﻿Improved 
  Form 
  of 
  Magnetometer. 
  

  

  157 
  

  

  from 
  the 
  needle 
  ; 
  (4) 
  that 
  the 
  operations 
  can 
  be 
  carried 
  out 
  

   with 
  great 
  rapidity 
  ; 
  unless 
  the 
  solenoid 
  is 
  very 
  close 
  up 
  to 
  

   the 
  magnetometer, 
  the 
  changing 
  over 
  o£ 
  the 
  apparatus 
  from 
  

   one 
  degree 
  o£ 
  sensibility 
  to 
  another 
  can 
  be 
  carried 
  out 
  within 
  

   the 
  space 
  of 
  two 
  minutes. 
  

  

  The 
  magnitude 
  oi: 
  the 
  directive 
  force 
  at 
  the 
  needle 
  is 
  

   easily 
  determined 
  by 
  passing 
  a 
  measured 
  current 
  through 
  

   one 
  of 
  the 
  balancing 
  coils 
  and 
  noting 
  the 
  deflexion 
  of 
  the 
  

   magnetometer-needle 
  produced. 
  The 
  value 
  of 
  the 
  directive 
  

   force 
  is 
  then 
  easily 
  calculated. 
  

  

  Fig. 
  4 
  (PL 
  II.) 
  is 
  a 
  photograph 
  of 
  the 
  apparatus 
  when 
  adjusted 
  

   for 
  the 
  examination 
  of 
  a 
  strongly 
  magnetic 
  specimen 
  ; 
  fig. 
  4 
  a 
  

   shows 
  the 
  arrangement 
  when 
  a 
  Eeebly 
  magnetic 
  specimen 
  is 
  

   being 
  dealt 
  with. 
  When 
  the 
  solenoid 
  has 
  to 
  be 
  placed 
  very 
  

   close 
  to 
  the 
  magnetometer-needle 
  to 
  allow 
  of 
  a 
  very 
  feebly 
  

   magnetic 
  specimen 
  being 
  examined, 
  the 
  coil 
  Ci 
  is 
  placed 
  on 
  

   the 
  opposite 
  side 
  of 
  the 
  needle 
  to 
  the 
  solenoid. 
  For 
  general 
  

   use, 
  however, 
  it 
  is 
  convenient 
  to 
  have 
  the 
  solenoid 
  and 
  coil 
  

   on 
  the 
  same 
  side. 
  It 
  is 
  worthy 
  of 
  remark, 
  in 
  passing, 
  that 
  

   even 
  if 
  Ci 
  is 
  placed 
  as 
  close 
  up 
  as 
  possible 
  to 
  the 
  end 
  of 
  the 
  

   solenoid, 
  it 
  cannot 
  alter 
  the 
  field 
  at 
  the 
  centre 
  of 
  the 
  specimen 
  

   by 
  so 
  much 
  as 
  \ 
  per 
  cent. 
  

  

  " 
  When 
  used 
  for 
  testing 
  specimens 
  at 
  temperatures 
  higher 
  

   than 
  than 
  that 
  of 
  the 
  room, 
  an 
  electric 
  furnace 
  of 
  a 
  type 
  

   similar 
  to 
  that 
  devised 
  by 
  Dr. 
  G. 
  E. 
  Allan 
  *, 
  is 
  placed 
  within 
  

   the 
  helix. 
  In 
  fig. 
  5 
  it 
  is 
  shown 
  in 
  position. 
  A 
  tube 
  E 
  of 
  

  

  Fig:, 
  o. 
  

  

  unglazed 
  porcelain 
  of 
  about 
  the 
  same 
  length 
  as 
  the 
  solenoid 
  

   having 
  an 
  internal 
  diameter 
  of 
  23*5 
  mm. 
  and 
  a 
  thickness 
  of 
  

   about 
  2 
  mm., 
  is 
  wound 
  non-inductively 
  with 
  fine 
  platinum 
  

   wire 
  ; 
  the 
  ends 
  of 
  this 
  wire 
  are 
  brought 
  out 
  to 
  two 
  terminals 
  

   mounted 
  on 
  a 
  slate 
  frame 
  at 
  F. 
  The 
  tube 
  is 
  enclosed 
  in 
  a 
  

   tube 
  G 
  of 
  Jena 
  glass, 
  which 
  fits 
  as 
  a 
  cartridge 
  within 
  the 
  

   magnetizing 
  solenoid. 
  The 
  space 
  HH 
  between 
  the 
  glass 
  and 
  

   porcelain 
  tubes 
  is 
  packed 
  with 
  dry 
  kaolin 
  clay, 
  which 
  per- 
  

   forms 
  the 
  double 
  duty 
  of 
  supporting 
  the 
  furnace 
  and 
  

  

  * 
  Phil. 
  Mag. 
  1904, 
  vol. 
  vii. 
  p. 
  46. 
  

  

  