﻿Horizontal 
  Intensity 
  of 
  the 
  Earth's 
  Magnetic 
  Field. 
  7 
  59 
  

  

  Placing 
  of 
  the 
  Solenoid 
  so 
  that 
  the 
  Field 
  given 
  by 
  the 
  Helix 
  

   is 
  exactly 
  parallel 
  to 
  the 
  Eartlis 
  Field. 
  

  

  Tm. 
  2. 
  

  

  

  D 
  -/• 
  

  

  L 
  

  

  

  

  

  n 
  / 
  

  

  

  E' 
  

  

  \ 
  n 
  

  

  

  ■=c 
  ;; 
  

  

  

  U 
  A 
  

  

  

  jk 
  u 
  

  

  

  

  , 
  N 
  

  

  A 
  

  

  F 
  

  

  ^H 
  

  

  T* 
  

  

  The 
  solenoid 
  tube 
  rests 
  symmetrically 
  upon 
  V 
  *s 
  in 
  the 
  

   pieces 
  of 
  wood 
  E, 
  E' 
  which 
  are 
  screwed 
  into 
  the 
  drawing- 
  

   board 
  B 
  in 
  the 
  positions 
  shown. 
  

  

  At 
  the 
  centre 
  of 
  this 
  drawing-board 
  is 
  a 
  small 
  circular 
  

   hole 
  0, 
  through 
  which 
  passes 
  a 
  peg 
  rigidly 
  attached 
  to 
  the 
  

   centre 
  of 
  the 
  slightly 
  larger 
  drawing-board 
  A. 
  A 
  large 
  

   V-groove, 
  circular 
  in 
  configuration, 
  is 
  cut 
  into 
  this 
  lower 
  

   drawing-board, 
  and 
  is 
  represented 
  in 
  the 
  diagram 
  by 
  the 
  

   dotted 
  circles 
  D. 
  Three 
  steel 
  balls, 
  ^ 
  in. 
  diameter, 
  roll 
  in 
  

   this 
  groove, 
  and 
  the 
  drawing-board 
  B 
  rests 
  on 
  these 
  steel 
  

   balls. 
  The 
  apparatus 
  rolls 
  easily 
  on 
  the 
  steel 
  balls 
  L 
  L 
  L,. 
  

   and 
  as 
  the 
  peg 
  at 
  the 
  centre 
  of 
  A 
  fits 
  exactly 
  into 
  the 
  hole 
  

   C, 
  the 
  motion 
  is 
  purely 
  rotational. 
  The 
  slightest 
  force 
  

   applied 
  to 
  the 
  drawing-board 
  B 
  is 
  sufficient 
  to 
  rotate 
  the 
  

   apparatus. 
  To 
  keep 
  the 
  apparatus 
  in 
  a 
  definite 
  position, 
  

   and 
  to 
  allow 
  of 
  very 
  small 
  rotations 
  being 
  made, 
  the 
  device 
  

   shown 
  at 
  F 
  was 
  employed. 
  G 
  is 
  a 
  piece 
  of 
  metal 
  firmly 
  

   screwed 
  into 
  the 
  side 
  of 
  the 
  drawing-board 
  A. 
  Through 
  this 
  

   metal 
  the 
  large-headed 
  screw 
  K 
  can 
  be 
  turned, 
  and 
  this 
  

   screw 
  is 
  of 
  such 
  a 
  length 
  as 
  to 
  press 
  against 
  the 
  side 
  of 
  the 
  

   board 
  B 
  with 
  its 
  point 
  and 
  to 
  allow 
  of 
  a 
  rotation 
  of 
  several 
  

   degrees. 
  When 
  the 
  point 
  of 
  the 
  screw 
  is 
  pressing 
  against 
  

   the 
  drawing-board 
  B, 
  a 
  slight 
  turning 
  of 
  the 
  screw 
  will 
  

   give 
  a 
  very 
  small 
  rotation. 
  This, 
  however, 
  only 
  allows 
  the 
  

   rotating 
  of 
  the 
  solenoid 
  in 
  one 
  direction, 
  and 
  to 
  remedy 
  this 
  

   a 
  small 
  brass 
  rod 
  H 
  was 
  screwed 
  into 
  the 
  board 
  B 
  and 
  an 
  

   elastic 
  band 
  passed 
  round 
  this 
  and 
  the 
  piece 
  of 
  metal 
  Gr. 
  

  

  3E 
  2 
  

  

  