﻿Barns 
  — 
  Trial 
  of 
  Interferential 
  Induction 
  balance. 
  107 
  

  

  Art. 
  IX. 
  — 
  Preliminary 
  Trial 
  of 
  an 
  Interferential 
  Induc- 
  

   tion 
  balance 
  ; 
  by 
  C. 
  Barus. 
  

  

  1. 
  The 
  following 
  device 
  is 
  capable, 
  I 
  believe, 
  of 
  a 
  variety 
  

   of 
  applications 
  in 
  relation 
  to 
  alternating 
  currents 
  and 
  to 
  mag- 
  

   netic 
  induction. 
  The 
  idea 
  underlying 
  the 
  apparatus 
  is 
  briefly 
  

   this 
  : 
  Let 
  the 
  slender 
  iron 
  cores 
  of 
  two 
  identical 
  helices 
  be 
  

   placed 
  at 
  right 
  angles 
  to 
  each 
  other, 
  in 
  the 
  same 
  (horizontal) 
  

   plane 
  and 
  at 
  like 
  distances 
  from 
  the 
  point 
  of 
  convergence. 
  

   Let 
  the 
  distant 
  ends 
  of 
  the 
  iron 
  cores 
  be 
  rigidly 
  fastened, 
  

   while 
  the 
  other 
  ends 
  are 
  free 
  to 
  move 
  (expand 
  and 
  contract) 
  

   in 
  the 
  direction 
  of 
  the 
  axes. 
  Then 
  it 
  is 
  possible 
  to 
  adapt 
  

   Michelson's 
  interferential 
  refractor 
  in 
  such 
  a 
  way 
  that 
  the 
  

   fringes 
  are 
  visible 
  whenever 
  the 
  excursions 
  of 
  the 
  free 
  ends 
  of 
  

   the 
  cores 
  are 
  either 
  zero 
  or 
  vibrating 
  in 
  the 
  same 
  phase, 
  ampli- 
  

   tude 
  and 
  period 
  to 
  and 
  from 
  the 
  point 
  of 
  convergence. 
  The 
  

   fringes 
  vanish 
  more 
  or 
  less 
  fully 
  for 
  all 
  other 
  phases. 
  

  

  Let 
  the 
  two 
  helices 
  be 
  traversed 
  by 
  the 
  same 
  alternating 
  

   current, 
  and 
  let 
  the 
  reduced 
  length 
  of 
  the 
  electrical 
  conductor 
  

   between 
  them 
  be 
  negligible 
  and 
  the 
  vibrations 
  in 
  the 
  same 
  

   phase 
  initially. 
  Then 
  if 
  the 
  reduced 
  length 
  of 
  the 
  conductor 
  

   be 
  gradually 
  increased, 
  the 
  two 
  vibrations 
  will 
  be 
  dephased 
  

   and 
  the 
  fringes 
  will 
  gradually 
  vanish. 
  If 
  the 
  lengthening 
  of 
  

   the 
  conductor 
  be 
  continued 
  until 
  the 
  excitement 
  of 
  one 
  helix 
  

   is 
  belated 
  one 
  complete 
  period 
  of 
  the 
  vibrating 
  core 
  with 
  ref- 
  

   erence 
  to 
  the 
  other, 
  the 
  fringes 
  will 
  reappear 
  caet. 
  par. 
  with 
  

   their 
  original 
  clearness. 
  A 
  method 
  of 
  estimating 
  the 
  speed 
  of 
  

   signalling 
  from 
  one 
  helix 
  to 
  the 
  other 
  is 
  thus 
  given 
  in 
  terms 
  of 
  

   the 
  period 
  of 
  longitudinal 
  vibration 
  of 
  the 
  vibrating 
  core. 
  

  

  If 
  the 
  helices 
  are 
  not 
  identical, 
  similar 
  deductions 
  apply 
  for 
  

   the 
  difference 
  of 
  time 
  lag 
  of 
  current 
  behind 
  the 
  electromotive 
  

   force 
  for 
  the 
  two 
  helices 
  respectively. 
  

  

  Finally 
  vanished 
  fringes 
  are 
  open 
  to 
  observation 
  by 
  strobo- 
  

   scope 
  methods. 
  

  

  § 
  2. 
  The 
  form 
  of 
  apparatus 
  used 
  is 
  shown 
  diagrammatic- 
  

   ally 
  and 
  largely 
  in 
  sectional 
  plan 
  in 
  the 
  figure. 
  A 
  and 
  B 
  

   are 
  tbe 
  two 
  helices 
  and 
  a 
  and 
  b 
  their 
  cores 
  of 
  soft 
  Norway 
  

   iron. 
  These 
  with 
  the 
  helices 
  are 
  rigidly 
  fixed 
  at 
  the 
  further 
  

   ends. 
  The 
  cores 
  pass 
  axially 
  through 
  the 
  helices 
  without 
  

   touching 
  them, 
  and 
  carry 
  small 
  light 
  plane 
  mirrors 
  m 
  and 
  n 
  at 
  

   their 
  free 
  ends. 
  With 
  the 
  thick 
  plane-parallel 
  plate 
  Jf, 
  these 
  

   mirrors 
  make 
  up 
  Michelson's 
  refractometer. 
  Light 
  (thus 
  far 
  

   furnished 
  by 
  a 
  sodium 
  flame) 
  is 
  emitted 
  by 
  the 
  distant 
  burner 
  

   ^provided 
  with 
  Cushman's 
  sodium 
  collar,* 
  and 
  condensed 
  by 
  

  

  *This 
  efficient 
  device, 
  due 
  to 
  Holbrook 
  Cushman, 
  consists 
  of 
  a 
  layer 
  of 
  filter 
  

   paper 
  impregnated 
  with 
  sodium 
  carbonate 
  and 
  wrapped 
  around 
  the 
  top 
  of 
  the 
  

   burner. 
  The 
  sodium 
  flame 
  so 
  obtained 
  is 
  very 
  dense 
  and 
  lasts 
  for 
  days. 
  

  

  