﻿112 
  Barus 
  — 
  Trial 
  of 
  Interferential 
  Induction 
  balance. 
  

  

  Taken 
  as 
  a 
  whole, 
  the 
  results 
  of 
  the 
  table 
  agree 
  in 
  character 
  

   with 
  Bidwell's* 
  interesting 
  investigations. 
  They 
  show 
  that 
  

   the 
  expansion 
  increases 
  to 
  a 
  maximum, 
  attaining 
  it 
  in 
  a 
  field 
  

   between 
  20 
  and 
  30 
  dynes, 
  at 
  which 
  the 
  iron 
  is 
  probably 
  satu- 
  

   rated. 
  After 
  this 
  the 
  data 
  are 
  obscured. 
  The 
  present 
  maxi- 
  

   mum 
  expansions 
  are 
  smaller 
  than 
  Bidwell's 
  and 
  occur 
  at 
  an 
  

   earlier 
  stage 
  of 
  magnetization. 
  Thus 
  an 
  elongation 
  of 
  1 
  to 
  1*5 
  

   millionths 
  was 
  rarely 
  exceeded. 
  

  

  Although 
  my 
  experiments 
  were 
  made 
  in 
  fields 
  as 
  large 
  as 
  

   600 
  dynes, 
  the 
  results 
  above 
  30 
  dynes 
  are 
  of 
  no 
  value. 
  For 
  in 
  

   larger 
  fields 
  than 
  this 
  the 
  fringes 
  showed 
  more 
  and 
  more 
  rota- 
  

   tion 
  intermixed 
  with 
  the 
  translation. 
  This 
  indicates 
  that 
  the 
  

   suspended 
  cores 
  were 
  flexed 
  by 
  the 
  field, 
  or 
  that 
  they 
  contained 
  

   inherent 
  torsion 
  which 
  in 
  a 
  field 
  gives 
  rise 
  to 
  a 
  Wiedemann 
  

   effect, 
  or 
  . 
  both. 
  Probably 
  flexure 
  supervened. 
  To 
  give 
  an 
  

   example 
  among 
  many 
  : 
  In 
  a 
  field 
  of 
  330 
  dynes 
  the 
  fringes 
  for 
  

   helix 
  A 
  shewed 
  clockwise 
  rotation, 
  the 
  fringes 
  for 
  helix 
  _B 
  

   counter-clockwise 
  rotation. 
  The 
  two 
  cores 
  together 
  showed 
  a 
  

   definite 
  translation 
  of 
  about 
  one 
  fringe, 
  without 
  rotatiop. 
  This 
  

   would 
  correspond 
  to 
  compensatory 
  flexures, 
  f 
  and 
  in 
  large 
  fields 
  

   the 
  real 
  phenomenon 
  was 
  masked 
  by 
  this 
  discrepancy. 
  

  

  If 
  the 
  results 
  are 
  studied 
  with 
  reference 
  to 
  weak 
  fields, 
  the 
  

   curve 
  is 
  found 
  to 
  terminate 
  abruptly 
  in 
  the 
  abscissa. 
  No 
  

   elongation 
  was 
  observable 
  in 
  fields 
  less 
  than 
  about 
  7 
  dynes. 
  

   For 
  fields 
  larger 
  than 
  this, 
  an 
  increment 
  of 
  field 
  less 
  than 
  a 
  

   single 
  dyne 
  produced 
  an 
  observable 
  change 
  of 
  elongation. 
  This 
  

   result 
  is 
  sufficiently 
  remarkable 
  to 
  deserve 
  detailed 
  study. 
  

  

  There 
  is 
  a 
  final 
  point 
  to 
  be 
  mentioned. 
  In 
  a 
  field 
  of 
  about 
  

   20, 
  in 
  which 
  the 
  elongation 
  of 
  the 
  above 
  rods 
  was 
  most 
  pro- 
  

   nounced, 
  the 
  observed 
  effect 
  with 
  a 
  single 
  helix 
  in 
  circuit 
  was 
  

   such 
  as 
  to 
  correspond 
  to 
  a 
  shifting 
  of 
  the 
  fringes 
  whenever 
  the 
  

   circuit 
  was 
  closed, 
  with 
  no 
  return 
  of 
  fringes 
  apparent 
  on 
  open- 
  

   ing. 
  On 
  closing, 
  about 
  1*5 
  fringes 
  visibly 
  and 
  invariably 
  moved 
  

   across 
  the 
  cross-hairs 
  of 
  the 
  telescope 
  ; 
  on 
  opening, 
  the 
  return 
  

   was 
  either 
  partial 
  or 
  (apparently) 
  absent 
  altogether. 
  I 
  was 
  

   even 
  able 
  to 
  make 
  over 
  100 
  fringes 
  move 
  across 
  the 
  field 
  by 
  

   continually 
  closing 
  and 
  opening 
  the 
  circuit. 
  On 
  inserting 
  the 
  

   other 
  helix, 
  alone 
  the 
  result 
  was 
  the 
  same 
  but 
  reversed 
  in 
  direc- 
  

   tion. 
  With 
  both 
  helices 
  in 
  circuit 
  there 
  was 
  but 
  little 
  motion. 
  

   I 
  was 
  at 
  first 
  inclined 
  to 
  refer 
  this 
  observation 
  to 
  an 
  increase 
  of 
  

   temperature 
  due 
  to 
  magnetization. 
  The 
  passage 
  of 
  -J 
  fringe 
  

   corresponds 
  to 
  an 
  elongation 
  of 
  5xl0 
  -7 
  . 
  The 
  equivalent 
  rise 
  

  

  *Bidwell: 
  See 
  Ewing's 
  Magnetic 
  induction 
  in 
  iron, 
  p. 
  238; 
  Phil. 
  Trans., 
  1888. 
  

   p. 
  205 
  ; 
  Proc. 
  Roy. 
  Soc, 
  xlvii, 
  p. 
  469, 
  1890. 
  

  

  f 
  Unfortunately 
  the 
  soft 
  Norway 
  iron 
  obtainable 
  here 
  is 
  not 
  quite 
  straight 
  ; 
  

   and 
  since 
  all 
  mechanical 
  treatment 
  hardens 
  and 
  injures 
  the 
  iron, 
  I 
  was 
  obliged 
  to 
  

   content 
  myself 
  with 
  the 
  best 
  samples 
  in 
  the 
  lot 
  at 
  my 
  disposal. 
  

  

  