﻿Bifilar 
  Vibration 
  Galvanometer, 
  

  

  173 
  

  

  permissible 
  to 
  quote 
  the 
  sensibility 
  in 
  millimetres 
  o£ 
  amplitude 
  

   at 
  a 
  metre 
  per 
  R.M.S. 
  microampere. 
  I 
  have 
  theretore 
  

   adopted 
  this 
  method. 
  

  

  Fio:. 
  6. 
  

  

  15 
  20 
  

  

  25 
  

  

  4C 
  

  

  In 
  fig. 
  7 
  (p. 
  174) 
  the 
  relationship 
  between 
  the 
  sensibility 
  of 
  

   the 
  instrument 
  in 
  millimetres 
  o£ 
  amplitude 
  per 
  R.M.S. 
  micro- 
  

   ampere, 
  and 
  the 
  frequency 
  of 
  the 
  alternating 
  current 
  is 
  given. 
  

   The 
  different 
  curves 
  refer 
  to 
  different 
  free 
  lengths 
  of 
  the 
  

   wires. 
  One 
  interesting 
  point 
  of 
  this 
  figure 
  is 
  that 
  where 
  it 
  

   is 
  possible 
  to 
  tune 
  this 
  vibration 
  galvanometer 
  to 
  a 
  given 
  

   frequency, 
  using 
  various 
  combinations 
  of 
  length 
  and 
  tension, 
  

   the 
  sensibility 
  so 
  obtained 
  is 
  very 
  nearly 
  the 
  same 
  as 
  long 
  as 
  the 
  

   wires 
  are 
  longer 
  than 
  the 
  pole 
  pieces. 
  The 
  practical 
  result 
  

   of 
  this 
  is 
  that 
  if 
  at 
  any 
  given 
  length 
  of 
  wire 
  or 
  tension, 
  one 
  

   can 
  tune 
  the 
  instrument 
  to 
  suit 
  the 
  frequency 
  of 
  the 
  alter- 
  

   nating 
  current, 
  then 
  no 
  further 
  adjustments 
  need 
  be 
  made 
  in 
  

   the 
  hope 
  of 
  finding 
  a 
  better 
  combination 
  of 
  length 
  and 
  tension 
  

   for 
  the 
  purpose. 
  

  

  