﻿Horizontal 
  Intensity 
  of 
  the 
  Kavih's 
  Magnetic 
  Field. 
  753 
  

  

  In 
  the 
  experiment 
  as 
  actually 
  carried 
  out 
  the 
  field 
  is 
  

   produced 
  by 
  an 
  accurately 
  wound 
  helix. 
  In 
  such 
  a 
  case 
  the 
  

   field 
  at 
  the 
  centre 
  is 
  given 
  by 
  *47niC 
  cos 
  a 
  7 
  where 
  n 
  is 
  the 
  

   number 
  of 
  turns 
  per 
  cm. 
  o£ 
  the 
  helix, 
  is 
  the 
  current 
  in 
  

   amperes, 
  and 
  2a 
  is 
  the 
  angle 
  subtended 
  by 
  the 
  diameter 
  of 
  

   the 
  end 
  of 
  the 
  helix 
  at 
  the 
  centre 
  of 
  the 
  helix. 
  

  

  It 
  will 
  be 
  at 
  once 
  seen 
  that 
  the 
  following 
  corrections, 
  

   necessary 
  in 
  the 
  Magnetometer 
  method, 
  can 
  be 
  eliminated 
  : 
  

  

  (a) 
  Correction 
  o£ 
  the 
  time 
  o£ 
  swing 
  owing 
  to 
  the 
  vibration 
  

  

  not 
  being 
  in 
  an 
  infinitely 
  small 
  arc. 
  

  

  (b) 
  Correction 
  for 
  the 
  torsion 
  of 
  the 
  fibre. 
  

  

  (c) 
  Correction 
  for 
  temperature 
  alteration. 
  

  

  (d) 
  Correction 
  due 
  to 
  the 
  temporary 
  increase 
  of 
  the 
  

  

  moment 
  of 
  the 
  magnet 
  owing 
  to 
  its 
  lying 
  in 
  the 
  

   earth's 
  field. 
  

  

  The 
  conditions 
  in 
  the 
  two 
  parts 
  of 
  the 
  proposed 
  method 
  

   are 
  exactly 
  the 
  same, 
  hence 
  (d) 
  does 
  not 
  enter, 
  while 
  (a) 
  is 
  

   made 
  of 
  non-effect 
  by 
  taking 
  the 
  initial 
  arc 
  o£ 
  swing 
  the 
  

   same 
  in 
  each 
  part 
  of 
  the 
  experiment. 
  

  

  By 
  turning 
  the 
  head 
  supporting 
  the 
  suspension 
  fibre 
  

   through 
  180° 
  the 
  torsion 
  when 
  the 
  magnet 
  is 
  vibrating 
  in 
  its 
  

   reversed 
  position 
  is 
  exactly 
  the 
  same 
  as 
  when 
  it 
  is 
  vibrating 
  

   under 
  the 
  influence 
  of 
  the 
  earth 
  alone, 
  and 
  therefore 
  no 
  

   correction 
  is 
  needed 
  on 
  account 
  of 
  (b). 
  As 
  will 
  be 
  shown 
  later, 
  

   the 
  torsion 
  may 
  influence 
  the 
  result 
  in 
  another 
  manner, 
  

   (c) 
  becomes 
  of 
  no 
  account 
  if 
  the 
  experiment 
  is 
  performed 
  in 
  

   a 
  room 
  kept 
  at 
  a 
  fairly 
  uniform 
  temperature, 
  for 
  the 
  experi- 
  

   ment 
  is 
  not 
  long. 
  If 
  it 
  were 
  found 
  necessary, 
  the 
  requisite 
  

   correction 
  could 
  easily 
  be 
  determined. 
  

  

  Preliminary 
  experiments 
  having 
  shown 
  that 
  the 
  method 
  

   was 
  practicable, 
  apparatus 
  was 
  devised 
  for 
  carrying 
  out 
  the 
  

   experiment 
  in 
  an 
  accurate 
  manner. 
  The 
  construction 
  and 
  

   working 
  of 
  the 
  different 
  parts 
  will 
  be 
  described, 
  then 
  the 
  

   conditions 
  of 
  accuracy 
  discussed, 
  and 
  lastly 
  the 
  method 
  of 
  

   making 
  the 
  experiment 
  will 
  be 
  detailed. 
  

  

  The 
  Helix. 
  

  

  As 
  the 
  field 
  at 
  the 
  centre 
  is 
  *47rnCcosa, 
  n 
  must 
  be 
  quite 
  

   definite 
  and, 
  if 
  great 
  accuracy 
  is 
  to 
  be 
  obtained, 
  cos 
  a 
  must 
  

   be 
  almost 
  1 
  in 
  order 
  to 
  ensure 
  the 
  field 
  near 
  the 
  centre 
  being 
  

   uniform. 
  A 
  helix 
  of 
  length 
  100 
  cm. 
  and 
  diameter 
  4 
  cm. 
  

   gives 
  cos« 
  = 
  *9992, 
  and 
  is 
  therefore 
  of 
  suitable 
  dimensions 
  as 
  

   will 
  be 
  shown 
  later. 
  A 
  preliminary 
  experiment 
  was 
  carried 
  

   out 
  using 
  a 
  helix 
  of 
  silk-covered 
  wire 
  wound 
  on 
  a 
  copper 
  

   tube 
  118 
  cm. 
  long 
  and 
  internal 
  diameter 
  4 
  cm. 
  The 
  results 
  

  

  