﻿L. 
  Page— 
  The 
  Photoelectric 
  Effect. 
  507 
  

  

  where 
  h 
  is 
  Planck's 
  radiation 
  constant, 
  v 
  is 
  the 
  frequency 
  of 
  

   the 
  light 
  iised, 
  and 
  P 
  is 
  the 
  work 
  done 
  by 
  the 
  electron 
  in 
  

   escaping 
  from 
  the 
  metal. 
  Using 
  6'55(10) 
  -27 
  erg 
  sec. 
  as 
  the 
  

   value 
  of 
  A, 
  

  

  L 
  = 
  8'97(10)- 
  12 
  ergs 
  — 
  P 
  for 
  X 
  = 
  2190 
  

  

  L 
  = 
  7'88(10)- 
  12 
  ergs 
  — 
  P 
  for 
  A 
  = 
  2460 
  

  

  From 
  this 
  result 
  values 
  of 
  P 
  as 
  follows 
  : 
  

  

  l 
  = 
  2190 
  / 
  = 
  2460 
  

  

  Scraped 
  Al 
  4-5(l0)~ 
  12 
  4-3(10)~ 
  12 
  

  

  Zn 
  5-3(l0)- 
  12 
  5-l(10)- 
  12 
  

  

  Ca 
  6'5(10)- 
  12 
  6-2(10)- 
  12 
  

  

  Unscraped 
  Al 
  .- 
  5'6(10)- 
  12 
  5-3(10)~ 
  12 
  

  

  Zn 
  6-0(10)" 
  12 
  5-7(10)- 
  12 
  

  

  Cu 
  7-4(10)" 
  12 
  7'0(10)- 
  12 
  

  

  indicating 
  a 
  value 
  of 
  h 
  somewhat 
  less 
  than 
  6'55(10) 
  -27 
  . 
  That 
  

   the 
  values 
  of 
  P 
  among 
  the 
  different 
  metals 
  differ 
  very 
  exactly 
  

   by 
  the 
  contact 
  difference 
  of 
  potential 
  between 
  these 
  metals 
  

   is 
  made 
  evident 
  by 
  the 
  fact 
  that 
  the 
  curves 
  for 
  different 
  metals 
  

   but 
  for 
  the 
  same 
  wave 
  length 
  meet 
  the 
  voltage 
  axis 
  at 
  the 
  

   same 
  point. 
  

  

  The 
  shift 
  of 
  the 
  point 
  of 
  saturation 
  to 
  the 
  right 
  after 
  scrap- 
  

   ing 
  seems 
  to 
  throw 
  some 
  light 
  on 
  the 
  nature 
  of 
  the 
  contact- 
  

   difference 
  of 
  potential. 
  Since 
  the 
  point 
  of 
  saturation 
  marks 
  

   the 
  true 
  zero 
  potential, 
  the 
  effect 
  of 
  scraping 
  must 
  have 
  been 
  

   to 
  make 
  the 
  metal 
  more 
  electropositive. 
  A 
  similar 
  effect 
  when 
  

   the 
  scraping 
  has 
  been 
  done 
  in 
  air 
  at 
  atmospheric 
  pressure 
  has 
  

   been 
  noted 
  by 
  other 
  observers, 
  and 
  is 
  commented 
  on 
  by 
  Lord 
  

   Kelvin 
  in 
  his 
  paper 
  on 
  "Contact 
  Electricity 
  of 
  Metals."* 
  

   Now, 
  on 
  the 
  surface 
  layer 
  theory 
  of 
  the 
  contact 
  potential 
  differ- 
  

   ence, 
  the 
  degree 
  to 
  which 
  a 
  metal 
  is 
  electropositive 
  depends 
  on 
  

   the 
  degree 
  to 
  which 
  it 
  is 
  covered 
  bj 
  the 
  surface 
  layer. 
  Hence 
  

   any 
  removal 
  of 
  the 
  surface 
  layer 
  would 
  make 
  the 
  metal 
  

   less 
  electropositive 
  or 
  more 
  electronegative. 
  Consequently 
  it 
  

   would 
  seem 
  that 
  the 
  contact 
  potential 
  difference 
  cannot 
  be 
  due 
  

   essentially 
  to 
  a 
  double 
  layer 
  on 
  the 
  surface 
  of 
  the 
  metal, 
  though 
  

   no 
  doubt 
  modified 
  by 
  the 
  presence 
  of 
  such 
  a 
  layer. 
  

  

  Conclusions. 
  

  

  (1) 
  By 
  replacing 
  the^copper 
  receiving 
  electrode 
  by 
  one 
  of 
  

   aluminium 
  it 
  is 
  shown 
  that 
  the 
  contact 
  potential 
  difference 
  

   should 
  be 
  corrected 
  for 
  as 
  well 
  when 
  the 
  metals 
  which 
  are 
  

   exposed 
  to 
  the 
  light 
  have 
  been 
  scraped 
  in 
  vacuum 
  as 
  when 
  

   they 
  have 
  been 
  scraped 
  in 
  air. 
  

  

  *Lord 
  Kelvin, 
  Phil. 
  Mag., 
  xlvi, 
  p. 
  95 
  et 
  seq., 
  1898. 
  

  

  