﻿the 
  Conductivity 
  of 
  Liquids 
  in 
  Thin 
  Layers. 
  259 
  

  

  driven 
  by 
  means 
  of 
  a 
  small 
  water-motor. 
  It 
  will 
  be 
  seen 
  

   from 
  the 
  diagram 
  that 
  the 
  current 
  through 
  the 
  galvanometer 
  

   was 
  always 
  continuous, 
  but 
  the 
  sensibility 
  was 
  reduced 
  slightly 
  

   owing 
  to 
  the 
  insulating 
  spaces 
  between 
  the 
  segments 
  of 
  the 
  

   commutator. 
  When 
  the 
  four 
  brushes 
  were 
  set 
  right 
  the 
  

   deflexion 
  was 
  quite 
  steady 
  and 
  was 
  not 
  affected 
  by 
  slight 
  

   changes 
  in 
  speed, 
  with 
  the 
  liquid 
  layer 
  in 
  circuit. 
  

  

  (3) 
  The 
  third 
  method 
  was 
  the 
  modification 
  of 
  the 
  Wheat- 
  

   stone 
  bridge 
  described 
  by 
  T. 
  0. 
  Fitzpatrick 
  *. 
  By 
  means 
  of 
  

   a 
  double 
  commutator, 
  designed 
  by 
  Prof. 
  Threlfall, 
  an 
  alter- 
  

   nating 
  current 
  is 
  supplied 
  to 
  two 
  corners 
  of 
  a 
  Wheatstone 
  

   bridge, 
  whilst 
  the 
  connexions 
  to 
  the 
  galvanometer 
  are 
  reversed 
  

   at 
  the 
  same 
  time 
  as 
  the 
  current, 
  so 
  that 
  any 
  current 
  through 
  

   the 
  galvanometer 
  is 
  always 
  in 
  one 
  direction. 
  The 
  commutator 
  

   employed 
  at 
  first 
  had 
  two 
  drums, 
  each 
  with 
  two 
  brass 
  segments 
  

   and 
  four 
  brushes. 
  It 
  is 
  shown 
  in 
  fig. 
  3, 
  with 
  the 
  connexions. 
  

   Driven 
  at 
  full 
  speed 
  the 
  commutator 
  gave 
  about 
  60 
  alterna- 
  

   tions 
  per 
  second. 
  

  

  Method 
  of 
  Experimenting. 
  

  

  With 
  the 
  direct 
  deflexion 
  methods 
  of 
  measuring 
  the 
  resis- 
  

   tance 
  the 
  upper 
  electrode 
  was 
  set 
  at 
  the 
  required 
  distance 
  

   from 
  the 
  lower 
  one, 
  the 
  current 
  switched 
  on, 
  and 
  a 
  reading 
  

   of 
  the 
  galvanometer 
  taken 
  at 
  a 
  definite 
  interval 
  of 
  time, 
  T, 
  

   afterwards 
  ; 
  usually 
  about 
  1 
  minute. 
  A 
  series 
  of 
  readings 
  

   was 
  taken, 
  increasing 
  the 
  thickness 
  of 
  the 
  layer 
  before 
  each 
  

   reading, 
  and 
  then 
  a 
  series 
  decreasing 
  the 
  thickness. 
  The 
  

   means 
  of 
  these 
  two 
  sets, 
  when 
  they 
  agreed 
  fairly 
  closely, 
  were 
  

   usually 
  taken. 
  

  

  In 
  the 
  case 
  of 
  the 
  bridge-method 
  the 
  measurements 
  were 
  

   made 
  as 
  quickly 
  as 
  possible 
  one 
  after 
  the 
  other, 
  so 
  as 
  to 
  avoid 
  

   any 
  changes 
  in 
  the 
  slate 
  of 
  the 
  liquid, 
  or 
  of 
  the 
  surfaces 
  of 
  

   the 
  electrodes. 
  

  

  The 
  specific 
  resistance 
  is 
  calculated 
  from 
  the 
  formula 
  

  

  Sp.E.= 
  'f 
  

  

  Results 
  with 
  First 
  Form 
  of 
  Apparatus. 
  

  

  The 
  first 
  liquids 
  tried 
  were 
  paraffin-oil, 
  turpentine, 
  benzol, 
  

   and 
  CS 
  2 
  , 
  using 
  method 
  (1) 
  to 
  find 
  the 
  resistance; 
  but 
  their 
  

   resistance 
  was 
  so 
  high 
  that 
  only 
  a 
  small 
  deflexion 
  could 
  be 
  

   obtained 
  with 
  the 
  thinnest 
  layer 
  (*317 
  mm.), 
  about 
  50 
  mm 
  

   at 
  the 
  most. 
  No 
  results 
  were 
  therefore 
  obtained 
  for 
  these 
  

   substances. 
  

  

  * 
  B. 
  A. 
  Report, 
  1886. 
  

  

  