﻿474 
  Prof. 
  Sir 
  W. 
  Thomson 
  on 
  the 
  Effects 
  of 
  [June 
  10, 
  

  

  result 
  — 
  the 
  effect 
  of 
  weights 
  on 
  and 
  off 
  shown 
  by 
  the 
  very 
  soft 
  iron 
  wire, 
  

   under 
  only 
  the 
  feeble 
  magnetizing 
  influence 
  of 
  the 
  earth, 
  being 
  many 
  

   times 
  (from 
  three 
  times 
  to 
  nine 
  or 
  ten 
  times) 
  as 
  great 
  as 
  the 
  effects 
  which 
  

   the 
  same 
  weights 
  on 
  and 
  off 
  produced 
  in 
  the 
  same 
  wires 
  when 
  under 
  

   vastly 
  greater 
  magnetizing 
  forces 
  of 
  the 
  currents 
  through 
  the 
  helix. 
  

  

  4. 
  But 
  by 
  reducing 
  the 
  strength 
  of 
  the 
  magnetizing 
  current 
  gradually, 
  

   it 
  was 
  clear 
  that 
  the 
  small 
  positive 
  effect 
  of 
  the 
  " 
  on" 
  with 
  the 
  positive 
  

   current 
  flowing 
  and 
  the 
  small 
  negative 
  effect 
  with 
  the 
  negative 
  current 
  

   must 
  be 
  gradually 
  brought 
  to 
  approximate 
  more 
  and 
  more 
  nearly 
  to 
  the 
  

   large 
  positive 
  effect 
  of 
  the 
  "on" 
  when 
  there 
  is 
  no 
  current 
  at 
  all. 
  Imme- 
  

   diately 
  after 
  my 
  former 
  communication 
  I 
  therefore 
  arranged 
  to 
  have 
  

   experiments 
  made 
  with 
  different 
  measured 
  strengths 
  of 
  current, 
  feebler 
  

   and 
  feebler, 
  until 
  the 
  law 
  of 
  the 
  continuity 
  thus 
  pointed 
  out 
  should 
  be 
  

   ascertained 
  ; 
  and 
  so 
  speedily 
  arrived 
  at 
  the 
  following 
  astonishing 
  conclu- 
  

   sions 
  : 
  — 
  

  

  5. 
  (1) 
  When 
  the 
  magnetizing 
  force 
  does 
  not 
  exceed 
  a 
  certain 
  critical 
  

   value 
  the 
  alternate 
  effects 
  of 
  pull 
  and 
  relaxation 
  are 
  respectively 
  to 
  aug- 
  

   ment 
  and 
  diminish 
  the 
  induced 
  magnetization. 
  

  

  (2) 
  When 
  the 
  magnetizing 
  force 
  exceeds 
  the 
  critical 
  value 
  the 
  

   effects 
  are 
  — 
  pull 
  diminishes, 
  relaxation 
  augments, 
  the 
  induced 
  magneti- 
  

   zation. 
  

  

  (3) 
  The 
  critical 
  value 
  of 
  the 
  magnetizing 
  force 
  for 
  the 
  annealed 
  

   Johnson 
  soft-iron 
  wire, 
  with 
  14 
  lbs. 
  on 
  and 
  off, 
  is 
  about 
  17 
  or 
  18, 
  if 
  (for 
  

   a 
  moment) 
  we 
  take 
  as 
  unity 
  the 
  vertical 
  component 
  of 
  the 
  terrestrial 
  mag- 
  

   netic 
  force 
  at 
  Glasgow. 
  

  

  (4) 
  The 
  maximum 
  positive 
  effect 
  of 
  the 
  pull 
  on 
  the 
  inductive 
  mag- 
  

   netism 
  is 
  obtained 
  when 
  the 
  magnetizing 
  force 
  is 
  about 
  4. 
  

  

  (5) 
  The 
  positive 
  effect 
  of 
  the 
  pull 
  when 
  the 
  magnetizing 
  force 
  is 
  3 
  

   is 
  about 
  eight 
  or 
  nine 
  times 
  the 
  amount 
  of 
  the 
  negative 
  effect 
  when 
  the 
  

   magnetizing 
  force 
  is 
  25. 
  

  

  6. 
  The 
  actual 
  results 
  of 
  the 
  experiments 
  which 
  proved 
  these 
  conclu- 
  

   sions 
  are 
  exhibited 
  graphically 
  in 
  the 
  accompanying 
  diagram. 
  The 
  hori- 
  

   zontal 
  scale 
  (abscissas) 
  shows 
  the 
  numbers 
  of 
  divisions 
  of 
  the 
  scale 
  of 
  the 
  

   steady-current 
  galvanometer 
  (called 
  for 
  brevity 
  the 
  " 
  battery-galvano- 
  

   meter 
  ") 
  used 
  to 
  measure 
  the 
  strengths 
  of 
  the 
  current 
  through 
  the 
  helix. 
  

   The 
  scale 
  of 
  ordinates 
  shows 
  the 
  numbers 
  of 
  divisions 
  of 
  the 
  scale 
  of 
  the 
  

   ballistic 
  galvanometer 
  by 
  which 
  the 
  sudden 
  changes 
  of 
  the 
  magnetism 
  of 
  

   the 
  wire 
  produced 
  by 
  14 
  lbs. 
  "on" 
  and 
  14 
  lbs. 
  "off" 
  were 
  measured. 
  

   The 
  ordinates 
  are 
  drawn 
  in 
  the 
  positive 
  direction 
  when 
  the 
  effect 
  of 
  " 
  on 
  " 
  

   is 
  to 
  increase 
  and 
  of 
  "off" 
  to 
  diminish 
  the 
  magnetism. 
  The 
  simple 
  

   round 
  spots 
  show 
  the 
  results 
  of 
  observations 
  with 
  currents 
  in 
  the 
  direc- 
  

   tion 
  called 
  negative 
  (being 
  those 
  which 
  gave 
  negative 
  deflections 
  of 
  the 
  

   battery-galvanometer) 
  . 
  The 
  spots 
  in 
  the 
  centre 
  of 
  signs 
  ( 
  + 
  ) 
  show 
  results 
  

   obtained 
  with 
  currents 
  in 
  the 
  direction 
  called 
  positive. 
  The 
  star 
  (*) 
  at 
  

   the 
  position 
  64 
  on 
  the 
  line 
  of 
  ordinates 
  through 
  the 
  zero 
  of 
  abscissas 
  

  

  