﻿Z. 
  Page—The 
  Photoelectric 
  Effect. 
  503 
  

  

  of 
  80 
  volts. 
  The 
  volt 
  sensitiveness 
  of 
  the 
  electrometer 
  was 
  

   about 
  135 
  centimeter 
  scale 
  divisions. 
  

  

  A 
  glass 
  tube 
  sealed 
  into 
  a 
  hole 
  in 
  the 
  plate 
  B 
  led 
  to 
  char- 
  

   coal 
  tube, 
  drying 
  tube, 
  and 
  pump. 
  Throughout 
  the 
  experi- 
  

   ment 
  the 
  charcoal 
  tube 
  was 
  kept 
  immersed 
  in 
  liquid 
  air, 
  and 
  

   the 
  vacuum 
  was 
  tested 
  before 
  each 
  set 
  of 
  readings. 
  

  

  The 
  source 
  of 
  light 
  was 
  a 
  Cooper 
  Hewitt 
  Mercury 
  Yapor 
  

   lamp. 
  The 
  light 
  passed 
  through 
  a 
  Hilger 
  spectrometer 
  with 
  

   quartz 
  train, 
  the 
  slit 
  of 
  which 
  was 
  focused 
  on 
  o 
  the 
  metal 
  at 
  G. 
  

   "Wave 
  lengths 
  used 
  were 
  24:60 
  and 
  2190 
  Angstrom 
  units. 
  

   The 
  lamp 
  was 
  found 
  to 
  burn 
  very 
  steadily, 
  giving 
  the 
  best 
  

   results 
  with 
  a 
  current 
  of 
  24 
  amperes, 
  the 
  maximum 
  fluctua- 
  

   tion 
  in 
  the 
  current 
  being 
  not 
  more 
  than 
  *03 
  amperes. 
  A 
  glass 
  

   shutter 
  operated 
  by 
  a 
  heavy 
  pendulum 
  was 
  found 
  to 
  be 
  a 
  very 
  

   satisfactory 
  instrument 
  for 
  regulating 
  the 
  length 
  of 
  exposures. 
  

   When 
  this 
  method 
  gave 
  too 
  small 
  a 
  deflection, 
  a 
  watch 
  was 
  

   used. 
  In 
  all 
  cases 
  the 
  current 
  was 
  measured 
  by 
  noting 
  the 
  

   deflection 
  of 
  the 
  electrometer 
  corresponding 
  to 
  a 
  given 
  ex- 
  

   posure. 
  

  

  Experimental 
  Procedure. 
  

  

  During 
  the 
  first 
  part 
  of 
  the 
  experiment 
  the 
  tube 
  H 
  was 
  of 
  

   copper. 
  Later 
  this 
  was 
  replaced 
  by 
  an 
  exactly 
  similar 
  tube 
  of 
  

   aluminium. 
  Each 
  tube 
  was 
  thoroughly 
  scraped 
  before 
  being 
  

   inserted. 
  The 
  metal 
  disks 
  on 
  which 
  the 
  light 
  was 
  allowed 
  to 
  

   fall 
  were 
  of 
  copper, 
  zinc, 
  and 
  aluminium. 
  The 
  design 
  of 
  the 
  

   apparatus 
  was 
  such 
  that 
  very 
  little 
  trouble 
  was 
  experienced 
  

   from 
  reflected 
  light. 
  In 
  fact 
  the 
  effect 
  due 
  to 
  reflected 
  light 
  

   was 
  inappreciable 
  when 
  the 
  copper 
  receiving 
  electrode 
  was 
  in 
  

   use. 
  Even 
  with 
  the 
  aluminium 
  electrode 
  this 
  effect 
  was 
  so 
  

   small 
  that 
  it 
  was 
  the 
  cause 
  of 
  no 
  trouble. 
  

  

  Immediately 
  after 
  scraping, 
  the 
  metals 
  used 
  showed 
  a 
  very 
  

   large 
  photoelectric 
  fatigue. 
  It 
  was 
  found 
  impossible 
  to 
  take 
  

   readings 
  until 
  this 
  fatigue 
  had 
  very 
  greatly 
  diminished 
  — 
  a 
  

   matter 
  of 
  some 
  ten 
  or 
  fifteen 
  minutes' 
  delay. 
  The 
  readings 
  

   were 
  taken 
  in 
  the 
  following 
  order 
  : 
  First, 
  the 
  point 
  of 
  satura- 
  

   tion, 
  which 
  had 
  been 
  previously 
  roughly 
  located, 
  was 
  deter- 
  

   mined 
  by 
  taking 
  readings 
  as 
  rapidly 
  as 
  possible 
  in 
  the 
  order 
  of 
  

   increasing 
  accelerating 
  potentials. 
  As 
  exposures 
  of 
  four 
  or 
  

   iive 
  seconds 
  would 
  usually 
  suffice, 
  these 
  readings 
  could 
  be 
  taken 
  

   very 
  quickly. 
  The 
  reason 
  for 
  taking 
  the 
  readings 
  in 
  the 
  order 
  

   of 
  increasing 
  accelerating 
  potentials 
  was 
  so 
  as 
  to 
  take 
  them 
  in 
  

   the 
  most 
  unfavorable 
  way, 
  as 
  regards 
  fatigue, 
  for 
  producing 
  

   the 
  shift 
  of 
  the 
  saturation 
  point, 
  which 
  was 
  noted 
  in 
  all 
  cases 
  

   of 
  scraping. 
  Second, 
  the 
  maximum 
  velocity 
  was 
  determined 
  

   by 
  exposures 
  of 
  ten 
  to 
  twenty 
  seconds. 
  Occasionally 
  the 
  

   metal 
  was 
  rescraped 
  and 
  the 
  maximum 
  velocity 
  determined 
  at 
  

  

  