﻿84 
  Electrical 
  Measurement 
  by 
  Alternating 
  Currents. 
  

  

  For 
  using 
  25 
  and 
  26, 
  a 
  key 
  to 
  make 
  instantaneous 
  change 
  

   of 
  connexions 
  is 
  almost 
  necessary. 
  

  

  Method 
  27. 
  

  

  To 
  measure 
  resistance 
  by 
  alternating 
  currents, 
  a 
  Wheat- 
  

   stone 
  bridge 
  is 
  often 
  used 
  with 
  a 
  telephone. 
  

  

  I 
  propose 
  to 
  increase 
  the 
  sensitiveness 
  of 
  the 
  method 
  by 
  

   using 
  my 
  method 
  of 
  passing 
  a 
  strong 
  current 
  through 
  the 
  

   fixed 
  coils 
  of 
  an 
  electrodynamometer 
  while 
  the 
  weaker 
  testing 
  

   current 
  goes 
  through 
  the 
  suspended 
  system. 
  

  

  Using 
  non-inductive 
  resistances, 
  methods 
  10, 
  13 
  A, 
  B, 
  C, 
  

   and 
  14 
  all 
  reduce 
  to 
  proper 
  ones. 
  10 
  or 
  14 
  is 
  specially 
  good 
  

   and 
  I 
  have 
  no 
  doubt 
  will 
  be 
  of 
  great 
  value 
  for 
  liquid 
  resist- 
  

   ances. 
  The 
  liquid 
  resistances 
  must, 
  however, 
  be 
  properly 
  

   designed 
  to 
  avoid 
  polarization 
  errors. 
  The 
  increase 
  of 
  ac- 
  

   curacy 
  over 
  using 
  the 
  electrodynamometer 
  in 
  the 
  usual 
  

   manner 
  is 
  of 
  the 
  order 
  of 
  magnitude 
  of 
  1000 
  times. 
  

  

  By 
  this 
  method, 
  a 
  resistance 
  of 
  10,000,000 
  ohms 
  can 
  be 
  

   measured 
  to 
  1 
  part 
  in 
  1000 
  or 
  even 
  more. 
  This 
  method 
  

   should 
  play 
  a 
  great 
  part 
  in 
  future 
  work 
  on 
  liquid 
  resistances. 
  

  

  Since 
  writing 
  the 
  above 
  I 
  have 
  tried 
  some 
  of 
  the 
  methods, 
  

   especially 
  6 
  and 
  12. 
  By 
  the 
  method 
  12, 
  results 
  to 
  1 
  in 
  1000 
  

   can 
  be 
  obtained. 
  Replacing 
  L' 
  by 
  an 
  equal 
  coil, 
  the 
  ratio 
  of 
  

   the 
  two, 
  all 
  other 
  errors 
  being 
  eliminated, 
  can 
  be 
  obtained 
  to 
  

   1 
  in 
  10,000, 
  or 
  even 
  more 
  accurately. 
  

  

  The 
  main 
  error 
  to 
  be 
  guarded 
  against 
  in 
  method 
  12, 
  or 
  any 
  

   other 
  where 
  large 
  inductances 
  or 
  resistances 
  are 
  included, 
  

   arises 
  from 
  twisting 
  the 
  wires 
  leading 
  to 
  these. 
  The 
  electro- 
  

   static 
  action 
  of 
  the 
  leads 
  or 
  the 
  twisted 
  wire 
  coils 
  of 
  an 
  

   ordinary 
  resistance-box, 
  may 
  cause 
  errors 
  of 
  several 
  per 
  cent. 
  

   Using 
  short 
  small 
  wire 
  leads 
  far 
  apart, 
  the 
  error 
  becomes 
  

   very 
  small. 
  

  

  Method 
  6 
  is 
  also 
  very 
  accurate, 
  but 
  the 
  electric 
  absorption 
  

   of 
  the 
  condensers 
  makes 
  much 
  accurac} 
  r 
  impossible 
  unless 
  a 
  

   series 
  of 
  experiments 
  is 
  made 
  to 
  determine 
  the 
  apparent 
  

   resistance 
  due 
  to 
  this 
  cause. 
  

  

  In 
  method 
  12 
  I 
  have 
  not 
  yet 
  detected 
  any 
  error 
  due 
  to 
  

   twisting 
  the 
  wires 
  of 
  coils 
  I. 
  However, 
  the 
  electrostatic 
  

   action 
  of 
  twisted 
  wire 
  coils 
  is 
  immense, 
  and 
  the 
  warning 
  

   against 
  their 
  use 
  which 
  I 
  have 
  given 
  above 
  has 
  been 
  well 
  

   substantiated 
  by 
  experiment. 
  Only 
  in 
  case 
  of 
  low 
  resistances 
  

   and 
  low 
  inductances, 
  or 
  in 
  cases 
  like 
  that 
  just 
  mentioned, 
  is 
  

   it 
  to 
  be 
  tolerated 
  for 
  a 
  moment. 
  Connecting 
  two 
  twisted 
  

   wires 
  in 
  a 
  coil 
  in 
  series 
  with 
  a 
  resistance 
  between 
  I 
  have 
  

  

  