﻿418 
  

  

  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  Table 
  IV. 
  — 
  Change 
  in 
  Contact-Potential 
  of 
  Zinc 
  and 
  Copper 
  

   due 
  to 
  Layers 
  of 
  Turpentine, 
  Rosin 
  Oil, 
  and 
  Indiarubber 
  

   Solution. 
  

  

  

  

  Potential, 
  volts. 
  

  

  

  Liquid. 
  

  

  Metal. 
  

  

  

  Remarks. 
  

  

  1 
  

  

  Change. 
  

  

  Wet 
  with 
  turpentine. 
  

  

  Copper, 
  polished 
  on 
  

  

  

  

  -•11 
  

  

  Note. 
  — 
  The 
  plates 
  were 
  

  

  

  glass-paper. 
  

   Zinc, 
  polished 
  on 
  glass- 
  

  

  

  +•20 
  

  

  compared 
  with 
  one 
  

   another, 
  and 
  not 
  with 
  

  

  

  paper. 
  

  

  

  

  the 
  standard 
  plate. 
  

  

  Wet 
  with 
  rosin-oil 
  . 
  . 
  

  

  Copper, 
  polished 
  on 
  

   glass-paper. 
  

  

  Zinc, 
  polished 
  on 
  glass- 
  

   paper. 
  

  

  

  -•12 
  

   -•02 
  

  

  Hence 
  only 
  the 
  change 
  

   of 
  potential 
  is 
  given. 
  

  

  Wet 
  with 
  indiarubber 
  

  

  Copper, 
  polished 
  on 
  

  

  

  +•02 
  

  

  

  solution. 
  

  

  glass-paper. 
  

   Zinc, 
  polished 
  on 
  glass- 
  

   paper. 
  

  

  

  +•07 
  

  

  

  VIII. 
  Films 
  formed 
  on 
  Metals 
  by 
  Gases. 
  

  

  § 
  54. 
  The 
  potential 
  of 
  a 
  metal 
  is 
  usually 
  altered 
  by 
  soaking 
  

   the 
  plate 
  in 
  a 
  gas 
  other 
  than 
  air. 
  If 
  the 
  gas 
  be 
  oxygen, 
  this 
  

   alteration 
  is 
  as 
  a 
  rule 
  only 
  temporary, 
  and 
  apparently 
  depends 
  

   on 
  the 
  formation 
  of 
  a 
  surface 
  film, 
  or 
  rather 
  on 
  a 
  change 
  in 
  

   the 
  film 
  which 
  doubtless 
  already 
  exists. 
  It 
  will 
  be 
  shown 
  

   that 
  the 
  change 
  of 
  potential 
  of 
  zinc 
  due 
  to 
  soaking 
  in 
  oxygen 
  

   is 
  nearly 
  equal 
  to 
  that 
  of 
  copper 
  ; 
  hence 
  the 
  film 
  of 
  oxygen 
  

   acts 
  like 
  a 
  liquid 
  film 
  (Chap. 
  VII.), 
  but 
  its 
  effect 
  is 
  less 
  

   permanent. 
  Previous 
  experimenters, 
  except 
  Lord 
  Kelvin, 
  

   appear 
  to 
  have 
  neglected 
  the 
  existence 
  of 
  these 
  films, 
  and 
  to 
  

   have 
  looked 
  only 
  to 
  the 
  nature 
  of 
  the 
  body 
  of 
  gas 
  between 
  

   the 
  plates. 
  Of 
  course 
  I 
  do 
  not 
  here 
  allude 
  to 
  the 
  " 
  double- 
  

   layer 
  " 
  which 
  has 
  been 
  offered 
  as 
  an 
  explanation 
  of 
  the 
  

   phenomenon, 
  but 
  to 
  a 
  layer 
  in 
  mechanical 
  and 
  electrical 
  

   contact 
  with 
  the 
  metal. 
  If 
  the 
  contact-potential 
  of 
  two 
  

   metals 
  immersed 
  in 
  a 
  gas 
  were 
  the 
  sum 
  of 
  the 
  potential- 
  

   differences 
  between 
  each 
  metal 
  and 
  a 
  skin 
  of 
  gas 
  close 
  to 
  it, 
  

   we 
  should 
  have 
  no 
  slope 
  of 
  potential 
  in 
  the 
  body 
  of 
  the 
  gas 
  

   between 
  the 
  plates. 
  That 
  a 
  slope 
  does 
  exist, 
  however, 
  is 
  

   proved 
  by 
  Lord 
  Kelvin's 
  earliest 
  experiments 
  with 
  the 
  divided 
  

   ring 
  of 
  copper 
  and 
  zinc. 
  In 
  this 
  connexion 
  Dr. 
  Bottomley's 
  

   research 
  on 
  contact-electricity 
  in 
  high 
  vacua 
  * 
  is 
  of 
  great 
  

   importance 
  as 
  showing 
  that 
  the 
  volta-potential 
  of 
  metals 
  is 
  

  

  * 
  B. 
  A. 
  Report, 
  1885. 
  

  

  