﻿990 
  Magnetic 
  Field 
  of 
  two 
  Electromagnets 
  in 
  Rotation. 
  

  

  The 
  results 
  obtained 
  with 
  the 
  magnet 
  in 
  the 
  upper 
  and 
  

   lower 
  positions 
  would 
  at 
  first 
  sight 
  appear 
  to 
  indicate 
  a 
  very 
  

   slight 
  twist 
  of 
  the 
  lines 
  of 
  induction 
  in 
  the 
  direction 
  to 
  be 
  

   expected 
  if 
  any 
  entrainment 
  of 
  the 
  lines 
  of 
  induction 
  were 
  

   actually 
  to 
  occur. 
  Many 
  other 
  observations, 
  similar 
  to 
  those 
  

   given 
  above, 
  were 
  obtained 
  with 
  the 
  magnet 
  in 
  the 
  upper 
  and 
  

   lower 
  positions, 
  and 
  the 
  results 
  were 
  always 
  consistent 
  with 
  

   those 
  given 
  here, 
  except 
  that 
  one 
  group 
  of 
  5 
  sets 
  with 
  

   magnet 
  above 
  gave 
  a 
  positive 
  result 
  (D 
  = 
  +0*03 
  mm. 
  + 
  0*02 
  

   mm.). 
  These 
  observations, 
  however, 
  were 
  obtained 
  under 
  

   unsatisfactory 
  conditions. 
  Nevertheless, 
  in 
  view 
  of 
  the 
  large 
  

   discrepancies 
  and 
  the 
  systematic 
  errors 
  possible, 
  the 
  only 
  

   result 
  of 
  which 
  one 
  can 
  be 
  confident 
  is 
  that 
  the 
  change 
  of 
  

   direction 
  of 
  a 
  line 
  near 
  the 
  edge 
  of 
  the 
  uniform 
  part 
  of 
  the 
  

   field 
  when 
  the 
  magnets 
  are 
  brought 
  from 
  rest 
  to 
  full 
  speed 
  

   is 
  not 
  greater 
  than 
  a 
  very 
  minute 
  quantity 
  (the 
  mean 
  from 
  

   the 
  above 
  table 
  is 
  about 
  V 
  of 
  arc). 
  

  

  In 
  the 
  hope 
  of 
  obtaining 
  still 
  more 
  accurate 
  results, 
  the 
  

   electromagnets 
  were 
  mounted 
  in 
  new 
  bearings, 
  fitting 
  as 
  

   closely 
  as 
  practicable 
  ; 
  the 
  bearings 
  were 
  bolted 
  directly 
  to 
  

   the 
  cement 
  floor, 
  the 
  wooden 
  bases 
  being 
  removed 
  ; 
  and 
  the 
  

   optical 
  arrangement 
  was 
  improved 
  by 
  substituting 
  for 
  the 
  

   old 
  mirror 
  a 
  new 
  and 
  more 
  accurately 
  ground 
  one 
  by 
  

   Petitdidier, 
  and 
  by 
  increasing 
  the 
  distance 
  from 
  eyepiece 
  to 
  

   mirror 
  to 
  5'4 
  metres. 
  This 
  distance 
  was 
  kept 
  constant 
  

   throughout 
  the 
  rest 
  of 
  the 
  work. 
  

  

  The 
  results 
  were 
  of 
  the 
  same 
  character 
  as 
  those 
  obtained 
  

   before. 
  Thus 
  5 
  sets 
  with 
  the 
  magnet 
  in 
  the 
  upper 
  position 
  

   gave 
  D=— 
  0*09 
  mm. 
  + 
  0*02 
  mm., 
  and 
  5 
  sets 
  with 
  the 
  

   magnet 
  in 
  the 
  lower 
  position 
  gave 
  D=-f0 
  , 
  16 
  mm. 
  

   + 
  O08 
  mm. 
  

  

  Mrs. 
  Barnett, 
  to 
  whom 
  I 
  am 
  indebted 
  for 
  assistance 
  

   throughout 
  the 
  observations 
  described 
  here, 
  having 
  suggested 
  

   that 
  the 
  distortion 
  of 
  the 
  lines, 
  if 
  any 
  occurred, 
  might 
  be 
  a 
  

   minimum 
  over 
  the 
  central 
  plane, 
  each 
  line 
  having 
  there 
  a 
  

   point 
  of 
  inflexion, 
  additional 
  observations 
  were 
  made 
  as 
  close 
  

   a» 
  practicable 
  to 
  one 
  of 
  the 
  pole-faces, 
  but 
  without 
  changing 
  

   the 
  result. 
  The 
  pole 
  distance 
  of 
  3 
  cm., 
  however, 
  was 
  too 
  

   small 
  to 
  permit 
  of 
  any 
  great 
  shift 
  of 
  the 
  magnet. 
  By 
  boring 
  

   partially 
  through 
  and 
  remounting 
  the 
  iron 
  disks, 
  the 
  pole 
  

   distance 
  was 
  therefore 
  increased 
  to 
  7 
  cm. 
  ; 
  and 
  observations 
  

   were 
  made 
  with 
  the 
  indicating 
  magnet 
  in 
  three 
  positions, 
  all 
  

   corresponding 
  in 
  altitude 
  to 
  the 
  upper 
  position 
  of 
  the 
  earlier 
  

   work, 
  but 
  one 
  close 
  to 
  the 
  left 
  pole, 
  one 
  in 
  the 
  middle 
  as 
  

   before, 
  and 
  one 
  close 
  to 
  the 
  right 
  pole. 
  The 
  increase 
  of 
  pole 
  

   distance 
  diminished 
  the 
  intensity 
  to 
  about 
  800 
  gausses 
  in 
  the 
  

  

  