﻿Relations 
  between 
  JEther, 
  Matter, 
  and 
  Electricity. 
  363 
  

  

  coils 
  against 
  sudden 
  changes 
  in 
  temperature 
  due 
  to 
  air- 
  

   currents, 
  while 
  at 
  the 
  same 
  time 
  they 
  served 
  as 
  yokes 
  to 
  the 
  

   magnets. 
  A 
  current 
  of 
  one-tenth 
  ampere 
  was 
  used, 
  which 
  

   insured 
  a 
  high 
  state 
  of 
  magnetization 
  in 
  the 
  iron 
  when 
  the 
  

   two 
  coils 
  were 
  in 
  series, 
  giving 
  5000 
  turns. 
  

  

  The 
  coils 
  were 
  connected 
  in 
  the 
  bridge 
  in 
  such 
  a 
  way 
  that 
  

   the 
  two 
  coils 
  in 
  one 
  case 
  formed 
  the 
  opposite 
  arms 
  of 
  the 
  

   bridge, 
  as 
  shown 
  in 
  the 
  diagram. 
  

  

  Fiff. 
  1. 
  

  

  A 
  and 
  B 
  are 
  the 
  two 
  cases 
  and 
  G 
  the 
  galvanometer. 
  At 
  S 
  

   was 
  a 
  reversing 
  switch 
  by 
  which 
  the 
  current 
  in 
  one 
  of 
  the 
  

   coils 
  could 
  be 
  reversed. 
  This 
  changed 
  the 
  field 
  which 
  might 
  

   affect 
  two 
  opposite 
  arms 
  of 
  the 
  bridge, 
  and 
  thus 
  doubled 
  the 
  

   effect 
  on 
  the 
  galvanometer. 
  Another 
  switch 
  might 
  have 
  been 
  

   inserted 
  in 
  the 
  other 
  pair 
  of 
  arms, 
  and 
  thus 
  the 
  effect 
  have 
  

   been 
  again 
  doubled 
  ; 
  but 
  errors 
  due 
  to 
  the 
  switches 
  would 
  

   also 
  have 
  been 
  doubled 
  and 
  no 
  advantage 
  gained. 
  The 
  

   reversing 
  switch 
  was 
  carefully 
  constructed 
  with 
  large 
  copper 
  

   rods 
  fitting 
  into 
  mercury 
  cups 
  made 
  of 
  copper. 
  The 
  contact 
  

   of 
  the 
  copper 
  on 
  copper 
  was 
  so 
  good 
  that 
  thermal 
  effects 
  were 
  

   probably 
  inappreciable. 
  However, 
  at 
  best, 
  the 
  inaccuracies 
  

   of 
  the 
  switch 
  limited 
  the 
  accuracy 
  of 
  the 
  experiment. 
  

  

  The 
  fine 
  adjustments 
  were 
  made 
  by 
  resistance-boxes 
  shunted 
  

   around 
  one 
  of 
  the 
  coils. 
  About 
  15,000 
  ohms 
  in 
  this 
  shunt 
  

   balanced 
  the 
  bridge. 
  A 
  change 
  of 
  one 
  ohm 
  in 
  the 
  shunt 
  

   gave 
  a 
  deflexion 
  of 
  two 
  millimetres, 
  and 
  indicated 
  a 
  change 
  

   in 
  the 
  resistance 
  of 
  the 
  arm 
  of 
  5/100,000 
  ohm. 
  The 
  whole 
  

   resistance 
  being 
  over 
  100 
  ohms, 
  this 
  would 
  give 
  a 
  determination 
  

   of 
  one 
  part 
  in 
  2,000,000 
  or, 
  since 
  the 
  deflexion 
  is 
  doubled, 
  

   one 
  part 
  in 
  4,000,000 
  for 
  each 
  arm. 
  

  

  2 
  B 
  2 
  

  

  