﻿402 
  Dr. 
  J. 
  Erskine-Murray 
  on 
  

  

  against 
  the 
  plate, 
  holding 
  it 
  firmly 
  in 
  the 
  hook 
  ; 
  by 
  this 
  

   means 
  the 
  upper 
  plate 
  can 
  be 
  set 
  parallel 
  to 
  the 
  lower 
  one 
  in 
  

   a 
  very 
  short 
  time. 
  This 
  apparatus, 
  which 
  is 
  the 
  same 
  as 
  that 
  

   used 
  by 
  Lord 
  Kelvin 
  in 
  his 
  experiments 
  many 
  years 
  ago, 
  

   was 
  found 
  to 
  be 
  very 
  convenient, 
  the 
  arrangements 
  enabling 
  

   us 
  to 
  take 
  out 
  and 
  replace 
  a 
  plate 
  very 
  rapidly, 
  a 
  matter 
  of 
  

   great 
  importance 
  when 
  observing 
  temporary 
  variations, 
  while 
  

   at 
  the 
  same 
  time 
  both 
  plates 
  were 
  firmly 
  attached 
  to 
  their 
  

   supports 
  when 
  in 
  position. 
  

  

  § 
  6. 
  The 
  divider, 
  whose 
  total 
  resistance 
  was 
  about 
  2400 
  

   ohms, 
  was 
  made 
  to 
  divide 
  the 
  potential-difference 
  between 
  its 
  

   outer 
  terminals 
  into 
  one 
  hundred 
  equal 
  parts. 
  The 
  potential 
  

   applied 
  was 
  usually 
  that 
  given 
  by 
  a 
  Daniell 
  cell 
  of 
  Lord 
  

   Kelvin's 
  gravity 
  type, 
  and 
  was 
  tested 
  frequently 
  by 
  comparison 
  

   with 
  a 
  standard 
  Clark 
  cell, 
  or 
  by 
  means 
  of 
  a 
  standard 
  resistance 
  

   and 
  a 
  current 
  balance. 
  

  

  § 
  7. 
  The 
  permanent 
  connexions 
  were 
  as 
  follows 
  : 
  — 
  (1) 
  The 
  

   lower 
  plate 
  to 
  the 
  insulated 
  pair 
  of 
  quadrants 
  of 
  the 
  electro- 
  

   meter 
  ; 
  (2) 
  The 
  terminals 
  of 
  the 
  Daniell 
  cell 
  to 
  the 
  ends 
  of 
  

   the 
  divider 
  (there 
  was 
  a 
  reversing 
  plug 
  in 
  this 
  circuit) 
  ; 
  (3) 
  

   One 
  end 
  of 
  the 
  divider 
  to 
  the 
  upper 
  plate 
  and 
  uninsulated 
  

   quadrants 
  of 
  the 
  electrometer. 
  

  

  The 
  temporary 
  connexion 
  (§ 
  5) 
  was 
  from 
  the 
  sliding 
  con- 
  

   tact-piece 
  of 
  the 
  divider 
  to 
  the 
  lower 
  plate. 
  

  

  § 
  8. 
  Each 
  experiment 
  was 
  as 
  a 
  rule 
  begun 
  by 
  polishing 
  a 
  

   metal 
  plate 
  on 
  clean 
  glass-paper 
  or 
  emery-cloth. 
  Its 
  con- 
  

   tact 
  potential 
  with 
  a 
  standard 
  plate, 
  generally 
  of 
  electrolytically 
  

   deposited 
  gold 
  washed 
  some 
  hours 
  previously 
  with 
  alcohol, 
  as 
  

   used 
  by 
  M. 
  Pellat, 
  was 
  then 
  measured. 
  The 
  plate 
  was 
  next 
  

   subjected 
  to 
  some 
  particular 
  treatment; 
  for 
  instance 
  it 
  was 
  

   filed, 
  or 
  burnished, 
  or 
  polished 
  on 
  leather 
  or 
  paper, 
  or 
  washed 
  

   with 
  water, 
  alcohol, 
  or 
  turpentine, 
  or 
  heated 
  in 
  air 
  and 
  

   oxidized, 
  or 
  not 
  oxidized, 
  or 
  exposed 
  to 
  steam, 
  or 
  oxygen, 
  or 
  

   fumes 
  of 
  iodine 
  or 
  hydrogen 
  sulphide, 
  or 
  simply 
  left 
  to 
  alter 
  

   under 
  the 
  influences 
  of 
  the 
  atmosphere 
  and 
  its 
  own 
  molecular 
  

   forces. 
  Its 
  potential 
  with 
  the 
  same 
  standard 
  plate 
  was 
  again 
  

   measured, 
  and 
  the 
  change 
  due 
  to 
  the 
  treatment 
  its 
  surface 
  

   had 
  undergone 
  noted. 
  

  

  § 
  9. 
  The 
  metal 
  which 
  requires 
  to 
  be 
  joined 
  to 
  the 
  zinc 
  end 
  

   of 
  the 
  battery, 
  in 
  order 
  to 
  effect 
  a 
  balance, 
  will 
  be 
  called 
  

   positive. 
  Thus 
  aluminium 
  is 
  positive 
  to 
  zinc 
  and 
  zinc 
  positive 
  

   to 
  copper. 
  Also, 
  when 
  a 
  plate 
  becomes 
  more 
  positive 
  its 
  

   potential 
  will 
  be 
  said 
  to 
  rise 
  ; 
  when 
  more 
  negative, 
  to 
  fall. 
  

   When 
  the 
  " 
  potential 
  5 
  ' 
  of 
  a 
  plate 
  is 
  mentioned, 
  without 
  other 
  

   qualification, 
  its 
  contact 
  potential-difference 
  with 
  a 
  standard 
  

   plate 
  (§ 
  8) 
  is 
  meant. 
  

  

  