﻿1875.] 
  • 
  

  

  On 
  Inductive 
  Magnetism 
  in 
  Soft 
  Iron. 
  

  

  473 
  

  

  VIII. 
  ^ 
  Effects 
  of 
  Stress 
  on 
  Inductive 
  Magnetism 
  in 
  Soft 
  Iron." 
  

   (Preliminary 
  Notice.) 
  By 
  Prof. 
  Sir 
  William 
  Thomson, 
  

   F.R.S. 
  Received 
  June 
  10, 
  1875. 
  

  

  1. 
  At 
  the 
  last 
  ordinary 
  meeting 
  of 
  the 
  Royal 
  Society 
  (May 
  27), 
  after 
  fully 
  

   describing 
  experiments 
  by 
  which 
  I 
  had 
  found 
  certain 
  remarkable 
  effects 
  

   of 
  stress 
  on 
  inductive 
  and 
  retained 
  magnetism 
  in 
  steel 
  and 
  soft 
  iron, 
  I 
  

   briefly 
  referred 
  to 
  seeming 
  anomalies 
  presented 
  by 
  soft 
  iron 
  which 
  had 
  

   much 
  perplexed 
  me 
  since 
  the 
  23rd 
  of 
  December. 
  Differences 
  presented 
  

   by 
  the 
  different 
  specimens 
  of 
  soft-iron 
  wire 
  which 
  I 
  tried 
  complicated 
  the 
  

   question 
  very 
  much 
  ; 
  but 
  one 
  of 
  them, 
  the 
  softest 
  of 
  all, 
  a 
  wire 
  specially 
  

   made 
  by 
  Messrs. 
  Richard 
  Johnson 
  and 
  Nephew, 
  of 
  Manchester, 
  for 
  this 
  

   investigation, 
  through 
  the 
  kindness 
  of 
  Mr. 
  "William 
  H. 
  Johnson, 
  gave 
  a 
  

   result 
  standing 
  clearly 
  out 
  from 
  the 
  general 
  confusion, 
  and 
  pointing 
  the 
  

   way 
  to 
  further 
  experiments, 
  by 
  which, 
  within 
  the 
  fortnight 
  which 
  has 
  

   intervened 
  since 
  my 
  former 
  communication, 
  I 
  have 
  arrived 
  at 
  a 
  complete 
  

   explanation 
  of 
  all 
  that 
  had 
  formerly 
  seemed 
  anomalous. 
  These 
  experi- 
  

   ments 
  have 
  been 
  performed 
  in 
  the 
  Physical 
  Laboratory 
  of 
  the 
  University 
  

   of 
  Glasgow 
  by 
  Mr. 
  Andrew 
  Gray 
  and 
  Mr. 
  Thomas 
  Gray, 
  according 
  to 
  

   instructions 
  which, 
  in 
  my 
  absence, 
  I 
  have 
  sent 
  them 
  from 
  day 
  to 
  day 
  by 
  

   post 
  and 
  telegraph. 
  

  

  2. 
  The 
  guiding 
  result 
  (described 
  near 
  the 
  end 
  of 
  my 
  former 
  paper, 
  and 
  

   referred 
  to 
  in 
  the 
  last 
  paragraph 
  but 
  one 
  of 
  the 
  Abstract 
  in 
  Proceedings 
  

   of 
  the 
  Royal 
  Society 
  for 
  May 
  27) 
  was, 
  that 
  the 
  softest 
  wire, 
  tried 
  with 
  

   weights 
  on 
  and 
  off 
  repeatedly, 
  after 
  it 
  had 
  been 
  magnetized 
  in 
  either 
  

   direction 
  by 
  making 
  the 
  current, 
  in 
  the 
  positive 
  or 
  negative 
  direction, 
  and 
  

   stopping 
  it, 
  gave 
  effects 
  on 
  the 
  ballistic 
  galvanometer 
  which 
  proved 
  a 
  

   shaking 
  out 
  of 
  residual 
  magnetism 
  by 
  the 
  first 
  two 
  or 
  three 
  ons 
  and 
  offs, 
  

   and 
  a 
  gradual 
  settlement 
  into 
  a 
  condition 
  in 
  which 
  the 
  effect 
  of 
  "on 
  " 
  was 
  

   an 
  augmentation, 
  and 
  the 
  effect 
  of 
  "off" 
  a 
  diminution, 
  of 
  the 
  inductive 
  

   magnetization 
  due 
  to 
  the 
  vertical 
  component 
  of 
  the 
  earth's 
  magnetizing 
  

   force. 
  "When 
  a 
  fresh 
  piece 
  of 
  the 
  same 
  wire 
  was 
  put 
  into 
  the 
  apparatus 
  

   and 
  tested 
  with 
  weights 
  on 
  and 
  off 
  it 
  gave 
  this 
  same 
  effect. 
  If 
  the 
  wire 
  had 
  

   been 
  turned 
  upper 
  end 
  down 
  and 
  tried 
  again 
  in 
  the 
  course 
  of 
  any 
  of 
  the 
  

   experiments, 
  still 
  this 
  same 
  effect 
  would 
  have 
  been 
  shown. 
  It 
  seemed 
  

   perfectly 
  clear 
  that 
  in 
  these 
  experiments 
  there 
  was 
  no 
  other 
  efficient 
  

   dipolar 
  quality 
  of 
  the 
  apparatus 
  by 
  which 
  the 
  positive 
  throw 
  of 
  the 
  bal- 
  

   listic 
  galvanometer 
  could 
  be 
  given 
  by 
  putting 
  on 
  the 
  weight, 
  and 
  the 
  

   negative 
  throw 
  by 
  taking 
  it 
  off, 
  than 
  the 
  vertical 
  component 
  of 
  the 
  earth's 
  

   magnetic 
  force. 
  

  

  3. 
  Yet 
  I 
  did 
  not 
  consider 
  that 
  I 
  had 
  explained 
  the 
  result 
  by 
  the 
  ter- 
  

   .restrial 
  influence, 
  because, 
  for 
  all 
  the 
  specimens 
  of 
  steel 
  and 
  soft 
  iron, 
  the 
  

  

  effect 
  of 
  weights 
  on 
  had 
  been 
  uniformly 
  to 
  diminish, 
  and 
  of 
  weights 
  off 
  

   to 
  augment 
  the 
  magnetism 
  when 
  the 
  magnetizing 
  current 
  was 
  kept 
  

   flowing. 
  And 
  I 
  was, 
  moreover, 
  perplexed 
  by 
  the 
  magnitude 
  of 
  the 
  

  

  