﻿Bifilar 
  Vibration 
  Galvanometer. 
  179 
  

  

  through 
  il, 
  and 
  so 
  appears, 
  as 
  far 
  as 
  the 
  outside 
  circuit 
  is 
  con- 
  

   cerned, 
  like 
  an 
  addition 
  to 
  the 
  resistance 
  of 
  the 
  instrument, 
  

   indicates 
  that 
  no 
  arrangement 
  of 
  condensers 
  is 
  Kkely 
  to 
  

   improve 
  matters 
  as 
  one 
  might 
  at 
  first 
  expect. 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  intercept 
  in 
  the 
  above 
  table 
  

   decreases 
  for 
  a 
  given 
  frequency 
  with 
  the 
  length 
  of 
  the 
  wires 
  

   in 
  use, 
  and 
  as 
  the 
  current 
  sensibility 
  does 
  not 
  change 
  at 
  all 
  

   rapidly 
  with 
  the 
  active 
  length 
  of 
  the 
  wires 
  there 
  is 
  some 
  

   advantage 
  in 
  using 
  the 
  wires 
  as 
  short 
  as 
  possible 
  when 
  

   measuring 
  small 
  P.D/s 
  in 
  a 
  low 
  resistance 
  circuit. 
  

  

  I 
  have 
  not 
  had 
  an 
  opportunity 
  of 
  testing 
  the 
  back 
  E.M.F. 
  

   of 
  other 
  vibration 
  galvanometers. 
  In 
  the 
  case 
  of 
  the 
  moving- 
  

   coil 
  vibration 
  galvanometer 
  I 
  should 
  expect 
  that 
  it 
  would 
  be 
  

   considerably 
  larger 
  than 
  in 
  the 
  bifilar 
  owing 
  to 
  the 
  coil 
  

   having 
  a 
  number 
  of 
  turns. 
  I 
  trust 
  that 
  the 
  discussion 
  on 
  the 
  

   paper 
  may 
  bring 
  out 
  some 
  information 
  on 
  this 
  point. 
  

  

  All 
  the 
  sensibilities 
  so 
  far 
  given 
  for 
  the 
  vibration 
  galvano- 
  

   meter 
  correspond 
  to 
  the 
  use 
  of 
  a 
  permanent 
  magnet 
  for 
  the 
  

   field. 
  The 
  instrument 
  has, 
  however, 
  been 
  tried 
  in 
  an 
  oscillo- 
  

   graph 
  electromagnet, 
  and 
  the 
  current 
  sensibility 
  was 
  found 
  to 
  

   be 
  increased 
  threefold. 
  

  

  This 
  corresponds 
  at 
  a 
  100 
  frequency 
  to 
  a 
  sensibility 
  of 
  not 
  

   less 
  than 
  160 
  millimetres 
  per 
  microampere 
  ; 
  as 
  a 
  small 
  

   fraction 
  of 
  a 
  millimetre 
  movement 
  of 
  the 
  spot 
  is 
  noticeable 
  we 
  

   may 
  reckon 
  that 
  at 
  a 
  100 
  frequency 
  with 
  this 
  instrument 
  we 
  

   can 
  detect 
  a 
  current 
  as 
  low 
  as 
  2 
  or 
  3 
  X 
  10~^ 
  ampere. 
  

  

  The 
  advantages 
  of 
  the 
  bifilar 
  galvanometer 
  may 
  be 
  

   summarized 
  as 
  follows: 
  — 
  Simplicity 
  — 
  ease 
  in 
  tuning 
  — 
  wide 
  

   range 
  of 
  frequency 
  for 
  which 
  it 
  can 
  be 
  tuned 
  — 
  high 
  sensi- 
  

   bilitv 
  — 
  negligible 
  self-induction 
  — 
  comparatively 
  small 
  back 
  

   E.M.F. 
  

  

  Its 
  main 
  defect 
  is 
  the 
  small 
  size 
  of 
  mirror 
  that 
  it 
  is 
  neces- 
  

   sary 
  to 
  use 
  on 
  the 
  instrument. 
  With 
  a 
  carefully 
  adjusted 
  

   optical 
  arrangement, 
  and 
  using 
  a 
  small 
  4 
  volt 
  metal 
  filament 
  

   lamp, 
  one 
  can 
  work 
  with 
  comfort 
  at 
  a 
  scale 
  distance 
  of 
  a 
  

   metre 
  in 
  a 
  room 
  which 
  is 
  not 
  too 
  well 
  lighted. 
  

  

  In 
  conclusion, 
  I 
  wish 
  to 
  express 
  my 
  indebtedness 
  to 
  my 
  

   assistant 
  Mr. 
  Neale, 
  for 
  the 
  painstaking 
  way 
  in 
  which 
  he 
  has 
  

   made 
  the 
  experiments 
  recorded 
  in 
  this 
  paper. 
  

  

  N2 
  

  

  