﻿536 
  

  

  Prof. 
  W. 
  G. 
  Adams 
  on 
  the 
  

  

  [June 
  17 
  } 
  

  

  heating 
  by 
  the 
  current. 
  In 
  most 
  of 
  the 
  experiments 
  a 
  battery 
  of 
  30 
  Le- 
  

   clanche 
  cells 
  was 
  employed. 
  

  

  It 
  was 
  found 
  that 
  the 
  higher 
  the 
  battery-power 
  the 
  less 
  is 
  the 
  resistance 
  

   of 
  the 
  selenium. 
  Experiments 
  with 
  5, 
  30, 
  and 
  35 
  cells 
  gave 
  the 
  follow- 
  

   ing 
  results 
  : 
  — 
  

  

  Resistance 
  R 
  with 
  5 
  cells 
  5400 
  ohms. 
  

  

  „ 
  35 
  „ 
  4400 
  „ 
  

  

  5 
  „ 
  5400 
  „ 
  

  

  „ 
  30 
  „ 
  4600 
  „ 
  — 
  

  

  After 
  some 
  hours 
  : 
  — 
  

  

  Resistance 
  R 
  with 
  30 
  „ 
  . 
  . 
  . 
  , 
  4800 
  „ 
  

  

  5J 
  J5 
  „ 
  5750 
  „ 
  

  

  This 
  diminution 
  of 
  resistance 
  with 
  increased 
  battery-power 
  may 
  be 
  

   accounted 
  for 
  in 
  part 
  by 
  leakage 
  from 
  the 
  rheocord 
  ; 
  and 
  there 
  may 
  be 
  

   also 
  an 
  opposing 
  electromotive 
  force 
  similar 
  to 
  polarization 
  brought 
  into 
  

   action 
  in 
  the 
  selenium 
  when 
  the 
  current 
  is 
  passing 
  which 
  increases 
  with 
  

   the 
  current. 
  

  

  Exposure 
  to 
  light 
  diminishes 
  the 
  resistance 
  of 
  selenium. 
  

   This 
  may 
  be 
  accounted 
  for 
  by 
  either 
  of 
  two 
  hypotheses 
  : 
  — 
  

  

  (1) 
  That 
  light 
  acting 
  on 
  the 
  selenium 
  sets 
  up 
  a 
  polarization 
  current 
  

   in 
  it 
  which 
  opposes 
  the 
  battery-current 
  passing 
  through 
  it. 
  

  

  (2) 
  That 
  light 
  makes 
  the 
  selenium 
  a 
  better 
  conductor 
  of 
  electricity 
  by 
  

   producing 
  a 
  change 
  in 
  its 
  surface 
  similar 
  to 
  the 
  change 
  which 
  it 
  produces 
  

   on 
  the 
  surface 
  of 
  -a 
  phosphorescent 
  body, 
  by 
  which 
  that 
  body 
  is 
  enabled 
  to 
  

   give 
  out 
  light 
  after 
  it 
  has 
  been 
  exposed. 
  

  

  "With 
  the 
  same 
  battery-power, 
  an 
  increase 
  in 
  the 
  temperature 
  causes 
  

   an 
  increase 
  in 
  the 
  resistance 
  of 
  the 
  selenium. 
  

  

  In 
  December, 
  before 
  the 
  extreme 
  cold, 
  the 
  resistance 
  R 
  with 
  30 
  cells 
  

   was 
  about 
  5200. 
  Throughout 
  the 
  extreme 
  cold, 
  from 
  December 
  18 
  to 
  

   January 
  4, 
  the 
  resistance 
  R 
  was 
  about 
  4400. 
  This 
  was 
  the 
  value 
  of 
  R 
  at 
  

   1 
  p.m. 
  on 
  January 
  1st, 
  a 
  bright 
  cold 
  day. 
  On 
  January 
  5 
  the 
  temperature 
  

   out 
  of 
  doors 
  changed 
  to 
  44° 
  E. 
  in 
  the 
  shade 
  at 
  12 
  o'clock, 
  and 
  the 
  value 
  

   of 
  R 
  was 
  found 
  to 
  be 
  5400. 
  These 
  values 
  were 
  obtained 
  before 
  the 
  box 
  

   was 
  opened, 
  and 
  were 
  the 
  first 
  experiments 
  made 
  on 
  the 
  days 
  named. 
  

  

  "When 
  first 
  exposed 
  after 
  being 
  closed 
  up 
  for 
  some 
  days 
  or 
  even 
  hours, 
  

   the 
  selenium 
  is 
  more 
  sensitive 
  to 
  light 
  : 
  this 
  sensitiveness 
  increases 
  with 
  

   the 
  time 
  during 
  which 
  the 
  selenium 
  has 
  been 
  kept 
  in 
  the 
  dark 
  ; 
  hence 
  

   the 
  first 
  experiment 
  is 
  generally 
  not 
  comparable 
  with 
  the 
  others. 
  

  

  On 
  exposure 
  to 
  light 
  the 
  resistance 
  is 
  diminished 
  ; 
  but 
  on 
  being 
  again 
  

   eclipsed, 
  the 
  selenium 
  returns 
  in 
  a 
  very 
  few 
  minutes 
  nearly 
  to 
  its 
  pre- 
  

   vious 
  resistance. 
  

  

  The 
  change 
  of 
  resistance 
  produced 
  by 
  exposure 
  -to 
  daylight 
  sometimes 
  

   amounts 
  to 
  one 
  fourth 
  of 
  the 
  whole 
  resistance 
  of 
  the 
  selenium. 
  

  

  The 
  experiments 
  with 
  various 
  absorbing 
  media 
  seem 
  to 
  show 
  that 
  the 
  

  

  