﻿156 
  Dr. 
  J. 
  G. 
  Gray 
  and 
  Mr. 
  A. 
  D. 
  Ross 
  on 
  an 
  

  

  When 
  a 
  .speciinen 
  has 
  to 
  be 
  tested 
  the 
  solenoid 
  H 
  is 
  moved 
  

   to 
  a 
  convenient 
  distance 
  from 
  the 
  magnetometer-needle 
  and 
  

   firmly 
  clamped. 
  The 
  coil 
  C2 
  is 
  placed 
  at 
  the 
  far 
  end 
  o£ 
  the 
  

   magnetometer 
  table, 
  and 
  a 
  current 
  two 
  or 
  three 
  times 
  greater 
  

   than 
  the 
  maximum 
  to 
  be 
  used 
  in 
  the 
  subsequent 
  test 
  is 
  sent 
  

   through 
  the 
  complete 
  circuit. 
  Coil 
  Ci 
  is 
  then 
  moved 
  until 
  

   it 
  just 
  falls 
  short 
  of 
  balancing 
  the 
  effect 
  of 
  the 
  solenoid 
  on 
  

   the 
  needle. 
  It 
  is 
  then 
  securely 
  clamped. 
  Coil 
  C2 
  is 
  next 
  

   slowly 
  moved 
  up 
  towards 
  the 
  magnetometer-needle 
  until 
  the 
  

   deflexion 
  of 
  the 
  latter 
  is 
  brought 
  exactly 
  to 
  zero 
  ; 
  C2 
  is 
  now 
  

   clamped, 
  and 
  the 
  accuracy 
  of 
  the 
  compensation 
  verified 
  by 
  

   suddenly 
  reversing 
  the 
  current 
  in 
  the 
  coils. 
  No 
  measurable 
  

   change 
  in 
  the 
  scale-reading 
  should 
  result. 
  The 
  current 
  

   having 
  been 
  interrupted, 
  a 
  small 
  permanent 
  magnet 
  is 
  next 
  

   placed 
  east 
  and 
  west 
  on 
  the 
  stand 
  E, 
  and 
  the 
  stand 
  moved 
  

   along 
  the 
  cross 
  channel 
  in 
  the 
  magnetometer 
  bed-plate 
  until 
  

   the 
  spot 
  rests 
  near 
  one 
  extremity 
  of 
  the 
  scale. 
  The 
  current 
  

   is 
  again 
  made 
  and 
  reversed, 
  and 
  if 
  any 
  appreciable 
  deflexion 
  

   of 
  the 
  spot 
  on 
  the 
  scale 
  is 
  observed 
  coil 
  C3 
  is 
  included 
  in 
  

   the 
  circuit, 
  the 
  current 
  being 
  so 
  directed 
  through 
  it 
  that 
  the 
  

   deviations 
  of 
  the 
  needle 
  from 
  its 
  equilibrium 
  position 
  are 
  

   diminished. 
  The 
  coil 
  is 
  gradually 
  moved 
  closer 
  to 
  the 
  

   magnetometer 
  until 
  the 
  Erhard 
  effect 
  is 
  completely 
  wiped 
  

   out, 
  and 
  is 
  then 
  clamped 
  in 
  position. 
  The 
  compensation 
  now 
  

   holds 
  for 
  all 
  parts 
  of 
  the 
  scale, 
  and 
  the 
  apparatus 
  is 
  ready 
  for 
  

   carrying 
  out 
  magnetic 
  tests. 
  

  

  The 
  several 
  sections 
  in 
  which 
  the 
  three 
  compensating 
  coils 
  

   are 
  built 
  allow 
  the 
  adjustment 
  to 
  be 
  completely 
  made 
  with 
  

   the 
  coils 
  in 
  several 
  different 
  positions. 
  This 
  is 
  a 
  great 
  

   advantage, 
  as 
  it 
  always 
  affords 
  a 
  means 
  of 
  escape 
  from 
  any 
  

   arrangements 
  of 
  the 
  coils 
  which 
  might 
  prove 
  awkward 
  when 
  

   specimens 
  are 
  in 
  the 
  solenoid. 
  

  

  With 
  reference 
  to 
  the 
  adjustment 
  of 
  the 
  coils 
  described 
  

   above, 
  it 
  should 
  be 
  noted 
  (1) 
  that 
  the 
  method 
  is 
  systematic, 
  

   and 
  that 
  there 
  is 
  no 
  possibility 
  of 
  failure 
  to 
  secure 
  a 
  balance 
  

   — 
  all 
  the 
  adjustments 
  are 
  carried 
  out 
  in 
  a 
  perfectly 
  definite 
  

   manner 
  ; 
  (2) 
  that 
  the 
  method 
  is 
  delicate, 
  for, 
  owing 
  to 
  the 
  

   great 
  distance 
  of 
  coil 
  C2 
  from 
  the 
  magnetometer-needle, 
  it 
  

   may 
  generally 
  be 
  moved 
  several 
  millimetres 
  without 
  causing 
  

   any 
  appreciable 
  error 
  in 
  the 
  compensation 
  ; 
  (3) 
  that 
  the 
  

   method 
  is 
  capable 
  of 
  furnishing 
  a 
  high 
  degree 
  of 
  accuracy 
  ; 
  

   with 
  the 
  near 
  end 
  of 
  the 
  solenoid 
  at 
  a 
  distance 
  of 
  only 
  12 
  cm. 
  

   from 
  the 
  magnetometer-needle 
  it 
  is 
  quite 
  possible 
  to 
  arrange 
  

   that 
  the 
  change 
  in 
  the 
  scale 
  - 
  reading 
  brought 
  about 
  by 
  

   reversing 
  a 
  current 
  of 
  15 
  amperes 
  in 
  the 
  circuit 
  is 
  only 
  a 
  

   fraction 
  of 
  1 
  mm. 
  with 
  the 
  scale 
  at 
  a 
  distance 
  of 
  175 
  cm. 
  

  

  