﻿Imp 
  roved 
  Fo 
  rm 
  of 
  Magn 
  etom 
  eter. 
  151 
  

  

  on 
  the 
  scale 
  SS. 
  H 
  is 
  the 
  mngnetizino; 
  solenoid 
  placed 
  due 
  

   east 
  or 
  west 
  o£ 
  the 
  magnetometer-needle 
  and 
  clamped 
  in 
  a 
  

   convenient 
  position. 
  Ci 
  and 
  Cg 
  are 
  compensating 
  coils 
  placed 
  

   with 
  their 
  axes 
  approximately 
  in 
  coincidence 
  with 
  that 
  of 
  the 
  

   solenoid. 
  In 
  adjusting 
  the 
  a})paratus 
  the 
  effect 
  of 
  the 
  current 
  

   in 
  H 
  on 
  the 
  needle 
  ns 
  is 
  first 
  approximately 
  annulled 
  by 
  

   means 
  of 
  Ci, 
  which 
  is 
  then 
  clamped 
  in 
  position. 
  The 
  final 
  

   adjustment 
  of 
  the 
  compensation, 
  so 
  far 
  as 
  the 
  undisturbed 
  

   position 
  of 
  the 
  needle 
  is 
  concerned, 
  is 
  carried 
  out 
  I'y 
  means 
  

   of 
  Co, 
  which 
  on 
  account 
  of 
  its 
  great 
  distance 
  from 
  the 
  needle 
  

   contributes 
  only 
  a 
  small 
  fraction 
  of 
  the 
  balancing 
  field, 
  and 
  

   thus 
  provides 
  an 
  adjustment 
  of 
  great 
  refinement. 
  The 
  

   position 
  of 
  C2 
  necessary 
  for 
  balance 
  having 
  been 
  obtained, 
  it 
  

   is 
  clamped 
  in 
  position 
  ; 
  obviously, 
  since 
  the 
  distance 
  of 
  C2 
  

   from 
  the 
  needle 
  is 
  great, 
  any 
  slioht 
  movement 
  caused 
  by 
  

   doing 
  so 
  produces 
  no 
  perceptible 
  effect 
  on 
  the 
  compensation. 
  

  

  If 
  the 
  axes 
  of 
  Ci, 
  H, 
  and 
  C2 
  were 
  coincident 
  and 
  passed 
  

   through 
  the 
  magnetometer-needle, 
  the 
  adjustment 
  would 
  

   now 
  be 
  complete. 
  If, 
  however, 
  the 
  needle 
  ns 
  is 
  deflected 
  by 
  

   means 
  of 
  a 
  permanent 
  magnet, 
  and 
  a 
  large 
  current 
  is 
  reversed 
  

   in 
  the 
  circuit, 
  in 
  oeneral 
  an 
  alteration 
  in 
  the 
  scale-readino; 
  

   on 
  S 
  S 
  will 
  be 
  observed. 
  A 
  coil 
  Cg, 
  placed 
  with 
  its 
  axis 
  in 
  

   the 
  magnetic 
  north 
  and 
  south 
  line 
  passing 
  through 
  the 
  needle, 
  

   is 
  now 
  included 
  in 
  the 
  circuit. 
  By 
  properly 
  adjusting 
  the 
  

   direction 
  of 
  the 
  current 
  in 
  C3, 
  and 
  altering 
  the 
  distance 
  of 
  C3 
  

   from 
  the 
  needle, 
  the 
  compensation 
  can 
  be 
  made 
  perfect 
  for 
  

   all 
  positions 
  of 
  ns"^ 
  . 
  In 
  a 
  magnetometer 
  where 
  11 
  s, 
  Ci, 
  H, 
  

   and 
  Co 
  are 
  all 
  carried 
  on 
  stands 
  moving 
  in 
  one 
  channel 
  in 
  

   the 
  bed-plate, 
  there 
  should 
  be 
  little 
  departure 
  of 
  the 
  axes 
  of 
  

   the 
  coils 
  from 
  coincidence. 
  Accordingly 
  the 
  resultant 
  mag- 
  

   netic 
  field, 
  due 
  to 
  the 
  coils 
  and 
  solenoid, 
  in 
  the 
  north 
  and 
  

   south 
  direction 
  will 
  be 
  small. 
  The 
  coil 
  C^ 
  is 
  therefore 
  made 
  

   of 
  Utile 
  power, 
  and 
  a 
  small 
  change 
  in 
  its 
  position 
  brings 
  

   about 
  only 
  a 
  very 
  slight 
  alteration 
  in 
  its 
  effect 
  upon 
  the 
  

   needle. 
  It 
  can 
  therefore 
  be 
  clamped 
  without 
  any 
  risk 
  of 
  

   upsetting 
  the 
  balance. 
  The 
  manner 
  of 
  making 
  the 
  adjustments 
  

   will 
  be 
  fully 
  explained 
  later. 
  

  

  'i'he 
  instrument 
  with 
  its 
  fittings 
  is 
  shown 
  in 
  plan 
  in 
  fig. 
  3 
  

   (p. 
  155). 
  The 
  bed-plate 
  is 
  in 
  the 
  form 
  of 
  a 
  cross, 
  and 
  is 
  built 
  

   of 
  well-seasoned 
  mahogany 
  planks 
  22 
  cm. 
  broad 
  and 
  2*5 
  cm. 
  

   thick. 
  The 
  length 
  over 
  all 
  is 
  350 
  cm., 
  and 
  the 
  breadth 
  from 
  

   end 
  to 
  end 
  of 
  the 
  arms 
  135 
  cm. 
  The 
  cross-piece 
  is 
  at 
  a 
  

   distance 
  of 
  100 
  cm. 
  from 
  one 
  end 
  of 
  the 
  main 
  length. 
  Like 
  

  

  * 
  A 
  side 
  coil 
  has 
  been 
  used 
  by 
  Dr. 
  G. 
  E. 
  Allan 
  in 
  his 
  nia^uetometric 
  

   work 
  for 
  jrivino' 
  compen^ation 
  throughout 
  the 
  scale, 
  but 
  it 
  does 
  not 
  

   permit 
  of 
  the 
  adjustment 
  here 
  described. 
  

  

  