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  VIII. 
  Notes 
  on 
  the 
  Plug 
  Permeameter. 
  

   By 
  Chakles 
  V. 
  Drysdale, 
  Ij.Sc* 
  

  

  [Plates 
  II. 
  & 
  III.] 
  

  

  IN 
  1901 
  the 
  writer 
  described 
  a 
  permeameter 
  |, 
  devised 
  with 
  

   the 
  object 
  of 
  enabling 
  permeability-tests 
  to 
  be 
  made 
  on 
  

   castings 
  and 
  forgings 
  for 
  dynamo-work. 
  The 
  apparatus 
  

   consisted 
  of 
  a 
  special 
  drill, 
  which 
  could 
  be 
  employed 
  to 
  bore 
  

   a 
  hole 
  in 
  the 
  casting 
  or 
  forging, 
  leaving, 
  however, 
  a 
  small 
  

   piece 
  of 
  the 
  metal 
  in 
  the 
  form 
  ol 
  a 
  rod 
  or 
  pin 
  standing 
  in 
  the 
  

   centre 
  of 
  the 
  hole. 
  A 
  split 
  iron 
  plug, 
  arranged 
  to 
  fit 
  both 
  

   the 
  conical 
  sides 
  of 
  the 
  hole 
  and 
  over 
  the 
  pin, 
  was 
  emploved 
  

   to 
  complete 
  the 
  magnetic 
  circuit 
  ; 
  and 
  this 
  plug 
  carried 
  a 
  

   bobbin 
  wound 
  with 
  magnetizing 
  and 
  search 
  coils. 
  A 
  special 
  

   portable 
  testing 
  set 
  was 
  also 
  designed 
  with 
  the 
  object 
  of 
  

   making 
  permeability-tests 
  by 
  direct 
  reading 
  of 
  H 
  and 
  B. 
  

  

  It 
  was 
  noticed 
  at 
  the 
  time 
  that 
  the 
  magnetization-curves 
  

   obtained 
  by 
  this 
  method 
  fell 
  below 
  those 
  obtained 
  by 
  mea- 
  

   surements 
  on 
  a 
  long 
  bar 
  of 
  the 
  same 
  material. 
  This 
  would 
  

   naturally 
  be 
  attributed 
  to 
  a 
  bad 
  magnetic 
  joint 
  between 
  the 
  

   pin 
  and 
  plug 
  ; 
  but 
  experiments 
  made 
  at 
  the 
  time 
  seemed 
  to 
  

   point 
  rather 
  to 
  the 
  effect 
  being 
  due 
  to 
  the 
  reluctance 
  of 
  the 
  

   return 
  path 
  at 
  the 
  point 
  of 
  entrance 
  of 
  the 
  flux, 
  due 
  to 
  

   the 
  concentration 
  of 
  the 
  lines 
  of 
  force 
  at 
  these 
  points. 
  

   This 
  seemed 
  probable 
  from 
  the 
  fact 
  that 
  the 
  readings 
  were 
  

   remarkably 
  consistent 
  when 
  the 
  plug 
  was 
  removed 
  and 
  

   replaced. 
  

  

  This 
  hypothesis 
  rendered 
  it 
  probable 
  that 
  there 
  might 
  be 
  

   a 
  more 
  or 
  less 
  definite 
  relation 
  between 
  the 
  value 
  of 
  the 
  

   true 
  magnetizing-force 
  H 
  in 
  the 
  specimen, 
  and 
  the 
  apparent 
  

  

  magnetizing-force 
  H^^— 
  -j- 
  ; 
  and 
  arrangements 
  were 
  made 
  

  

  to 
  test 
  this 
  relation. 
  The 
  most 
  perfect 
  method 
  was 
  suggested 
  

   by 
  the 
  double-yoke 
  method 
  of 
  Prof. 
  Ewing. 
  It 
  consisted 
  in 
  

   finding 
  the 
  relation 
  of 
  the 
  induction 
  density 
  and 
  the 
  mag- 
  

   netizing 
  current 
  for 
  the 
  ordinary 
  drilled 
  specimen, 
  and 
  after- 
  

   wards 
  when 
  the 
  length 
  of 
  the 
  specimen 
  was 
  doubled 
  by 
  drilling 
  

   to 
  a 
  greater 
  depth 
  with 
  a 
  second 
  drill. 
  The 
  increase 
  of 
  the 
  

   magnetizing 
  current 
  for 
  the 
  same 
  induction 
  should 
  give 
  

   the 
  true 
  H. 
  A 
  Large 
  number 
  ol' 
  tests 
  were 
  made 
  in 
  tlii- 
  

   manner, 
  but 
  proved 
  ol' 
  little 
  value, 
  a-, 
  although 
  the 
  tirsl 
  

  

  drilling 
  presented 
  no 
  difficulties, 
  it 
  was 
  I', 
  and 
  impossible 
  to 
  

  

  perform 
  the 
  second 
  drilling 
  accurately 
  enough 
  to 
  preserve 
  

   parallelism 
  of 
  the 
  pin. 
  

  

  * 
  Communicated 
  by 
  the 
  Physical 
  Society: 
  read 
  March 
  27, 
  L908. 
  

   t 
  Proc 
  Inst. 
  Elector. 
  Eng. 
  vol. 
  \\\i. 
  p. 
  288. 
  

  

  