﻿L. 
  Page 
  — 
  The 
  Photoelectric 
  Effect. 
  501 
  

  

  Art. 
  XLY. 
  — 
  The 
  Photoelectric 
  Effect 
  ; 
  by 
  Leigh 
  Page. 
  

  

  Compton* 
  has 
  shown 
  that 
  in 
  measuring 
  photoelectric 
  currents 
  

   corresponding 
  to 
  different 
  accelerating 
  potentials 
  the 
  contact 
  

   difference 
  of 
  potential 
  between 
  the 
  metal 
  exposed 
  to 
  the 
  ultra- 
  

   violet 
  light 
  and 
  the 
  receiving 
  electrode 
  must 
  be 
  taken 
  into 
  

   account. 
  As 
  this 
  contact 
  potential 
  difference 
  may 
  amount 
  to 
  

   one 
  volt 
  or 
  more, 
  and 
  as 
  the 
  potentials 
  used 
  to 
  retard 
  or 
  

   accelerate 
  the 
  emission 
  of 
  the 
  electrons 
  are 
  generally 
  not 
  more 
  

   than 
  two 
  volts, 
  it 
  is 
  obvious 
  that 
  serious 
  misapprehensions 
  may 
  

   result 
  from 
  the 
  failure 
  to 
  make 
  correction 
  for 
  it. 
  Richardson 
  

   and 
  Comptonf 
  have 
  shown 
  that 
  when 
  this 
  contact 
  potential 
  

   difference 
  is 
  taken 
  account 
  of, 
  the 
  saturation 
  point 
  is 
  indepen- 
  

   dent 
  of 
  the 
  wave 
  length 
  of 
  the 
  ultra-violet 
  light 
  and 
  of 
  the 
  

   nature 
  of 
  the 
  metal, 
  and 
  always 
  falls 
  on 
  the 
  corrected 
  axis 
  of 
  

   ordinates-zero 
  volts. 
  This 
  leads 
  to 
  the 
  conclusion 
  that 
  no 
  

   helping 
  field 
  is 
  necessary 
  to 
  cause 
  the 
  maximum 
  emission 
  of 
  

   electrons 
  under 
  the 
  influence 
  of 
  ultra-violet 
  light, 
  but 
  that 
  any 
  

   hindering 
  field, 
  however 
  small, 
  will 
  diminish 
  the 
  photoelectric 
  

   current. 
  

  

  The 
  following 
  investigation 
  was 
  undertaken 
  to 
  verify 
  the 
  

   results 
  obtained 
  by 
  Richardson 
  and 
  Compton 
  when 
  a 
  photo- 
  

   electric 
  chamber 
  of 
  somewhat 
  different 
  design 
  was 
  used, 
  to 
  

   investigate 
  the 
  effect 
  of 
  scraping 
  in 
  vacuum 
  the 
  metals 
  which 
  

   were 
  to 
  be 
  exposed 
  to 
  the 
  light, 
  and 
  to 
  obtain, 
  if 
  possible, 
  

   experimental 
  evidence 
  as 
  to 
  the 
  nature 
  of 
  the 
  contact 
  difference 
  

   of 
  potential 
  between 
  metals. 
  

  

  Apparatus. 
  

   The 
  photoelectric 
  chamber 
  consisted 
  of 
  the 
  glass 
  jar, 
  

   A 
  (fig. 
  1), 
  the 
  end 
  of 
  which 
  was 
  closed 
  by 
  the 
  brass 
  plate 
  B. 
  

   The 
  light, 
  after 
  passing 
  through 
  the 
  quartz 
  window, 
  C, 
  

   traversed 
  the 
  tube, 
  D, 
  12*8 
  cm 
  in 
  length 
  and 
  3'5 
  cm 
  in 
  diameter. 
  

   The 
  end 
  of 
  this 
  tube 
  was 
  covered 
  by 
  the 
  plate, 
  E, 
  of 
  the 
  same 
  

   metal, 
  in 
  the 
  center 
  of 
  which 
  was 
  the 
  circular 
  aperture, 
  F, 
  8 
  mm 
  

   in 
  diameter. 
  The 
  beam 
  of 
  light 
  used 
  was 
  at 
  this 
  point 
  not 
  

   more 
  than 
  4 
  mm 
  in 
  diameter, 
  and 
  passed 
  as 
  nearly 
  as 
  possible 
  

   through 
  the 
  center 
  of 
  this 
  opening. 
  To 
  the 
  circular 
  plate, 
  Gr, 
  

   were 
  soldered 
  disks 
  of 
  the 
  metals 
  on 
  which 
  the 
  light 
  was 
  

   allowed 
  to 
  fall. 
  By 
  means 
  of 
  the 
  tube, 
  H, 
  which 
  could 
  turn 
  

   in 
  the 
  conical 
  bearing 
  L, 
  G 
  could 
  be 
  rotated 
  so 
  as 
  to 
  bring 
  the 
  

   various 
  metals 
  in 
  front 
  of 
  the 
  aperture, 
  F. 
  A 
  steel 
  scraper, 
  I, 
  

   was 
  used 
  to 
  scrape 
  the 
  surfaces 
  of 
  the 
  metallic 
  disks 
  before 
  

   exposing 
  them 
  to 
  the 
  light. 
  The 
  scraper 
  was 
  similar 
  to 
  that 
  

  

  *K. 
  T. 
  Compton, 
  Phil. 
  Mag., 
  xxiii, 
  p. 
  579, 
  1912. 
  

  

  fO. 
  W. 
  Eichardson 
  and 
  K. 
  T. 
  Compton, 
  ibid., 
  xxiv, 
  p. 
  575, 
  1912. 
  

  

  