﻿Volta 
  Electricity 
  of 
  Metals. 
  401 
  

  

  9 
  centim. 
  in 
  diameter 
  and 
  2 
  centim. 
  thickness, 
  is 
  insulated 
  

   and 
  in 
  permanent 
  connexion 
  with 
  the 
  insulated 
  pair 
  of 
  

   quadrants 
  of 
  an 
  electrometer. 
  A 
  similar 
  plate 
  of 
  the 
  other 
  

   metal 
  placed 
  parallel 
  to 
  the 
  first, 
  at 
  a 
  distance 
  of 
  a 
  few 
  milli- 
  

   metres 
  from 
  it, 
  is 
  uninsulated 
  and 
  in 
  connexion 
  with 
  the 
  

   uninsulated 
  pair 
  of 
  quadrants. 
  While 
  in 
  this 
  position 
  they 
  

   are 
  temporarily 
  connected 
  with 
  one 
  another 
  through 
  a 
  simple 
  

   form 
  of 
  potential-divider 
  in 
  which 
  a 
  slope 
  of 
  potential 
  is 
  

   maintained 
  by 
  a 
  Daniell 
  cell, 
  in 
  the 
  direction 
  opposite 
  to 
  that 
  

   given 
  by 
  the 
  experimental 
  plates. 
  The 
  temporary 
  connexion 
  

   is 
  now 
  broken 
  and 
  the 
  plates 
  are 
  separated. 
  In 
  doing 
  so 
  the 
  

   capacity 
  of 
  the 
  condenser 
  is 
  reduced, 
  hence 
  if 
  there 
  be 
  any 
  

   electric 
  charge 
  on 
  them 
  it 
  will 
  be 
  indicated 
  by 
  a 
  further 
  

   deflexion 
  of 
  the 
  electrometer. 
  This 
  operation 
  is 
  repeated 
  

   with 
  different 
  values 
  of 
  the 
  counterpotential 
  until 
  separation 
  

   of 
  the 
  plates 
  produces 
  no 
  change 
  in 
  the 
  deflexion 
  of 
  the 
  

   electrometer. 
  Since 
  it 
  has 
  annulled 
  the 
  charge, 
  the 
  counter- 
  

   potential 
  must 
  now 
  be 
  equal 
  and 
  opposite 
  to 
  the 
  natural 
  

   potential-difference. 
  

  

  § 
  5. 
  During 
  most 
  of 
  the 
  experiments 
  a 
  piece 
  of 
  apparatus 
  

   made 
  by 
  Lord 
  Kelvin 
  in 
  or 
  about 
  1861 
  was 
  used. 
  In 
  it 
  the 
  

   plates 
  were 
  surrounded 
  by 
  a 
  cylindrical 
  zinc 
  case. 
  This 
  was 
  

   made 
  in 
  two 
  parts 
  for 
  convenience 
  in 
  manipulation, 
  the 
  lower 
  

   fixed 
  to 
  a 
  heavy 
  cast-iron 
  base 
  plate, 
  while 
  the 
  upper 
  stood 
  

   upon 
  small 
  brackets 
  attached 
  to 
  the 
  top 
  edge 
  of 
  the 
  lower 
  

   part, 
  its 
  position 
  being 
  regulated 
  by 
  a 
  hole-slot-and-plane 
  

   arrangement. 
  Openings 
  in 
  the 
  sides 
  of 
  the 
  upper 
  part 
  

   facilitated 
  the 
  adjustment 
  of 
  the 
  experimental 
  plates, 
  but 
  these 
  

   were 
  closed 
  by 
  a 
  sliding 
  cover 
  during 
  a 
  measurement 
  of 
  

   potential. 
  

  

  The 
  lower 
  plate 
  was 
  supported 
  by 
  an 
  insulating 
  glass 
  stem 
  

   which 
  was 
  kept 
  dry 
  by 
  means 
  of 
  pumice 
  and 
  sulphuric 
  acid. 
  

   On 
  the 
  top 
  of 
  this 
  stem 
  was 
  a 
  brass 
  cap 
  screwed 
  to 
  fit 
  the 
  

   sockets 
  which 
  were 
  soldered 
  to 
  the 
  backs 
  of 
  the 
  plates, 
  and 
  

   from 
  the 
  brass 
  cap 
  a 
  stiff 
  wire 
  passed 
  out 
  through 
  a 
  hole 
  in 
  

   the 
  case 
  and 
  formed 
  the 
  connexion 
  between 
  the 
  insulated 
  

   plate 
  and 
  the 
  electrometer. 
  

  

  A 
  piece 
  of 
  platinum 
  foil 
  was 
  soldered 
  round 
  a 
  part 
  of 
  this 
  

   wire 
  to 
  give 
  a 
  clean 
  contact 
  for 
  the 
  temporary 
  connexion 
  

   between 
  the 
  plates 
  through 
  the 
  potential-divider, 
  and 
  the 
  wire 
  

   from 
  the 
  divider 
  had 
  likewise 
  a 
  platinum 
  end 
  piece. 
  

  

  The 
  upper 
  plate 
  hung, 
  by 
  a 
  ring 
  at 
  the 
  centre 
  of 
  its 
  back, 
  

   on 
  a 
  hook 
  at 
  the 
  lower 
  end 
  of 
  a 
  vertical 
  metal 
  rod 
  which 
  

   could 
  be 
  drawn 
  up 
  so 
  as 
  to 
  increase 
  the 
  distance 
  between 
  the 
  

   plates. 
  A 
  small 
  disk 
  fixed 
  on 
  the 
  rod 
  at 
  right 
  angles 
  to 
  it 
  just 
  

   above 
  the 
  hook 
  had 
  three 
  screws 
  passed 
  through 
  it 
  and 
  pressed 
  

  

  