﻿Variable 
  Frequency 
  Alternating 
  Currents. 
  491 
  

  

  Telephonic 
  Measurements. 
  — 
  The 
  special 
  form 
  of 
  vibrating 
  

   wire 
  described 
  was 
  primarily 
  designed 
  for 
  telephonic 
  measure- 
  

   ments 
  ; 
  and 
  although 
  the 
  tests 
  carried 
  out 
  in 
  this 
  direction 
  

   are 
  far 
  too 
  extensive 
  to 
  describe 
  in 
  any 
  detail 
  in 
  this 
  paper, 
  

   a 
  brief 
  outline 
  may 
  perhaps 
  be 
  given. 
  

  

  Telephonic 
  measurements 
  are 
  now 
  mainly 
  carried 
  out 
  by 
  

   expressing 
  the 
  attenuation 
  of 
  speech-waves 
  in 
  any 
  line 
  circuit 
  

   or 
  apparatus 
  in 
  terms 
  of 
  the 
  attenuation 
  over 
  a 
  given 
  length 
  

   of 
  a 
  certain 
  type 
  of 
  cable 
  line 
  settled 
  on 
  as 
  a 
  standard. 
  

  

  It 
  has 
  been 
  found 
  that 
  the 
  words 
  representing 
  the 
  numbers 
  

   1, 
  2, 
  3, 
  4, 
  5, 
  embody 
  all 
  the 
  frequencies 
  of 
  telephonic 
  im- 
  

   portance, 
  and 
  these 
  numbers 
  are 
  invariably 
  used 
  for 
  testing- 
  

   purposes. 
  

  

  Inspection 
  of 
  the 
  oscillograms 
  of 
  these 
  five 
  words 
  as 
  spoken 
  

   by 
  a 
  number 
  of 
  persons 
  shows 
  that 
  they 
  may 
  be 
  approximately 
  

   represented 
  by 
  fig. 
  19 
  (PL 
  XVI.), 
  which 
  is 
  given 
  by 
  the 
  

   equation 
  

  

  y 
  = 
  '29 
  sin 
  pt 
  + 
  '32 
  sin 
  2pi5 
  + 
  '39 
  sin 
  3 
  ^ 
  + 
  '55 
  sin 
  4: 
  p>t 
  

  

  + 
  1-06 
  sin 
  5^ 
  + 
  6-5 
  sin 
  Upt- 
  |j- 
  1*06 
  sin 
  

  

  1 
  pt 
  — 
  -56 
  sin 
  8pt— 
  *39 
  sin 
  9 
  ^—-32 
  sin 
  10 
  pt 
  

   -•29 
  sin 
  11 
  pt 
  9 
  

   where 
  p-=2irx 
  145. 
  

  

  The 
  general 
  resemblance 
  of 
  this 
  wave 
  to 
  some 
  of 
  those 
  

   produced 
  artificially, 
  see 
  figs. 
  7 
  and 
  12, 
  is 
  obvious 
  on 
  

   inspection. 
  

  

  It 
  has 
  been 
  found 
  that 
  by 
  replacing 
  the 
  human 
  voice 
  and 
  

   transmitter 
  by 
  the 
  vibrating 
  wire 
  producing 
  these 
  waves, 
  

   and 
  by 
  using 
  the 
  correct 
  frequencies, 
  similar 
  results 
  to 
  those 
  

   produced 
  by 
  the 
  voice 
  can 
  be 
  obtained. 
  

  

  Having 
  obtained 
  a 
  satisfactory 
  substitute 
  for 
  the 
  human 
  

   voice, 
  it 
  is 
  next 
  necessaiy 
  to 
  replace 
  the 
  ear 
  and 
  receiver 
  by 
  

   some 
  measuring 
  instrument. 
  

  

  Simple 
  measurements 
  of 
  the 
  comparative 
  current 
  attenua- 
  

   tion 
  over 
  the 
  standard 
  line 
  and 
  the 
  line 
  or 
  apparatus 
  under 
  

   test 
  do 
  not 
  give 
  accurate 
  results, 
  and 
  it 
  was 
  suggested 
  by 
  my 
  

   assistant 
  Mr. 
  A. 
  J. 
  Aldridge, 
  to 
  whom 
  I 
  take 
  this 
  opportunity 
  

   for 
  expressing 
  my 
  indebtedness 
  for 
  considerable 
  help 
  given 
  

   in 
  carrying 
  out 
  the 
  investigations 
  embodied 
  in 
  this 
  paper, 
  

   that 
  direct 
  measurements 
  of 
  the 
  comparative 
  volumes 
  of 
  

   sound 
  issuing 
  from 
  the 
  telephone 
  receiver 
  could 
  be 
  made 
  by 
  

   allowing 
  the 
  receiver 
  to 
  sound 
  into 
  a 
  transmitter 
  connected 
  

   up 
  in 
  the 
  usual 
  manner 
  to 
  a 
  battery 
  and 
  induction-coil, 
  and 
  

   to 
  measure 
  the 
  current 
  in 
  the 
  secondary 
  of 
  the 
  induction- 
  

   coil. 
  

  

  