﻿220 
  C. 
  Barus 
  — 
  Excursions 
  of 
  a 
  Telephone 
  Diaphragm. 
  

  

  They 
  found 
  that 
  if 
  x 
  is 
  excursion 
  or 
  amplitude 
  of 
  vibration 
  in 
  

   10" 
  4 
  cm 
  and 
  y 
  the 
  current 
  in 
  milliamperes, 
  

  

  y 
  2 
  = 
  0-305a? 
  3 
  - 
  26 
  

  

  in 
  very 
  close 
  agreement 
  with 
  their 
  results. 
  They 
  select 
  y 
  = 
  2 
  

   as 
  a 
  strong 
  telephone 
  current 
  and 
  thus 
  find 
  the 
  corresponding 
  

   excursion 
  to 
  be 
  220xl0~ 
  6cm 
  , 
  with 
  the 
  remark 
  that 
  the 
  true 
  tel- 
  

   ephone 
  current 
  is 
  probably 
  somewhat 
  less 
  than 
  this, 
  inasmuch 
  

   as 
  the 
  field-strength 
  for 
  which 
  the 
  equations 
  hold 
  is 
  somewhat 
  

   greater 
  than 
  the 
  telephone 
  field. 
  

  

  Now 
  if 
  the 
  current 
  of 
  minimum 
  change 
  of 
  audibility 
  is«10~ 
  4 
  

   milliamperes, 
  as 
  I 
  found, 
  the 
  corresponding 
  change 
  of 
  excur- 
  

   sion 
  as 
  called 
  for 
  by 
  the 
  above 
  equation 
  would 
  be 
  5X10" 
  7 
  cm 
  . 
  

   Furthermore 
  if 
  the 
  extent 
  of 
  the 
  interval 
  of 
  silence 
  is 
  ±5 
  X 
  10" 
  4 
  

   milliamperes, 
  the 
  smallest 
  excursion 
  obtainable 
  is 
  1*36 
  xlO 
  -6 
  cm 
  . 
  

   Hence 
  if 
  all 
  these 
  divers 
  results 
  hang 
  correctly 
  together, 
  the 
  

   smallest 
  audible 
  excursions 
  of 
  the 
  telephone 
  should 
  be 
  immeas- 
  

   urable 
  even 
  with 
  Michelson's 
  refractometer 
  using 
  sodium 
  light. 
  

   In 
  other 
  words, 
  the 
  sounds 
  should 
  emerge 
  out 
  of 
  the 
  region 
  of 
  

   silence 
  without 
  producing 
  any 
  disturbance 
  of 
  the 
  interference 
  

   fringes 
  due 
  to 
  the 
  mirror 
  attached 
  to 
  the 
  telephone 
  diaphragm. 
  

   This 
  is 
  the 
  case. 
  

  

  2. 
  Putting 
  the 
  telephone 
  in 
  circuit 
  with 
  the 
  secondary 
  of 
  a 
  

   small 
  induction 
  coil, 
  the 
  primary 
  was 
  supplied 
  with 
  the 
  

   smallest 
  current 
  sufficient 
  to 
  actuate 
  the 
  interruptor. 
  The 
  tele- 
  

   phone 
  was 
  heard 
  to 
  respond 
  quite 
  audibly 
  even 
  when 
  the 
  sec- 
  

   ondary 
  was 
  left 
  open, 
  for 
  the 
  capacity 
  of 
  the 
  wires 
  was 
  favor- 
  

   able. 
  By 
  closing 
  the 
  secondary 
  circuit 
  with 
  a 
  very 
  large 
  

   resistance, 
  preferably 
  by 
  putting 
  the 
  fingers 
  in 
  the 
  gap, 
  the 
  

   sound 
  could 
  be 
  increased 
  to 
  any 
  degree 
  by 
  greater 
  or 
  less 
  pres- 
  

   sure 
  of 
  the 
  hand 
  at 
  the 
  contact. 
  

  

  The 
  result 
  with 
  one 
  telephone 
  in 
  circuit 
  showed 
  no 
  change 
  

   of 
  clearness 
  of 
  the 
  interference 
  fringes 
  when 
  the 
  circuit 
  was 
  

   left 
  open, 
  although 
  the 
  signal 
  in 
  the 
  telephone 
  (about 
  50 
  vibra- 
  

   tions 
  per 
  second) 
  was 
  distinct 
  to 
  the 
  ear. 
  I 
  then 
  moved 
  the 
  

   telescope 
  for 
  observing 
  the 
  fringes 
  to 
  about 
  6 
  feet 
  from 
  the 
  

   telephone 
  and 
  controlled 
  the 
  circuit 
  from 
  this 
  distance. 
  The 
  

   fringes 
  did 
  not 
  disappear 
  nor 
  indicate 
  motion 
  even 
  when 
  I 
  

   heard 
  the 
  responding 
  telephone 
  at 
  the 
  telescope 
  or 
  even 
  further 
  

   away. 
  They 
  became 
  gradually 
  blurred 
  when 
  the 
  circuit 
  was 
  

   more 
  and 
  more 
  closed 
  with 
  the 
  fingers, 
  but 
  they 
  vanished 
  only 
  

   for 
  sonorous 
  noises. 
  On 
  the 
  other 
  hand, 
  shouting 
  or 
  loud 
  sing- 
  

   ing 
  at 
  the 
  distance 
  of 
  the 
  telescope 
  always 
  made 
  the 
  fringes 
  

   disappear, 
  particularly 
  at 
  certain 
  notes. 
  Thinking 
  that 
  the 
  

   two 
  telephones 
  (although 
  but 
  one 
  was 
  in 
  circuit) 
  might 
  favor 
  

   visibility 
  by 
  vibrating 
  sympathetically, 
  I 
  replaced 
  one 
  telephone 
  

   by 
  a 
  plate 
  mirror 
  without 
  affecting 
  the 
  result. 
  

  

  