﻿988 
  Prof. 
  S. 
  J. 
  Barnett 
  : 
  Experiments 
  on 
  the 
  

  

  For 
  these 
  experiments 
  two 
  similar 
  cylindrical 
  electro- 
  

   magnets 
  were 
  mounted 
  with 
  their 
  axes 
  in 
  the 
  same 
  straight 
  

   line, 
  and 
  were 
  so 
  magnetized 
  that 
  the 
  lines 
  of 
  induction 
  

   stretched 
  across 
  from 
  one 
  of 
  the 
  adjacent 
  poles 
  to 
  the 
  other. 
  

   A 
  small 
  elongated 
  cylinder 
  of 
  iron 
  was 
  so 
  mounted 
  at 
  the 
  

   point 
  of 
  the 
  field 
  under 
  investigation 
  as 
  always 
  to 
  follow 
  

   any 
  change 
  (of 
  the 
  sort 
  under 
  investigation) 
  in 
  the 
  direction 
  

   of 
  the 
  intensity 
  at 
  the 
  point, 
  and 
  a 
  mirror 
  attached 
  to 
  the 
  

   magnet's 
  support 
  and 
  other 
  optical 
  appliances 
  made 
  it 
  

   possible 
  to 
  determine 
  any 
  change 
  in 
  the 
  magnet's 
  orientation 
  

   with 
  great 
  precision. 
  

  

  In 
  the 
  principal 
  series 
  of 
  experiments 
  the 
  magnets 
  were 
  

   rotated 
  in 
  opposite 
  directions, 
  and 
  the 
  change 
  in 
  the 
  direction 
  

   of 
  the 
  intensity 
  produced 
  by 
  reversing 
  both 
  rotations 
  was 
  

   observed. 
  The 
  apparatus 
  as 
  arranged 
  for 
  these 
  experiments 
  

   will 
  be 
  briefly 
  described. 
  

  

  The 
  chief 
  part 
  of 
  the 
  core 
  of 
  each 
  electromagnet 
  was 
  a 
  

   rod 
  of 
  steel 
  shafting 
  7*5 
  cm. 
  in 
  diameter. 
  This 
  was 
  capped 
  

   at 
  one 
  end 
  with 
  a 
  disk 
  of 
  soft 
  iron 
  12*3 
  cm. 
  in 
  diameter 
  and 
  

   over 
  2'5 
  cm. 
  thick. 
  At 
  the 
  other 
  end 
  the 
  rod 
  was 
  turned 
  

   down 
  to 
  a 
  cylinder 
  of 
  3*8 
  cm. 
  diameter. 
  The 
  bearings 
  were 
  

   made 
  of 
  brass 
  and 
  bronze, 
  and 
  were 
  0*5 
  metre 
  apart, 
  with 
  

   the 
  coil 
  and 
  contact 
  rings 
  between 
  them 
  and 
  a 
  pulley 
  on 
  the 
  

   other 
  side 
  of 
  the 
  bearing 
  of 
  smaller 
  diameter. 
  The 
  bearings 
  

   were 
  firmly 
  screwed 
  to 
  a 
  heavy 
  wooden 
  base, 
  itself 
  bolted 
  

   to 
  the 
  cement 
  floor. 
  The 
  two 
  magnets 
  were 
  mounted 
  with 
  

   their 
  axes, 
  as 
  stated 
  above, 
  in 
  the 
  same 
  straight 
  line 
  and 
  the 
  

   iron 
  disks 
  adjacent 
  and 
  3 
  cm. 
  apart. 
  

  

  The 
  magnets 
  were 
  driven 
  with 
  belts 
  by 
  an 
  electric 
  motor 
  

   at 
  a 
  speed 
  of 
  about 
  30 
  revolutions 
  per 
  second. 
  The 
  strength 
  

   of 
  the 
  field 
  between 
  the 
  poles 
  was 
  about 
  1200 
  gausses 
  near 
  

   the 
  edge, 
  and 
  somewhat 
  greater 
  at 
  the 
  centre. 
  

  

  The 
  small 
  indicating 
  magnet 
  was 
  mounted 
  symmetrically 
  

   at 
  the 
  centre 
  of 
  a 
  much 
  longer 
  thin 
  brass 
  rod, 
  the 
  two 
  being 
  

   at 
  right 
  angles. 
  The 
  ends 
  of 
  the 
  brass 
  rod 
  were 
  turned 
  to 
  

   fit 
  bearings 
  in 
  the 
  caps 
  of 
  a 
  narrow 
  brass 
  tube, 
  which 
  en- 
  

   closed 
  the 
  rod 
  and 
  its 
  attachments. 
  These 
  included 
  the 
  

   magnet, 
  the 
  mirror, 
  which 
  was 
  plane, 
  and 
  a 
  small 
  brass 
  vane 
  

   near 
  the 
  bottom. 
  The 
  tube 
  was 
  mounted 
  in 
  a 
  clamp 
  of 
  brass 
  

   and 
  wood 
  on 
  the 
  laboratory 
  wall 
  with 
  its 
  axis 
  approximately 
  

   vertical, 
  parallel 
  to 
  the 
  disks, 
  and 
  in 
  a 
  plane 
  passing 
  through 
  

   the 
  centres 
  of 
  the 
  disks. 
  The 
  lower 
  part 
  of 
  the 
  tube 
  con- 
  

   tained 
  either 
  castor-oil 
  or 
  glycerine 
  ; 
  the 
  brass 
  vane, 
  sub- 
  

   merged 
  in 
  this 
  fluid, 
  served 
  to 
  damp 
  any 
  motion 
  of 
  the 
  

   magnet 
  and 
  mirror. 
  A 
  hole 
  was 
  cut 
  in 
  the 
  tube 
  opposite 
  

  

  