﻿1048 
  Dr. 
  Norman 
  Campbell 
  on 
  the 
  Electrical 
  

  

  observation 
  which 
  has 
  led 
  some 
  experimenters 
  to 
  imagine 
  

   that 
  these 
  liquid 
  resistances 
  show 
  polarization. 
  As 
  a 
  matter 
  

   of 
  fact 
  this 
  " 
  zero 
  E.M.F/' 
  has 
  nothing 
  to 
  do 
  with 
  the 
  mixture 
  

   in 
  the 
  cell; 
  it 
  arises 
  from 
  a 
  P.D. 
  between 
  the 
  external 
  

   electrode 
  of 
  the 
  cell 
  which 
  is 
  connected 
  to 
  the 
  electrometer 
  

   and 
  the 
  " 
  earthed 
  " 
  surroundings 
  ; 
  it 
  is 
  very 
  much 
  less 
  when 
  

   care 
  is 
  taken 
  to 
  obtain 
  good 
  insulation 
  between 
  that 
  electrode 
  

   and 
  its 
  surroundings 
  by 
  means 
  of 
  the 
  sealing-wax 
  (fig. 
  1). 
  

   Even 
  if 
  this 
  zero 
  E.M.F. 
  is 
  (as 
  it 
  may 
  be 
  in 
  bad 
  cases) 
  com- 
  

   parable 
  with 
  1 
  volt, 
  it 
  need 
  not 
  vitiate 
  the 
  measurements 
  of 
  

   resistance 
  by 
  means 
  of 
  the 
  bridge. 
  If 
  r/, 
  r 
  2 
  are 
  the 
  values 
  

   of 
  rj, 
  r 
  2 
  necessary 
  for 
  a 
  balance 
  when 
  the 
  current 
  flows 
  in 
  

   one 
  direction, 
  r" 
  , 
  r 
  2 
  " 
  those 
  when 
  it 
  flows 
  in 
  the 
  other, 
  then 
  

  

  R 
  

  

  2\r 
  l 
  ' 
  + 
  r 
  2 
  '^r 
  1 
  " 
  + 
  r 
  2 
  ") 
  

  

  It 
  was 
  also 
  found 
  that 
  it 
  was 
  usually 
  impossible 
  to 
  balance 
  

   the 
  bridge 
  permanently 
  and 
  exactly 
  ; 
  the 
  position 
  of 
  balance 
  

   varied 
  slightly 
  with 
  the 
  time 
  which 
  had 
  elapsed 
  since 
  the 
  

   battery 
  circuit 
  was 
  closed 
  or 
  reversed. 
  The 
  ratio 
  of 
  the 
  

   resistances 
  measured 
  2 
  minutes 
  after 
  reversal 
  was 
  about 
  2 
  

   or 
  3 
  parts 
  in 
  a 
  1000 
  different 
  from 
  that 
  measured 
  10 
  minutes 
  

   after 
  reversal, 
  the 
  change 
  being 
  in 
  such 
  a 
  direction 
  that 
  the 
  

   resistances 
  tended 
  to 
  become 
  more 
  nearly 
  equal 
  with 
  lapse 
  of 
  

   time. 
  This 
  drift 
  of 
  the 
  position 
  of 
  the 
  balance 
  was 
  traced 
  

   almost 
  certainly 
  to 
  induction 
  effects 
  in 
  the 
  glass 
  of 
  the 
  cells 
  

   or 
  other 
  insulator 
  surrounding 
  the 
  conductor 
  ; 
  it 
  could 
  just 
  

   ba 
  detected 
  even 
  when 
  two 
  wire-resistances 
  were 
  compared. 
  

   If 
  readings 
  were 
  always 
  taken 
  at 
  the 
  same 
  interval 
  after 
  

   reversal 
  perfect 
  consistency 
  was 
  obtained 
  and 
  the 
  measured 
  

   ratio 
  of 
  the 
  resistances 
  was 
  wholly 
  independent 
  of 
  the 
  

   E.M.F. 
  of 
  the 
  battery. 
  The 
  values 
  given 
  are 
  in 
  all 
  cases 
  

   those 
  3 
  minutes 
  after 
  reversal. 
  

  

  3. 
  Experiments 
  were 
  made 
  first 
  on 
  the 
  variation 
  of 
  the 
  

   conductivity 
  of 
  the 
  mixtures 
  with 
  their 
  composition, 
  the 
  tem- 
  

   perature 
  being 
  in 
  all 
  cases 
  25°' 
  6. 
  The 
  xylol 
  and 
  alcohol 
  

   used 
  in 
  preparing 
  the 
  mixtures 
  had 
  been 
  each 
  twice 
  re- 
  

   distilled 
  from 
  calcium 
  ; 
  the 
  same 
  specimens 
  were 
  used 
  in 
  

   preparing 
  all 
  these 
  mixtures. 
  The 
  following 
  table 
  gives 
  the 
  

   specific 
  conductivity 
  (7) 
  for 
  various 
  values 
  of 
  v, 
  where 
  y 
  is 
  

   the 
  ratio 
  of 
  the 
  weight 
  of 
  the 
  alcohol 
  to 
  the 
  total 
  weight 
  of 
  

   the 
  mixture; 
  y 
  = 
  l 
  corresponds 
  to 
  pure 
  alcohol, 
  y 
  = 
  to 
  pure 
  

   xylol. 
  It 
  will 
  be 
  observed 
  that 
  the 
  conductivity 
  varies 
  

   extremely 
  rapidly 
  with 
  y, 
  the 
  variation 
  being 
  the 
  more 
  

   rapid 
  the 
  smaller 
  the 
  value 
  of 
  y 
  9 
  and 
  that 
  the 
  conductivity 
  

  

  