﻿Horizontal 
  Intensity 
  of 
  the 
  Earth's 
  Magnetic 
  Field. 
  763 
  

  

  order 
  to 
  ascertain 
  the 
  accuracy 
  with 
  which 
  the 
  adjustment 
  

   of 
  the 
  position 
  of 
  the 
  solenoid 
  could 
  be 
  made. 
  

  

  The 
  arrangement 
  used 
  is 
  depicted 
  in 
  figure 
  2. 
  Two 
  

   vertical 
  copper 
  wires 
  were 
  fixed, 
  one 
  at 
  N 
  and 
  the 
  other 
  at 
  

   C, 
  the 
  point 
  round 
  which 
  the 
  apparatus 
  rotates. 
  A 
  micro- 
  

   scope 
  M 
  was 
  arranged 
  so 
  that 
  its 
  length 
  was 
  in 
  the 
  same 
  

   straight 
  line 
  as 
  the 
  wires, 
  and 
  the 
  scale 
  in 
  the 
  eyepiece 
  was 
  

   focussed 
  on 
  the 
  copper 
  wire 
  N. 
  The 
  number 
  of 
  scale- 
  

   divisions 
  per 
  millimetre 
  was 
  determined 
  and 
  the 
  slightest 
  

   motion 
  of 
  the 
  wire 
  N 
  could 
  therefore 
  be 
  accurately 
  measured. 
  

   The 
  distance 
  through 
  which 
  the 
  wire 
  N 
  moves 
  divided 
  by 
  the 
  

   distance 
  between 
  the 
  two 
  wires 
  at 
  N 
  and 
  C 
  gives 
  the 
  angle 
  

   of 
  rotation. 
  By 
  this 
  means 
  it 
  was 
  found 
  that 
  the 
  screw 
  

   arrangement 
  at 
  F, 
  G, 
  H, 
  K 
  was 
  sensitive 
  enough 
  to 
  rotate 
  

  

  the 
  apparatus 
  through 
  an 
  angle 
  as 
  small 
  as 
  sin 
  g^ 
  . 
  The 
  

   solenoid 
  tube's 
  position 
  was 
  now 
  adjusted 
  until, 
  on 
  sending 
  

   a 
  large 
  current 
  through 
  the 
  helix, 
  so 
  as 
  to 
  produce 
  a 
  field 
  

   in 
  the 
  same 
  direction 
  as 
  the 
  earth's, 
  no 
  deflexion 
  of 
  the 
  

   magnet 
  took 
  place. 
  

  

  The 
  magnetic 
  axis 
  of 
  the 
  solenoid 
  is 
  now, 
  as 
  far 
  as 
  the 
  

   delicacy 
  of 
  the 
  experiment 
  will 
  allow, 
  exactly 
  parallel 
  to 
  

   the 
  earth's 
  field. 
  The 
  image 
  of 
  the 
  Nernst 
  lamp 
  filament 
  

   was 
  now 
  coincident 
  with 
  the 
  intersection 
  of 
  the 
  cross-wires 
  

   of 
  the 
  telescope 
  viewing 
  the 
  mirror 
  on 
  the 
  magnet. 
  The 
  

   table 
  was 
  now 
  rotated 
  until 
  on 
  sending 
  a 
  current 
  through 
  

   the 
  solenoid 
  the 
  image 
  of 
  the 
  filament 
  was 
  displaced 
  

   through 
  about 
  1 
  mm. 
  The 
  angle 
  through 
  which 
  the 
  table 
  

   had 
  been 
  rotated 
  was 
  measured 
  by 
  means 
  of 
  the 
  copper 
  

  

  wires 
  and 
  microscope, 
  and 
  found 
  to 
  be 
  sin 
  ^tq. 
  

  

  As 
  a 
  movement 
  of 
  much 
  less 
  than 
  1 
  mm. 
  could 
  easily 
  

   have 
  been 
  noticed 
  it 
  will 
  be 
  seen 
  that 
  the 
  adjustment 
  

   can 
  be 
  made 
  with 
  much 
  greater 
  accuracy 
  than 
  the 
  

  

  other 
  quantities 
  can 
  be 
  determined, 
  as 
  sin 
  -1 
  ^ 
  gives 
  

  

  cos 
  a 
  = 
  1 
  — 
  2 
  x 
  10" 
  G 
  or 
  an 
  error 
  of 
  2 
  in 
  10 
  5 
  . 
  

  

  (3) 
  Irregularity 
  in 
  the 
  winding 
  causing 
  the 
  formula 
  

   H= 
  '4:7rnC 
  cos 
  a 
  to 
  be 
  at 
  fault. 
  

   Any 
  irregularity 
  in 
  the 
  winding 
  near 
  the 
  centre 
  will 
  have 
  

   a 
  much 
  greater 
  effect 
  than 
  any 
  irregularity 
  near 
  the 
  ends. 
  

   In 
  the 
  examination 
  of 
  the 
  screw-thread 
  it 
  was 
  found 
  that 
  

   between 
  two 
  fixed 
  microscopes 
  the 
  number 
  of 
  threads 
  was 
  

   perfectly 
  constant. 
  The 
  distance 
  between 
  the 
  microscopes 
  

   was, 
  however, 
  about 
  3 
  cm., 
  and 
  it 
  is 
  quite 
  possible 
  that 
  

   while 
  the 
  sum 
  total 
  of 
  the 
  threads 
  in 
  any 
  3 
  cm. 
  is 
  constant, 
  

  

  