﻿THE 
  

   LONDON, 
  EDINBURGH, 
  and 
  DUBLIN 
  

  

  PHILOSOPHICAL 
  

  

  AND 
  

  

  AND 
  DUBLIN 
  '<^.^^ 
  

  

  magazine"^ 
  

  

  JOURNAL 
  OF 
  SCIENCE. 
  

  

  [SIXTH 
  SERIES.] 
  

  

  AUGUST 
  1909. 
  

  

  XXYII. 
  Tlie 
  Discontinuity 
  of 
  Potential 
  at 
  the 
  Surf 
  ace* 
  ,oj 
  

   Glowing 
  Carbon. 
  By 
  J. 
  A. 
  Pollock, 
  A. 
  B^S^^Ranclauo, 
  

   and 
  E. 
  P. 
  Norman 
  *. 
  -^ 
  ^^/ 
  

  

  OWING 
  to 
  the 
  projection 
  of 
  ions 
  by 
  hot 
  substances, 
  a 
  

   discontinnity 
  of 
  potential 
  will 
  occur 
  at 
  the 
  surfaces 
  of 
  

   the 
  electrodes 
  in 
  any 
  circuit 
  in 
  which 
  these 
  latter 
  are 
  formed 
  

   of 
  heated 
  materials. 
  The 
  value 
  of 
  potential 
  discontinuity 
  has 
  

   been 
  calculated 
  by 
  Professor 
  Richardson 
  t 
  from 
  considerations 
  

   connected 
  with 
  the 
  gas 
  theory 
  of 
  metallic 
  conduction, 
  and 
  has 
  

   been 
  found 
  in 
  the 
  case 
  of 
  carbon 
  by 
  Mr. 
  Duddell 
  %■> 
  who 
  

   observed 
  at 
  the 
  surface 
  of 
  the 
  cathode 
  of 
  the 
  ordinary 
  carbon 
  

   arc 
  a 
  forward 
  electromotive 
  force 
  of 
  6*1 
  volts, 
  and 
  a 
  back 
  

   electromotive 
  force 
  of 
  16*7 
  volts 
  at 
  the 
  surface 
  of 
  the 
  anode. 
  

   In 
  a 
  circuit 
  with 
  one 
  heated 
  electrode 
  in 
  air 
  at 
  ordinary 
  

   pressure, 
  the 
  projection 
  of 
  ions 
  from 
  the 
  hot 
  surface 
  

   necessitates 
  the 
  establishment 
  of 
  a 
  potential-difference 
  

   between 
  the 
  electrodes 
  if 
  the 
  current 
  in 
  the 
  circuit 
  is 
  to 
  be 
  

   zero. 
  If 
  electrons 
  alone 
  w^ere 
  projected, 
  a 
  layer 
  of 
  gas 
  close 
  

   to 
  the 
  hot 
  surface 
  would 
  become 
  negative 
  to 
  the 
  sohd 
  and 
  

   the 
  potential-difference 
  between 
  the 
  electrodes 
  for 
  zero 
  

   current 
  might 
  be 
  taken 
  as 
  a 
  measure 
  of 
  the 
  surface 
  discon- 
  

   tinuity 
  of 
  potential. 
  When 
  positive 
  ions 
  are 
  emitted 
  as 
  well 
  

   as 
  electrons, 
  owing 
  to 
  the 
  differ^. 
  2e 
  in 
  the 
  distances 
  from 
  

   the 
  electrode 
  at 
  which 
  the 
  two 
  classes 
  of 
  ions 
  are 
  stopped 
  by 
  

  

  * 
  Communicated 
  by 
  the 
  Authors. 
  Read 
  before 
  the 
  Royal 
  Society 
  of 
  

   Xew 
  South 
  AVales. 
  

  

  t 
  Richardson, 
  Phil. 
  Trans. 
  A. 
  cci. 
  p. 
  497 
  (1903). 
  

   X 
  Duddell, 
  Phil. 
  Trans. 
  A. 
  cciii. 
  p. 
  305 
  (1904). 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  18. 
  No. 
  104. 
  Aug. 
  1909. 
  R 
  

  

  