﻿150 
  Dr. 
  J. 
  G. 
  Gray 
  and 
  Mr. 
  A. 
  D. 
  Ross 
  on 
  an 
  

  

  The 
  necessity 
  for 
  attending 
  to 
  this 
  source 
  o£ 
  inaccuracy 
  

   was 
  first 
  pointed 
  out 
  by 
  Erhard*, 
  who 
  investigated 
  the 
  

   magnitude 
  o£ 
  the 
  errors 
  which 
  were 
  involved 
  by 
  n(iglecting 
  

   it. 
  In 
  the 
  case 
  of 
  a 
  magnetometer 
  of 
  the 
  usual 
  type 
  ex- 
  

   amined 
  by 
  him, 
  it 
  was 
  found 
  that 
  with 
  a 
  magnetizing 
  field 
  

   of 
  128*3 
  c.G.s. 
  units 
  in 
  the 
  solenoid 
  there 
  was 
  a 
  change 
  of 
  

   6*8 
  per 
  cent, 
  in 
  the 
  directive 
  force 
  on 
  the 
  needle 
  on 
  reversal 
  

   of 
  the 
  current, 
  Erhard 
  advised 
  that 
  the 
  magnitudes 
  of 
  the 
  

   errors 
  introduced 
  should 
  be 
  determined 
  for 
  various 
  parts 
  of 
  

   the 
  scale 
  and 
  allowed 
  for 
  in 
  the 
  results. 
  

  

  While 
  carrying 
  out 
  a 
  research 
  on 
  certain 
  feebly 
  magnetic 
  

   alloys, 
  the 
  authors 
  found 
  that 
  the 
  elimination 
  of 
  the 
  afore- 
  

   mentioned 
  sources 
  of 
  error 
  caused 
  very 
  considerable 
  delay 
  in 
  

   the 
  progress 
  of 
  the 
  work. 
  An 
  attempt 
  was 
  therefore 
  made 
  

   to 
  design 
  a 
  form 
  of 
  magnetometer 
  which 
  would 
  overcome 
  

   these 
  disadvantages 
  which 
  are 
  common 
  to 
  instruments 
  of 
  the 
  

   usual 
  type. 
  In 
  planning 
  the 
  apparatus 
  the 
  following 
  re- 
  

   quirements 
  were 
  kept 
  constantly 
  in 
  view 
  : 
  (1) 
  the 
  magneto- 
  

   meter 
  must 
  be 
  capable 
  of 
  accurate 
  and 
  rapid 
  adjustment; 
  

   (2) 
  there 
  must 
  be 
  no 
  resultant 
  Erhard 
  effect; 
  (3) 
  the 
  instru- 
  

   ment 
  must 
  be 
  suited 
  for 
  testing 
  specimens 
  at 
  all 
  tempera- 
  

   tures 
  from 
  that 
  of 
  liquid 
  hydrogen 
  to 
  the 
  critical 
  temperature; 
  

   (4) 
  it 
  must 
  be 
  alike 
  efficient 
  for 
  testing 
  strongly 
  magnetic 
  

   and 
  feebly 
  magnetic 
  specimens; 
  (5) 
  the 
  magnetizing 
  solenoid 
  

   must 
  be 
  capable 
  of 
  furnishing 
  fields 
  up 
  to 
  at 
  least 
  400 
  c.G.s. 
  

   units; 
  (6) 
  the 
  instrument 
  must 
  be 
  rigid, 
  all 
  parts 
  being 
  fitted 
  

   on 
  one 
  bed-plate, 
  and 
  the 
  coils 
  must 
  be 
  capable 
  of 
  being 
  

   clamped 
  without 
  danger 
  of 
  destroying 
  the 
  compensation 
  in 
  

   so 
  doins. 
  

  

  Fig. 
  2. 
  

  

  L 
  n 
  

  

  « 
  I 
  

  

  8 
  

  

  I 
  

  

  The 
  general 
  principle 
  of 
  the 
  instrument 
  which 
  has 
  been 
  

   evolved 
  will 
  be 
  seen 
  from 
  fig. 
  2. 
  ns 
  represents 
  the 
  magneto- 
  

   meter-needle 
  provided 
  with 
  a 
  concave 
  mirror, 
  by 
  means 
  of 
  

   which 
  and 
  a 
  source 
  of 
  light 
  L, 
  its 
  movements 
  are 
  observed 
  

  

  * 
  ** 
  Eine 
  Fehlerquelle 
  bei 
  niagnetometrisclieii 
  Messungen/' 
  ^?^?^ 
  c?er 
  

   P^?/«. 
  1902, 
  p. 
  724. 
  

  

  