﻿C. 
  Barus 
  — 
  Excursions 
  of 
  a 
  Telephone 
  Diaphragm. 
  219 
  

  

  Art. 
  XX. 
  — 
  Note 
  on 
  the 
  Excursions 
  of 
  the 
  Diaphragm 
  of 
  a 
  

   Telephone; 
  by 
  C. 
  Baeus. 
  

  

  1. 
  Sometime 
  ago,* 
  while 
  attempting 
  to 
  measure 
  the 
  mini- 
  

   mum 
  current 
  audible 
  in 
  a 
  telephone, 
  I 
  encountered 
  a 
  rather 
  

   remarkable 
  phenomenon: 
  With 
  a 
  continually 
  decreasing 
  cur- 
  

   rent 
  the 
  sound 
  was 
  found 
  to 
  terminate 
  abruptly. 
  A 
  definite 
  

   interval 
  of 
  silence 
  could 
  be 
  mapped 
  out 
  for 
  gradually 
  increas- 
  

   ing 
  currents 
  on 
  either 
  side 
  of 
  zero, 
  and 
  the 
  extent 
  of 
  this 
  inter- 
  

   val 
  proved 
  to 
  be 
  5 
  to 
  even 
  15 
  times 
  the 
  interval 
  of 
  current 
  

   within 
  which 
  change 
  of 
  sound 
  intensity 
  is 
  appreciable 
  when 
  

   the 
  limits 
  of 
  the 
  field 
  in 
  question 
  are 
  exceeded. 
  Thus 
  at 
  the 
  

   margins 
  of 
  the 
  field 
  shades 
  of 
  intensity 
  of 
  about 
  10" 
  7 
  ampere 
  

   were 
  acoustically 
  distinguishable, 
  whereas 
  the 
  silent 
  field 
  

   extended 
  in 
  an 
  average 
  case 
  from 
  — 
  5xl0~ 
  7 
  to 
  + 
  5xl0~ 
  7 
  

   amperes. 
  

  

  I 
  have 
  since 
  been 
  endeavoring 
  to 
  find 
  some 
  reason 
  for 
  this 
  

   result, 
  and 
  it 
  occurred 
  to 
  me 
  that 
  the 
  interference 
  apparatus 
  

   described 
  in 
  my 
  last 
  paperf 
  would 
  be 
  adapted 
  for 
  the 
  purpose. 
  

   It 
  would 
  merely 
  be 
  necessary 
  to 
  replace 
  

   the 
  helices 
  shown 
  in 
  the 
  figure 
  there 
  given 
  

   by 
  two 
  ordinary 
  Bell 
  telephones 
  and 
  to 
  

   -attach 
  a 
  very 
  light 
  mirror 
  to 
  the 
  center 
  r*" 
  1 
  ! 
  

  

  of 
  the 
  diaphragm 
  of 
  each, 
  in 
  order 
  to 
  

   meet 
  the 
  conditions 
  of 
  the 
  experiment. 
  

  

  These 
  attached 
  mirrors 
  were 
  but 
  0*05 
  s 
  in 
  M 
  

  

  weight 
  and 
  less 
  than 
  l 
  cm 
  in 
  diameter, 
  

   ^ence 
  they 
  did 
  not 
  hamper 
  the 
  motion 
  

   of 
  the 
  diaphragm 
  in 
  any 
  way. 
  

  

  Interference 
  measurements 
  of 
  the 
  ex- 
  Q 
  

  

  eursions 
  of 
  the 
  plate 
  are 
  not 
  new 
  ; 
  they 
  ^ 
  ^ 
  Telephones. 
  

   were 
  made 
  by 
  SaletJ 
  and 
  more 
  recently 
  M, 
  Refractometer 
  

   by 
  Franckeg 
  with 
  divergent 
  results 
  (20 
  X 
  plate, 
  Burner 
  beyoud 
  

   10- 
  6 
  toSOxlO" 
  6 
  - 
  in 
  the 
  first 
  case 
  and 
  A 
  Telescope 
  beyond 
  <?. 
  

   20-7 
  cm 
  j 
  n 
  f^g 
  i 
  as 
  (. 
  case 
  ^ 
  j 
  n 
  both 
  instances, 
  however, 
  New- 
  

   tonian 
  interferences 
  were 
  produced, 
  which 
  for 
  the 
  present 
  

   purposes 
  are 
  manifestly 
  inferior 
  to 
  the 
  Michelson 
  refractometer. 
  

  

  The 
  subject 
  as 
  a 
  whole 
  has 
  been 
  overhauled 
  in 
  a 
  series 
  of 
  

   interesting 
  researches 
  due 
  to 
  Prof. 
  C. 
  R. 
  Cross, 
  Cross 
  and 
  

   Mansfield 
  and 
  others 
  [ 
  using 
  a 
  stroboscopic 
  micrometer 
  method. 
  

  

  * 
  Barus 
  : 
  Phil. 
  Mag. 
  (5), 
  xxxviii, 
  p. 
  558, 
  1894. 
  

   f 
  Barus: 
  This 
  Journal, 
  February, 
  1897. 
  

   X 
  Salet: 
  Comp. 
  Rend., 
  xcv, 
  p. 
  178, 
  1882. 
  

   gFranke: 
  Electrotech. 
  Zeitschrift, 
  xi, 
  pp. 
  288, 
  1890. 
  

  

  I 
  Cross 
  and 
  Mansfield: 
  Proc. 
  American 
  Acad., 
  xxviii, 
  p. 
  93, 
  1892; 
  cf. 
  Experi- 
  

   ment 
  with 
  the 
  microphone, 
  ibid., 
  xxv, 
  p. 
  69, 
  1890. 
  

  

  