﻿Bifilar 
  Vibration 
  Galvanometer. 
  

  

  169 
  

  

  the 
  sensibility 
  to 
  alternating 
  currents 
  depends 
  very 
  greatly 
  

   on 
  the 
  magnification 
  one 
  can 
  obtain 
  by 
  bringing 
  the 
  instru- 
  

   ment 
  into 
  tune 
  or 
  resonance 
  with 
  the 
  alternating 
  current 
  to 
  

   be 
  measured. 
  These 
  considerations 
  have 
  led 
  me 
  to 
  construct 
  

   a 
  vibration 
  galvanometer 
  in 
  which 
  the 
  mass 
  o£ 
  the 
  moving 
  

   parts 
  is 
  reduced 
  to 
  a 
  minimum, 
  the 
  moving 
  coil 
  being 
  reduced 
  

   to 
  the 
  two 
  wires 
  forming 
  its 
  two 
  sides^ 
  similar 
  to 
  a 
  bifilar 
  

   oscillograph, 
  but 
  with 
  this 
  difference 
  : 
  — 
  Whereas 
  the 
  bifilar 
  

   oscillograph 
  is 
  designed 
  so 
  as 
  to 
  make 
  the 
  damping 
  aperiodic, 
  

   the 
  bifilar 
  vibration 
  galvanometer 
  is 
  designed 
  so 
  as 
  to 
  keep 
  

   the 
  damping 
  as 
  small 
  as 
  possible. 
  

  

  The 
  design 
  of 
  the 
  instrument* 
  is 
  shown 
  in 
  fig. 
  1, 
  in 
  which 
  

   a 
  J 
  b, 
  c, 
  6?, 
  is 
  a 
  fine 
  bronze 
  wire 
  passing 
  

   over 
  a 
  pulley 
  p, 
  and 
  stretched 
  tight 
  by 
  

   means 
  of 
  a 
  spring, 
  the 
  tension 
  on 
  the 
  

   spring 
  being 
  capable 
  of 
  variation 
  by 
  a 
  

   milled 
  head. 
  The 
  wires 
  carry 
  a 
  mirror 
  M 
  

   in 
  the 
  centre 
  and 
  are 
  placed 
  in 
  a 
  strong 
  

   magnetic 
  field 
  between 
  the 
  poles 
  N 
  and 
  S 
  

   of 
  a 
  magnet. 
  The 
  wires 
  pass 
  over 
  two 
  

   bridge 
  pieces 
  B, 
  B, 
  which 
  limit 
  the 
  length 
  

   of 
  the 
  wires 
  which 
  is 
  free 
  to 
  vibrate. 
  

   These 
  two 
  bridge 
  pieces 
  can 
  be 
  moved 
  

   nearer 
  together 
  or 
  further 
  apart 
  by 
  means 
  

   of 
  a 
  right 
  and 
  left 
  handed 
  screw 
  as 
  

   required. 
  The 
  current 
  to 
  be 
  measured 
  

   passes 
  up 
  one 
  wire 
  and 
  down 
  the 
  other, 
  

   causing 
  one 
  wire 
  to 
  tend 
  to 
  move 
  forward 
  

   and 
  the 
  other 
  back 
  in 
  the 
  magnetic 
  field 
  

   and 
  so 
  tilts 
  the 
  mirror 
  M 
  through 
  a 
  small 
  

   angle. 
  

  

  The 
  periodic 
  time 
  of 
  the 
  wires 
  depends 
  

   on 
  their 
  mass, 
  length, 
  and 
  tension, 
  as 
  

   well 
  as 
  upon 
  the 
  moment 
  of 
  inertia 
  of 
  the 
  

   mirror. 
  In 
  a 
  completed 
  instrument 
  the 
  

   moment 
  of 
  inertia 
  of 
  the 
  mirror 
  and 
  the 
  

   mass 
  of 
  the 
  v/ires 
  are 
  fixed, 
  but 
  their 
  

   length 
  and 
  tension 
  can 
  be 
  altered 
  in 
  order 
  

   to 
  adjust 
  the 
  periodic 
  time. 
  Fig. 
  2 
  shows 
  the 
  relationship 
  

   between 
  the 
  free 
  length 
  of 
  the 
  wires 
  and 
  the 
  frequency 
  of 
  the 
  

   free 
  vibrations 
  of 
  the 
  instrument 
  for 
  different 
  tensions, 
  and 
  fig. 
  3 
  

   gives 
  the 
  relationship 
  between 
  the 
  frequency 
  and 
  the 
  tension 
  

   for 
  a 
  series 
  of 
  different 
  lengths. 
  It 
  will 
  be 
  seen 
  from 
  the 
  curves 
  

   that 
  the 
  total 
  range 
  of 
  frequency 
  obtainable 
  with 
  the 
  instru- 
  

   ment 
  is 
  very 
  large, 
  namely, 
  from 
  about 
  90-^ 
  per 
  second 
  up 
  

   * 
  Messrs. 
  Nalder 
  Bros, 
  are 
  manufacturing 
  the 
  instruments. 
  

  

  arf 
  

  

  