﻿1875.] 
  On 
  the 
  Action 
  of 
  Light 
  on 
  Selenium. 
  535 
  

  

  current 
  there 
  was 
  found 
  to 
  be 
  also 
  a 
  diminution 
  of 
  resistance 
  in 
  the 
  case 
  

   of 
  hard 
  steel, 
  and 
  an 
  increase 
  of 
  resistance 
  in 
  the 
  case 
  of 
  soft 
  iron 
  and 
  

   soft 
  steel. 
  

  

  Thus 
  the 
  effects 
  produced 
  are 
  the 
  same 
  as 
  those 
  due 
  to 
  transverse 
  

   magnetization 
  by 
  a 
  neighbouring 
  current. 
  

  

  Conclusions 
  to 
  be 
  drawn 
  from 
  the 
  experiments 
  : 
  — 
  

  

  (1) 
  The 
  effect 
  of 
  passing 
  any 
  current 
  through 
  a 
  bar 
  of 
  hard 
  steel 
  is 
  to 
  

   diminish 
  its 
  resistance, 
  and 
  through 
  a 
  bar 
  of 
  soft 
  iron 
  or 
  soft 
  steel 
  is 
  to 
  

   increase 
  its 
  resistance. 
  

  

  (2) 
  When 
  a 
  bar 
  of 
  hard 
  steel 
  is 
  magnetized 
  by 
  sending 
  a 
  current 
  through 
  

   a 
  coil 
  which 
  encloses 
  it, 
  there 
  is 
  a 
  diminution 
  of 
  resistance 
  which 
  is 
  

   directly 
  proportional 
  to 
  the 
  square 
  of 
  the 
  magnetizing 
  current 
  up 
  to 
  a 
  

   certain 
  limit. 
  

  

  (3) 
  "When 
  soft 
  steel 
  or 
  soft 
  iron 
  is 
  magnetized 
  longitudinally 
  or 
  trans- 
  

   versely, 
  there 
  is 
  an 
  increase 
  of 
  resistance 
  which 
  is 
  nearly 
  proportional 
  to 
  

   the 
  square 
  of 
  the 
  magnetizing 
  current. 
  

  

  XII. 
  " 
  The 
  Action 
  of 
  Light 
  on 
  Selenium." 
  By 
  Prof. 
  W. 
  G. 
  Adams, 
  

   M.A., 
  F.R.S. 
  Received 
  June 
  17, 
  1875. 
  

   (Abstract.) 
  

  

  The 
  paper 
  contains 
  an 
  account 
  of 
  several 
  series 
  of 
  experiments 
  made 
  

   in 
  December 
  and 
  January 
  last 
  on 
  this 
  subject 
  with 
  the 
  view 
  : 
  — 
  

  

  (1) 
  To 
  determine 
  whether 
  the 
  change 
  in 
  the 
  electrical 
  resistance 
  of 
  the 
  

   selenium 
  is 
  due 
  to 
  radiant 
  heat, 
  light, 
  or 
  chemical 
  action. 
  

  

  (2) 
  To 
  measure 
  the 
  amount 
  of 
  the 
  change 
  of 
  resistance 
  due 
  to 
  exposure 
  

   to 
  light 
  from 
  different 
  sources 
  and 
  through 
  various 
  absorbing 
  media. 
  

  

  (3) 
  To 
  determine 
  whether 
  the 
  action 
  is 
  instantaneous 
  or 
  gradual, 
  and, 
  

   if 
  possible, 
  to 
  measure 
  the 
  rate 
  at 
  which 
  the 
  action 
  takes 
  place. 
  

  

  The 
  selenium 
  formed 
  one 
  of 
  the 
  four 
  resistances 
  in 
  a 
  Wheatstone's 
  

   bridge, 
  and 
  its 
  average 
  resistance 
  was 
  about 
  2| 
  megohms. 
  

  

  The 
  two 
  resistances 
  in 
  the 
  bridge, 
  which 
  were 
  kept 
  constant, 
  were 
  4 
  

   and 
  2000, 
  so 
  that 
  the 
  resistance 
  of 
  the 
  selenium 
  was 
  500 
  times 
  the 
  vari- 
  

   able 
  resistance 
  required 
  to 
  balance 
  it. 
  

  

  R 
  is 
  taken 
  to 
  represent 
  this 
  resistance 
  required 
  to 
  balance 
  the 
  selenium. 
  

   The 
  box 
  containing 
  the 
  selenium 
  was 
  laid 
  on 
  its 
  side 
  and 
  had 
  a 
  draw- 
  

   lid, 
  which 
  was 
  kept 
  closed 
  except 
  when 
  exposure 
  was 
  made. 
  In 
  front 
  of 
  

   the 
  draw-lid 
  was 
  a 
  black 
  screen 
  with 
  an 
  opening 
  opposite 
  to 
  the 
  selenium 
  

   6 
  centims. 
  by 
  3| 
  centims., 
  into 
  or 
  in 
  front 
  of 
  which 
  various 
  absorbing 
  

   media 
  could 
  be 
  placed. 
  

  

  The 
  absorbing 
  media 
  employed 
  were 
  bichromate 
  of 
  potash, 
  sulphate 
  of 
  

   copper, 
  ruby, 
  orange, 
  green, 
  and 
  blue 
  glasses. 
  Plates 
  of 
  rock-salt, 
  alum, 
  

   mica, 
  and 
  quartz 
  were 
  also 
  employed. 
  

  

  With 
  the 
  lid 
  of 
  the 
  box 
  on, 
  the 
  resistance 
  of 
  the 
  selenium 
  was 
  measured, 
  

   and 
  was 
  found 
  to 
  increase 
  slowly 
  and 
  regularly 
  in 
  consequence 
  of 
  the 
  

  

  