﻿172 
  Mr. 
  W. 
  Duddell 
  on 
  a 
  

  

  of 
  the 
  deflexion 
  where 
  an 
  alternating 
  current, 
  having 
  a 
  

   constant 
  R.M.S. 
  value, 
  is 
  passed 
  through 
  the 
  instrument, 
  the 
  

   frequency 
  of 
  the 
  alternating 
  current 
  being 
  varied. 
  In 
  fig. 
  4 
  

   the 
  instrument 
  was 
  set 
  so 
  as 
  to 
  be 
  in 
  resonance 
  at 
  a 
  fre- 
  

   quency 
  of 
  595^- 
  per 
  second, 
  and 
  in 
  fig. 
  5 
  at 
  a 
  frequency 
  of 
  

   290^- 
  per 
  second. 
  The 
  small 
  irregularity 
  in 
  the 
  curve 
  fig. 
  5 
  

   at 
  a 
  frequency 
  of 
  240^- 
  per 
  second 
  was 
  not 
  an 
  error 
  of 
  

   observation, 
  but 
  was, 
  I 
  think, 
  due 
  to 
  the 
  intrument 
  resonating 
  

   one 
  of 
  the 
  higher 
  harmonics 
  of 
  the 
  wave 
  form 
  of 
  the 
  

   alternator. 
  

  

  To 
  measure 
  the 
  sensibility 
  of 
  the 
  instrument 
  the 
  following 
  

   method 
  was 
  used 
  : 
  — 
  A 
  current 
  generally 
  0*1 
  ampere 
  from 
  a 
  

   small 
  high-frequency 
  alternator, 
  having 
  a 
  very 
  nearly 
  sinu- 
  

   soidal 
  wave-form, 
  was 
  passed 
  through 
  a 
  small 
  non-inductive 
  

   resistance 
  and 
  an 
  accurate 
  dynamometer. 
  The 
  vibration 
  

   galvanometer 
  in 
  series 
  with 
  a 
  high 
  non-inductive 
  resistance 
  

   was 
  connected 
  as 
  a 
  shunt 
  to 
  the 
  terminals 
  of 
  the 
  small 
  

   resistance 
  in 
  the 
  main 
  circuit. 
  The 
  current 
  of 
  0*1 
  ampere 
  

   flowing 
  through 
  the 
  main 
  resistance 
  produced 
  a 
  small 
  known 
  

   difference 
  of 
  potential 
  applied 
  to 
  the 
  galvanometer 
  circuit 
  

   from 
  which 
  the 
  current 
  through 
  the 
  galvanometer 
  could 
  be 
  

   calculated. 
  The 
  vibration 
  galvanometer 
  it 
  must 
  be 
  remem- 
  

   bered, 
  has 
  when 
  in 
  operation 
  a 
  back 
  E.M.F., 
  so 
  that 
  it 
  is 
  very 
  

   important 
  to 
  keep 
  the 
  resistance 
  in 
  series 
  with 
  the 
  instrument 
  

   very 
  high 
  to 
  prevent 
  the 
  back 
  E.M.F. 
  from 
  falsifying 
  the 
  

   calculation 
  ; 
  25,000 
  ohms 
  was 
  used 
  for 
  most 
  of 
  the 
  tests. 
  

   This 
  method 
  of 
  testing 
  the 
  sensibility 
  of 
  the 
  vibration 
  

   galvanometer 
  really 
  calibrates 
  the 
  vibration 
  galvanometer 
  in 
  

   terms 
  of 
  a 
  standard 
  dynamometer 
  and 
  known 
  resistances. 
  

   The 
  vibration 
  galvanometer 
  is 
  practically 
  insensitive 
  to 
  any- 
  

   thing 
  except 
  the 
  fundamental 
  of 
  the 
  wave-form 
  of 
  the 
  

   alternating 
  current 
  for 
  which 
  it 
  is 
  tuned, 
  whereas 
  the 
  dyna- 
  

   mometer 
  reads 
  the 
  mean 
  squared 
  current 
  which 
  is 
  equal 
  to 
  

   the 
  sum 
  of 
  the 
  squares 
  of 
  the 
  amplitude 
  of 
  all 
  the 
  harmonics 
  

   including 
  the 
  fundamental. 
  The 
  two 
  instruments 
  are 
  there- 
  

   fore 
  not 
  strictly 
  comparable 
  unless 
  the 
  amplitude 
  of 
  all 
  the 
  

   upper 
  harmonics 
  is 
  zero, 
  that 
  is 
  to 
  say, 
  that 
  the 
  current 
  used 
  

   has 
  a 
  pure 
  sine 
  wave-form. 
  

  

  With 
  an 
  instrument 
  giving 
  a 
  very 
  sharp 
  resonance 
  there 
  

   is 
  some 
  difliculty 
  in 
  determining 
  the 
  exact 
  value 
  of 
  the 
  

   maximum 
  amplitude, 
  a 
  l^ct 
  which 
  prevents 
  such 
  consistent 
  

   results 
  being 
  obtained 
  as 
  would 
  otherwise 
  be 
  the 
  case. 
  When 
  

   the 
  instrument 
  is 
  tuned 
  so 
  as 
  to 
  be 
  in 
  resonance 
  with 
  the 
  

   alternating 
  current 
  to 
  be 
  measured 
  the 
  amplitude 
  of 
  the 
  

   vibration 
  is 
  practically 
  proportional 
  to 
  the 
  R.M.S. 
  current, 
  

   as 
  is 
  show^n 
  by 
  the 
  tests 
  recorded 
  in 
  fig. 
  6, 
  hence 
  it 
  is 
  

  

  