﻿the 
  Photoelectric 
  Effect. 
  

  

  559 
  

  

  number 
  of 
  electrons 
  liberated 
  per 
  unit 
  time 
  by 
  light 
  of 
  each 
  

   wave-length 
  was 
  divided 
  by 
  the 
  energy 
  of 
  the 
  light 
  of 
  the 
  

   corresponding 
  wave-length, 
  the 
  quotient 
  represented 
  the 
  

   relative 
  number 
  of 
  electrons 
  liberated 
  in 
  unit 
  time 
  by 
  a 
  unit 
  

   amount 
  of 
  light-energy 
  for 
  each 
  wave-length. 
  These 
  values 
  

   may 
  be 
  used 
  directly 
  to 
  test 
  equation 
  (6). 
  They 
  are 
  called 
  

   N 
  in 
  the 
  curves 
  and 
  are 
  plotted 
  as 
  ordinates. 
  

  

  The 
  Energy 
  Calibration. 
  

  

  The 
  relative 
  energies 
  in 
  different 
  parts 
  of 
  the 
  spectrum 
  

   were 
  measured 
  by 
  means 
  of 
  a 
  bismuth-silver 
  linear 
  thermo- 
  

   pile 
  made 
  by 
  W. 
  W. 
  Coblentz 
  *. 
  This 
  thermopile 
  was 
  adapted 
  

   to 
  the 
  monochromatic 
  illuminator 
  and 
  was 
  placed 
  directly 
  

   back 
  of 
  the 
  slit 
  in 
  the 
  telescope 
  arm. 
  This 
  slit 
  was 
  adjusted 
  

   to 
  the 
  exact 
  width 
  and 
  position 
  of 
  the 
  strip 
  which 
  had 
  been 
  

   tested 
  photoelectrically. 
  The 
  energy 
  was 
  determined 
  by 
  a 
  

   compensation 
  method 
  illustrated 
  in 
  fig. 
  3. 
  The 
  thermopile 
  

  

  Fig-. 
  3. 
  

  

  T 
  was 
  connected 
  in 
  series 
  with 
  a 
  shunt 
  S 
  and 
  a 
  Thomson 
  

   astatic 
  galvanometer 
  Gr. 
  A 
  second 
  circuit 
  contained, 
  in 
  

   series, 
  the 
  shunt 
  S, 
  a 
  battery 
  B, 
  and 
  a 
  variable 
  resistance 
  R. 
  

   The 
  battery 
  was 
  connected 
  so 
  that 
  its 
  electromotive 
  force 
  

   opposed 
  that 
  generated 
  by 
  the 
  thermopile. 
  The 
  resistance 
  R 
  

   was 
  adjusted 
  so 
  that 
  no 
  current 
  flowed 
  through 
  the 
  galva- 
  

   nometer. 
  Under 
  these 
  conditions 
  the 
  radiation 
  energy 
  falling 
  

   on 
  the 
  thermopile 
  was 
  inversely 
  proportional 
  to 
  the 
  resistance 
  

   R. 
  For 
  the 
  current 
  through 
  the 
  battery 
  is 
  

  

  1 
  = 
  

  

  E 
  

  

  K 
  + 
  6 
  + 
  

  

  where 
  E 
  is 
  the 
  electromotive 
  force 
  of 
  the 
  battery, 
  and 
  b, 
  s, 
  t, 
  

   and 
  g 
  are 
  the 
  resistances 
  of 
  the 
  battery, 
  shunt, 
  thermopile, 
  

   and 
  galvanometer 
  respectively. 
  R 
  varied 
  from 
  20,000 
  to 
  

   1,200,000 
  ohms 
  in 
  different 
  parts 
  of 
  the 
  spectrum, 
  b 
  and 
  s 
  

   were 
  each 
  0*01 
  ohm, 
  t 
  was 
  8 
  ohms 
  and 
  g 
  was 
  10'5 
  ohms. 
  

   * 
  Franklin 
  Inst. 
  Journ. 
  clxxv. 
  p. 
  151 
  (1913). 
  

  

  