﻿Discontimdty 
  of 
  Potential 
  at 
  Surface 
  of 
  Gloiuing 
  Carbon. 
  231 
  

  

  values 
  of 
  the 
  potential-difference 
  for 
  zero 
  current 
  with 
  hot 
  

   electrodes 
  of 
  different 
  makes 
  of 
  carbon 
  do 
  not 
  seriously 
  

   differ. 
  

  

  At 
  3120° 
  absolute, 
  with 
  50 
  volts 
  between 
  the 
  electrodes, 
  

   the 
  ratio 
  of 
  the 
  flow 
  of 
  negative 
  electricity 
  from 
  the 
  hot 
  carbon 
  

   to 
  the 
  flow 
  of 
  positive, 
  when 
  the 
  sign 
  of 
  the 
  potential- 
  

   difference 
  was 
  reversed, 
  was 
  as 
  20 
  to 
  1. 
  

  

  On 
  the 
  assumption 
  that 
  at 
  high 
  temperatures 
  the 
  potential- 
  

   difference 
  for 
  zero 
  current 
  measures 
  the 
  surface 
  discontinuity 
  

   of 
  potential, 
  as 
  the 
  number 
  of 
  electrons 
  then 
  projected 
  per 
  

   second 
  far 
  exceeds 
  that 
  of 
  positive 
  ions, 
  the 
  curve 
  in 
  the 
  figure 
  

   has 
  been 
  continued 
  to 
  the 
  value, 
  16" 
  7 
  volts, 
  found 
  by 
  

   Mr. 
  Duddell 
  (loc. 
  cit.), 
  for 
  the 
  back 
  electromotive 
  force 
  at 
  

   the 
  anode 
  of 
  an 
  arc 
  between 
  solid 
  Conradty 
  horis 
  carbons. 
  

   This 
  measure 
  has 
  been 
  plotted 
  for 
  the 
  temperature 
  of 
  the 
  

   crater^ 
  3690° 
  absolute^ 
  determined 
  by 
  Messrs. 
  Waidner 
  and 
  

   Burgess 
  * 
  from 
  observations 
  with 
  a 
  Holborn-Kurlbaum 
  

   optical 
  pyrometer, 
  a 
  similar 
  instrument 
  to 
  that 
  with 
  which 
  

   we 
  have 
  estimated 
  the 
  other 
  temperatures. 
  Mr. 
  Duddell 
  

   gives 
  6'1 
  \olts 
  as 
  the 
  measure 
  of 
  the 
  forward 
  electromotive 
  

   force 
  at 
  the 
  arc 
  cathode 
  ; 
  this 
  value 
  corresponds, 
  on 
  the 
  curve 
  

   o£ 
  potential-differences 
  for 
  zero 
  current, 
  to 
  a 
  temperature 
  of 
  

   3375° 
  absolute, 
  which, 
  if 
  the 
  assumption 
  already 
  stated 
  is 
  

   legitimate, 
  may 
  be 
  taken 
  as 
  an 
  estimate 
  of 
  the 
  temperature 
  of 
  

   the 
  cathode 
  of 
  the 
  carbon 
  arc. 
  

  

  With 
  a 
  heated 
  iron 
  wire 
  in 
  the 
  place 
  of 
  the 
  hot 
  carbon 
  of 
  

   the 
  previous 
  experiments, 
  the 
  potential-difference 
  for 
  zero 
  

   current 
  was 
  0'85 
  volt 
  at 
  1410° 
  absolute 
  and 
  0'25 
  volt 
  at 
  

   1570°, 
  the 
  hot 
  wire 
  being, 
  in 
  both 
  cases, 
  negative 
  to 
  the 
  

   cooler 
  electrode. 
  The 
  potential-difference 
  with 
  a 
  platinum 
  

   wire, 
  not 
  specially 
  treated, 
  at 
  1580° 
  absolute, 
  was 
  0'40 
  volt, 
  

   the 
  hot 
  wire 
  being 
  again 
  negative. 
  With 
  a 
  Nernst 
  filament, 
  

   designed 
  for 
  0*25 
  ampere 
  at 
  90 
  volts 
  and 
  used 
  under 
  these 
  

   cijnditions, 
  the 
  potential-difference 
  for 
  zero 
  current 
  was 
  0*25 
  

   volt. 
  In 
  this 
  case 
  the 
  filament 
  was 
  positive 
  to 
  the 
  cooler 
  

   electrode, 
  and 
  with 
  a 
  potential-difference 
  between 
  the 
  elec- 
  

   trodes 
  of 
  45 
  volts, 
  the 
  ratio 
  of 
  the 
  flow 
  of 
  negative 
  electricity 
  

   from 
  the 
  hot 
  filament 
  to 
  the 
  flow 
  of 
  positive, 
  when 
  the 
  sign 
  

   of 
  the 
  potential-difference 
  was 
  reversed, 
  was 
  as 
  33 
  to 
  1. 
  

  

  Professor 
  Richardson 
  (loc. 
  cit.) 
  gives, 
  for 
  the 
  number 
  of 
  

  

  corpuscles 
  shot 
  off 
  from 
  unit 
  area 
  of 
  a 
  hot 
  conductor 
  per 
  

  

  _ 
  

   second, 
  the 
  expression 
  A^^e 
  « 
  , 
  in 
  which 
  A 
  depends 
  on 
  the 
  

   number 
  of 
  corpuscles 
  per 
  unit 
  volume 
  of 
  the 
  conductor, 
  and 
  

   h 
  on 
  the 
  work 
  done 
  by 
  a 
  corpuscle 
  in 
  passing 
  through 
  the 
  

  

  * 
  Waidner 
  & 
  Burgess, 
  Phys. 
  Kev. 
  xix. 
  p. 
  255 
  (1904). 
  

   R 
  2 
  

  

  