﻿

  the 
  Photoelectric 
  Effect. 
  557 
  

  

  Heraeus 
  quartz 
  mercury 
  arc-lamp 
  Q 
  entered 
  the 
  collimator- 
  

   slit 
  o£ 
  a 
  Hilger 
  monochromatic 
  ultra-violet 
  illuminator 
  and 
  

   entered 
  the 
  photoelectric 
  cell 
  through 
  a 
  quartz 
  window 
  P. 
  

   The 
  apparatus 
  was 
  adjusted 
  so 
  that 
  the 
  light 
  from 
  the 
  slit 
  

   was 
  exactly 
  focussed 
  on 
  the 
  strip 
  S 
  of 
  metal 
  to 
  be 
  tested, 
  so 
  

   that 
  the 
  strip 
  and 
  the 
  image 
  of 
  the 
  slit 
  exactly 
  coincided 
  for 
  

   each 
  wave-length 
  of 
  light. 
  The 
  wave-length 
  of 
  light 
  falling 
  

   on 
  the 
  strip 
  was 
  shown 
  directly 
  by 
  the 
  graduated 
  drum 
  W. 
  

   The 
  degree 
  of 
  adjustment 
  could 
  be 
  examined 
  by 
  looking 
  into 
  

   the 
  bulb 
  at 
  Gr. 
  The 
  requisite 
  vacuum 
  was 
  obtained 
  by 
  first 
  

   using 
  a 
  (laede 
  pump, 
  then 
  sealing 
  off 
  the 
  pump 
  connexion 
  

   at 
  F 
  and 
  immersing 
  the 
  tube 
  of 
  coconut 
  charcoal 
  B 
  x 
  in 
  

   liquid 
  air. 
  Under 
  the 
  influence 
  of 
  the 
  light, 
  electrons 
  

   escaped 
  from 
  the 
  strip 
  S 
  to 
  the 
  inside 
  silvered 
  surface 
  of 
  the 
  

   bulb 
  B. 
  The 
  rate 
  of 
  emission 
  of 
  electrons 
  was 
  measured 
  by 
  

   observing 
  the 
  rate 
  of 
  deflexion 
  of 
  the 
  electrometer 
  E, 
  which 
  

   was 
  connected 
  with 
  the 
  strip 
  S. 
  The 
  bulb 
  B 
  was 
  maintained 
  

   at 
  a 
  positive 
  potential 
  of 
  5 
  volts 
  (contact 
  difference 
  of 
  

   potential 
  being 
  allowed 
  for). 
  This 
  gave 
  complete 
  saturation 
  

   and 
  prevented 
  any 
  reverse 
  current 
  from 
  B 
  to 
  S 
  due 
  to 
  light 
  

   reflected 
  from 
  the 
  strip 
  8 
  to 
  the 
  walls 
  of 
  the 
  bulb 
  B. 
  

  

  It 
  has 
  been 
  mentioned 
  that 
  the 
  theory 
  is 
  based 
  on 
  the 
  

   assumption 
  that 
  isotropic 
  radiation 
  falls 
  upon 
  the 
  metal 
  

   strip. 
  Since 
  the 
  light 
  falling 
  upon 
  the 
  strip 
  in 
  our 
  apparatus 
  

   was 
  practically 
  a 
  parallel 
  beam, 
  we 
  secured 
  the 
  equivalent 
  of 
  

   the 
  incidence 
  of 
  isotropic 
  light 
  by 
  bending 
  the 
  strip 
  around 
  

   a 
  lengthwise 
  axis 
  into 
  such 
  a 
  shape 
  that, 
  with 
  a 
  parallel 
  

   beam 
  of 
  light 
  striking 
  it, 
  there 
  would 
  be 
  the 
  same 
  proportion 
  

   of 
  light 
  incident 
  at 
  each 
  angle 
  as 
  if 
  isotropic 
  radiation 
  were 
  

   incident 
  on 
  a 
  plane 
  surface. 
  We 
  determined 
  graphically 
  

   the 
  shape 
  of 
  cross-section 
  of 
  the 
  strip 
  necessary 
  to 
  fulfil 
  these 
  

   conditions, 
  and 
  found 
  that 
  it 
  is 
  approximately 
  elliptical, 
  with 
  

   the 
  major 
  axis 
  twice 
  the 
  minor 
  axis, 
  and 
  pointing 
  in 
  the 
  

   direction 
  of 
  the 
  light. 
  

  

  In 
  order 
  to 
  obtain 
  absolutely 
  fresh 
  metal 
  surface 
  for 
  the 
  

   photoelectric 
  tests 
  we 
  adopted 
  the 
  following 
  device. 
  The 
  

   metal 
  strip 
  S, 
  of 
  elliptical 
  cross-section 
  and 
  just 
  the 
  size 
  of 
  

   the 
  image 
  of 
  the 
  slit, 
  was 
  fixed 
  to 
  the 
  end 
  of 
  a 
  light 
  aluminium 
  

   rod 
  R 
  which 
  was 
  capable 
  of 
  a 
  motion 
  up 
  and 
  down 
  in 
  a 
  

   closely 
  fitting 
  brass 
  tube. 
  Stops 
  D 
  were 
  arranged 
  so 
  that 
  

   the 
  upper 
  limit 
  of 
  motion 
  brought 
  the 
  strip 
  exactly 
  in 
  the 
  

   path 
  of 
  the 
  light, 
  while 
  the 
  lower 
  limit 
  left 
  the 
  strip 
  inside 
  

   of 
  a 
  small 
  bulb 
  T 
  directly 
  below 
  the 
  main 
  bulb 
  B. 
  The 
  

   brass 
  tube 
  was 
  firmly 
  waxed 
  into 
  the 
  outer 
  glass 
  tube 
  at 
  A. 
  

   A 
  soft 
  iron 
  block 
  I 
  was 
  attached 
  to 
  the 
  upper 
  end 
  of 
  the 
  

   aluminium 
  rod, 
  so 
  that 
  the 
  strip 
  could 
  be 
  moved 
  up 
  or 
  down 
  

  

  