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  Lord 
  Kelvin 
  

  

  true 
  explanation 
  of 
  the 
  old-known 
  fact 
  that 
  ground 
  glass 
  is 
  

   resinous 
  relatively 
  to 
  polished 
  glass. 
  The 
  process 
  of 
  polishing 
  

   might 
  be 
  expected 
  to 
  smooth 
  clown 
  the 
  smaller 
  atoms, 
  and 
  to 
  

   leave 
  the 
  larger 
  atoms 
  more 
  effective 
  in 
  the 
  surface. 
  

  

  § 
  12. 
  It 
  probably 
  contains 
  also 
  the 
  principle 
  of 
  the 
  explana- 
  

   tion 
  of 
  Erskine 
  Murray's* 
  experimental 
  discovery 
  that 
  sur- 
  

   faces 
  of 
  metals, 
  well 
  cleaned 
  by 
  rubbing 
  with 
  glass-paper 
  or 
  

   emery-paper, 
  become 
  more 
  positive 
  or 
  less 
  negative 
  in 
  the 
  

   Volta 
  contact 
  electricity 
  scale 
  by 
  being 
  burnished 
  with 
  a 
  

   smooth 
  round 
  hard 
  steel 
  burnisher. 
  Thus 
  a 
  zinc 
  plate 
  bright- 
  

   ened 
  by 
  rubbing 
  on 
  glass-paper 
  rose 
  by 
  '23 
  volt 
  by 
  repeated 
  

   burnishing 
  with 
  a 
  hard 
  steel 
  burnisher, 
  and 
  fell 
  again 
  by 
  the 
  

   same 
  difference 
  when 
  rubbed 
  again 
  with 
  glass-paper. 
  Copper 
  

   plates 
  showed 
  differences 
  of 
  about 
  the 
  same 
  amount 
  and 
  in 
  

   the 
  same 
  direction 
  when 
  similarly 
  treated. 
  Between 
  highly 
  

   burnished 
  zinc 
  and 
  emery-cleaned 
  copper, 
  Murray 
  found 
  a 
  

   Volta-difference 
  of 
  1*13 
  volts, 
  which 
  is, 
  I 
  believe, 
  considerably 
  

   greater 
  than 
  the 
  greatest 
  previously 
  found 
  Volta-difference 
  

   between 
  pure 
  metallic 
  surfaces 
  of 
  zinc 
  and 
  copper. 
  

  

  § 
  13. 
  To 
  further 
  illustrate 
  the 
  tendency 
  (§ 
  9) 
  of 
  the 
  smaller 
  

   atom 
  to 
  take 
  electrions 
  from 
  the 
  larger, 
  consider 
  two 
  atoms 
  : 
  

   A', 
  of 
  radius 
  a', 
  the 
  greater, 
  having 
  an 
  electrion 
  in 
  it 
  to 
  begin 
  

   with 
  ; 
  and 
  A, 
  radius 
  a, 
  the 
  smaller, 
  void. 
  

  

  By 
  ideal 
  forces 
  applied 
  to 
  the 
  atoms 
  while 
  the 
  electrion 
  is 
  

   free 
  let 
  them 
  approach 
  gradually 
  from 
  a 
  very 
  great 
  distance 
  

   apart. 
  The 
  attraction 
  of 
  A 
  draws 
  the 
  electrion 
  from 
  the 
  

   centre 
  of 
  A'; 
  at 
  first 
  very 
  slightly, 
  but 
  farther 
  and 
  farther 
  as 
  

   the 
  distance 
  between 
  the 
  atoms 
  is 
  diminished. 
  What 
  will 
  be 
  

  

  Fig-. 
  4 
  

  

  0'C 
  = 
  2 
  

  

  Fiff. 
  

  

  C'C 
  = 
  l-89. 
  C'E 
  = 
  0-63. 
  

  

  the 
  position 
  of 
  the 
  electrion 
  when 
  the 
  distance 
  between 
  the 
  

   centres 
  is, 
  as 
  in 
  fig. 
  4, 
  2a.'? 
  Without 
  calculation 
  we 
  see 
  that 
  

   the 
  electrion 
  would 
  be 
  in 
  equilibrium 
  if 
  placed 
  at 
  the 
  point 
  

  

  * 
  "On 
  Contact 
  Electricity 
  of 
  Metals/' 
  Proc. 
  Roy. 
  Soc. 
  vol. 
  lxiii. 
  

   p. 
  113 
  (1898). 
  See 
  also 
  Lord 
  Kelvin, 
  " 
  Contact 
  Electricity 
  of 
  Metals," 
  

   Phil. 
  Mag. 
  vol. 
  xlvi. 
  pp. 
  96-98 
  (1898). 
  

  

  