﻿Improved 
  Form 
  of 
  Magnetometer, 
  149 
  

  

  practice, 
  the 
  axes 
  of 
  the 
  solenoid 
  and 
  compensating 
  coils 
  are 
  

   in 
  general 
  slightly 
  inclined 
  to 
  one 
  another 
  and 
  to 
  the 
  east 
  

   and 
  west 
  line 
  passing 
  through 
  the 
  needle. 
  The 
  effect 
  of 
  this 
  

   is 
  to 
  increase 
  the 
  directive 
  force 
  on 
  the 
  needle 
  for 
  one 
  direc- 
  

   tion 
  of 
  the 
  current 
  and 
  to 
  diminish 
  it 
  for 
  the 
  other. 
  That 
  

   this 
  is 
  the 
  case 
  will 
  be 
  seen 
  from 
  fig. 
  1, 
  in 
  which 
  the 
  want 
  

  

  of 
  alignment 
  of 
  the 
  coil 
  and 
  solenoid 
  has 
  been 
  greatly 
  ex- 
  

   aggerated. 
  The 
  magnetometer-needle 
  is 
  situated 
  at 
  the 
  

   point 
  P, 
  and 
  it 
  has 
  been 
  assumed 
  that 
  the 
  solenoid 
  and 
  coil 
  

   are 
  so 
  placed 
  that 
  they 
  produce 
  fields 
  at 
  P 
  in 
  the 
  directions 
  

   PS 
  and 
  PC 
  respectively. 
  If 
  the 
  intensity 
  of 
  the 
  field 
  due 
  to 
  

   the 
  solenoid 
  be 
  denoted 
  bv 
  Fs- 
  and 
  that 
  due 
  to 
  the 
  coil 
  by 
  

   Fc, 
  then 
  since 
  the 
  coils 
  balance 
  for 
  the 
  undisturbed 
  position 
  

   of 
  the 
  needle 
  it 
  follows 
  that 
  Fscos 
  ^i 
  = 
  FcCos 
  6 
  2. 
  There 
  are 
  

   left, 
  however, 
  the 
  components 
  of 
  the 
  intensities 
  in 
  the 
  north 
  

   and 
  south 
  direction, 
  and 
  it 
  is 
  evident 
  from 
  the 
  figure 
  that 
  if 
  

   H 
  is 
  the 
  horizontal 
  component 
  of 
  the 
  earth's 
  magnetic 
  field 
  

   at 
  P, 
  the 
  total 
  directive 
  force 
  at 
  the 
  needle 
  is 
  H-j-(Fs 
  sin 
  61 
  

   -fFcsin^2)* 
  ^f 
  the 
  current 
  is 
  reversed 
  in 
  the 
  circuit 
  the 
  

   directions 
  of 
  Fs 
  and 
  Fc 
  change, 
  and 
  the 
  directive 
  force 
  at 
  

   the 
  needle 
  becomes 
  H 
  — 
  (Fs 
  sin 
  ^1 
  -hFc 
  sin 
  62). 
  

  

  The 
  presence 
  of 
  the 
  effect 
  referred 
  to 
  may 
  be 
  made 
  appa- 
  

   rent 
  by 
  placing 
  a 
  permanent 
  magnet 
  close 
  to 
  the 
  magneto- 
  

   meter, 
  and 
  thus 
  deflecting 
  the 
  needle. 
  On 
  reversing 
  a 
  

   current 
  in 
  the 
  circuit, 
  a 
  change 
  in 
  the 
  deflexion 
  will 
  in 
  

   general 
  be 
  observed. 
  The 
  magnitude 
  of 
  the 
  errors 
  intro- 
  

   duced 
  may 
  be 
  determined 
  in 
  this 
  way 
  for 
  various 
  parts 
  of 
  

   the 
  scale 
  and 
  allowed 
  for 
  iu 
  the 
  results, 
  or 
  the 
  coils 
  may 
  be 
  

   rotated 
  until 
  the 
  effect 
  disappears. 
  If 
  the 
  former 
  method 
  is 
  

   adopted, 
  the 
  labour 
  of 
  computing 
  the 
  results 
  is 
  much 
  in- 
  

   creased, 
  and, 
  further, 
  it 
  is 
  difficult 
  to 
  make 
  a 
  proper 
  correc- 
  

   tion, 
  since 
  the 
  allowance 
  to 
  be 
  made 
  is 
  a 
  function 
  both 
  of 
  

   the 
  angle 
  of 
  deflexion 
  and 
  the 
  strength 
  of 
  the 
  current- 
  The 
  

   second 
  method 
  can 
  only 
  be 
  used 
  if 
  the 
  coils 
  are 
  capable 
  of 
  

   being 
  rotated 
  on 
  their 
  stands, 
  and 
  the 
  adjustments 
  would 
  be 
  

   difficult 
  and 
  troublesome 
  to 
  carry 
  out. 
  

  

  