﻿Long-wave 
  Limits 
  of 
  the 
  Normal 
  Photoelectric 
  Effect. 
  1021 
  

  

  well 
  with 
  our 
  series 
  of 
  observations, 
  in 
  which 
  the 
  limit 
  for 
  

   the 
  but 
  little 
  more 
  electronegative 
  Sn 
  is 
  given 
  as 
  X 
  = 
  370 
  /jl/jl. 
  

   Recently 
  Richardson 
  and 
  Gompton 
  have 
  noted 
  a 
  similar 
  

   regular 
  it}^, 
  although 
  the 
  absolute 
  values 
  for 
  the 
  limiting 
  

   wave-lengths 
  lie 
  sensibly 
  more 
  toward 
  the 
  ultraviolet, 
  pro- 
  

   bably 
  because 
  the 
  metallic 
  surfaces 
  investigated 
  by 
  them 
  

   were 
  not 
  so 
  free 
  from 
  impurities. 
  

  

  Although 
  such 
  a 
  relation 
  between 
  the 
  position 
  of 
  a 
  metal 
  

   in 
  the 
  Volta 
  series 
  and 
  its 
  limit 
  of 
  photoelectric 
  sensitiveness, 
  

   that 
  is, 
  the 
  energy 
  of 
  separation 
  of 
  the 
  electrons, 
  certainly 
  

   does 
  exist, 
  it 
  seems 
  to 
  us 
  premature 
  to 
  attempt 
  to 
  build 
  up 
  a 
  

   quantitative 
  theory 
  from 
  the 
  absolute 
  measured 
  values 
  of 
  

   these 
  limits. 
  First 
  because, 
  as 
  already 
  noted, 
  it 
  is 
  almost 
  

   impossible 
  to 
  determine 
  when 
  the 
  surfaces 
  of 
  the 
  metals 
  

   under 
  observation 
  are 
  actually 
  free 
  from 
  impurities. 
  To 
  

   illustrate, 
  a 
  Ca 
  surface 
  prepared 
  by 
  Hughes 
  by 
  distillation 
  

   in 
  a 
  vacuum 
  ceased 
  to 
  be 
  photoelectrieally 
  sensitive 
  at 
  

   X 
  = 
  365yu,/A, 
  whereas 
  a 
  Ca 
  surface 
  prepared 
  by 
  us 
  in 
  an 
  

   exactly 
  similar 
  manner 
  gave 
  a 
  copious 
  emission 
  of 
  electrons 
  

   with 
  visible 
  light. 
  But 
  not 
  only 
  do 
  the 
  absolute 
  values 
  vary 
  

   widely 
  from 
  surface 
  to 
  surface, 
  but 
  they 
  are 
  not 
  always 
  

   constant 
  for 
  the 
  same 
  surface. 
  We 
  reported 
  such 
  a 
  varia- 
  

   tion 
  in 
  the 
  surface 
  of 
  Mg 
  and 
  Al 
  mirrors 
  recently 
  *. 
  Imme- 
  

   diately 
  after 
  their 
  production 
  these 
  mirrors 
  always 
  ceased 
  to 
  

   be 
  sensitive 
  at 
  the 
  boundary 
  of 
  the 
  visible 
  spectrum, 
  but 
  

   after 
  standing 
  for 
  some 
  time 
  in 
  a 
  vacuum 
  the 
  same 
  surfaces 
  

   would 
  emit 
  electrons 
  when 
  illuminated 
  with 
  even 
  infra-red 
  

   rays. 
  

  

  We 
  have 
  also 
  described 
  a 
  similar 
  time 
  displacement 
  of 
  the 
  

   sensitivity 
  limit 
  through 
  almost 
  an 
  octave 
  — 
  from 
  about 
  

   400 
  /x/x 
  to 
  over 
  700 
  jjl/jl— 
  in 
  the 
  case 
  of 
  distilled 
  Oaf 
  . 
  Quite 
  

   recently 
  we 
  have 
  found 
  that 
  a 
  liquid 
  Oa 
  amalgam 
  possessed 
  

   this 
  same 
  property. 
  When 
  such 
  an 
  amalgam 
  is 
  freshly 
  put 
  

   into 
  the 
  cell, 
  its 
  photoelectric 
  sensitiveness 
  ceases 
  at 
  about 
  

   \ 
  = 
  350 
  fjbfjb. 
  Within 
  a 
  few 
  minutes, 
  however, 
  the 
  limit 
  of 
  

   sensitiveness 
  moves 
  toward 
  the 
  visible 
  spectrum, 
  and 
  after 
  

   a 
  somewhat 
  longer 
  time 
  (about 
  one 
  hour) 
  has 
  reached 
  into 
  

   the 
  infra-red. 
  If, 
  then, 
  by 
  shaking 
  the 
  cell 
  a 
  new 
  surface 
  is 
  

   exposed, 
  it 
  exhibits 
  again 
  the 
  original 
  sensitiveness 
  only 
  to 
  

   pass 
  through 
  the 
  same 
  changes 
  as 
  before. 
  

  

  Measurements 
  showing 
  the 
  progress 
  of 
  the 
  sensitivity 
  

   limit 
  of 
  such 
  a 
  Ca 
  surface 
  are 
  given 
  in 
  Table 
  I. 
  and 
  fig. 
  2. 
  

   Energetic 
  heating 
  accelerated 
  the 
  process. 
  The 
  addition 
  

   of 
  foreign 
  gases 
  to 
  the 
  cell 
  (which 
  naturally 
  during 
  the 
  

  

  * 
  R 
  Pohl 
  & 
  P. 
  Pringsheim, 
  Verh. 
  d. 
  D. 
  Phys. 
  Ges. 
  xiv. 
  p. 
  546 
  (1912). 
  

   t 
  K. 
  Pohl 
  & 
  P. 
  Pringsheiin, 
  Verh. 
  d. 
  D. 
  Phys. 
  Ges. 
  xv. 
  p. 
  Ill 
  (1913). 
  

  

  