potential 
  were 
  increased 
  sufficiently, 
  how- 
  

   ever, 
  a 
  crackle 
  could 
  be 
  heard 
  and 
  a 
  luminous 
  spark 
  was 
  

   obtained, 
  the 
  voltmeter-needle 
  falling 
  to 
  zero. 
  The 
  potentials 
  

   given 
  in 
  the 
  preceding 
  tables 
  are 
  those 
  indicating 
  the 
  

   existence 
  of 
  the 
  dark 
  discharge. 
  

  

  For 
  potentials 
  less 
  than 
  the 
  least 
  ionizing 
  potential 
  it 
  

   was 
  extremely 
  difficult 
  to 
  secure 
  readings. 
  The 
  potential 
  

   gradient 
  is 
  extremely 
  steep, 
  hence 
  variations 
  of 
  ■£$ 
  of 
  a 
  wave- 
  

   length 
  would 
  be 
  accompanied 
  by 
  large 
  changes 
  in 
  potential- 
  

   difference 
  readings. 
  It 
  is 
  not 
  difficult, 
  however, 
  to 
  explore 
  

   the 
  region 
  for 
  potential-differences 
  which 
  ionize 
  the 
  gas, 
  and 
  

   by 
  taking 
  numerous 
  readings 
  for 
  these 
  potentials 
  one 
  is 
  

   enable 
  to 
  locate 
  quite 
  accurately 
  the 
  place 
  where 
  the 
  gradient 
  

   changes 
  from 
  one 
  of 
  great 
  inclination 
  to 
  one 
  parallel 
  to 
  the 
  

   axis 
  of 
  abscissae. 
  

  

  My 
  readings 
  indicate 
  that 
  the 
  gradient 
  changes 
  at 
  a 
  

   potential 
  of 
  338 
  volts 
  for 
  a 
  negatively 
  electrified 
  point. 
  The 
  

   distance 
  corresponding 
  to 
  this 
  potential- 
  difference 
  is 
  one 
  

   wave-length 
  of 
  Na 
  light. 
  It 
  may 
  also 
  be 
  remarked 
  that 
  for 
  

   distances 
  greater 
  than 
  this, 
  the 
  potential 
  gradient 
  for 
  a 
  

   positively 
  electrified 
  point 
  shifts. 
  For 
  distances 
  less 
  than 
  

   this, 
  the 
  discharge 
  is 
  carried 
  by 
  negative 
  corpuscles 
  or 
  even 
  

   positive 
  ions 
  — 
  metal 
  ions, 
  perhaps, 
  for 
  throughout 
  this 
  limited 
  

   region 
  the 
  gradient 
  is 
  not 
  determined 
  by 
  the 
  polarity 
  o£ 
  

   the 
  point. 
  

  

  