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  987 
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  LXXXVIII. 
  Some 
  'Experiments 
  on 
  the 
  Magnetic 
  Field 
  of 
  two 
  

   Electromagnets 
  in 
  Rotation. 
  By 
  S. 
  J. 
  Baenett, 
  Professor 
  

   of 
  Physics 
  in 
  the 
  Ohio 
  State 
  University*. 
  

  

  IN 
  1903 
  E. 
  Hoppe 
  t 
  made 
  some 
  experiments 
  with 
  rotating 
  

   magnets 
  and 
  iron 
  filings 
  which 
  he 
  thought 
  conclusive 
  

   in 
  favour 
  of 
  the 
  hypothesis 
  that 
  a 
  magnet 
  in 
  rotation 
  carries 
  

   with 
  it 
  the 
  lines 
  of 
  induction 
  of 
  its 
  field. 
  In 
  these 
  experi- 
  

   ments 
  a 
  glass 
  plate 
  or 
  a 
  piece 
  of 
  paper 
  was 
  supported 
  

   horizontal 
  just 
  above 
  a 
  vertical 
  magnet, 
  and 
  iron 
  filings 
  

   were 
  let 
  fall 
  upon 
  the 
  paper 
  or 
  glass 
  while 
  the 
  magnet 
  was 
  

   rotated 
  about 
  a 
  vertical 
  line 
  through 
  its 
  centre. 
  Hoppe 
  

   found 
  that 
  when 
  the 
  field 
  was 
  very 
  intense, 
  the 
  iron 
  filings 
  

   became 
  arranged 
  in 
  curves 
  which 
  were 
  not 
  radial, 
  but 
  were 
  

   curved 
  as 
  if 
  the 
  rotating 
  magnet 
  dragged 
  the 
  lines 
  of 
  in- 
  

   duction 
  with 
  it, 
  the 
  nearer 
  parts 
  ahead 
  of 
  the 
  more 
  remote. 
  

   He 
  found 
  also 
  that 
  the 
  effect 
  was 
  increased 
  by 
  surrounding 
  

   the 
  upper 
  pole 
  of 
  the 
  magnet 
  with 
  an 
  iron 
  ring 
  at 
  a 
  small 
  

   distance. 
  With 
  this 
  arrangement 
  and 
  an 
  electromagnet 
  

   with 
  a 
  round 
  core 
  3*2 
  cm. 
  in 
  diameter, 
  driven 
  at 
  a 
  speed 
  of 
  

   10 
  revolutions 
  per 
  second, 
  he 
  found 
  that 
  the 
  lines 
  at 
  the 
  edge 
  

   of 
  the 
  magnet 
  deviated 
  from 
  radial 
  directions 
  b}^ 
  as 
  much 
  as 
  

   2 
  mm. 
  

  

  S. 
  Yalentiner 
  J 
  repeated 
  some 
  of 
  these 
  experiments 
  with 
  

   a 
  thin 
  but 
  strongly 
  magnetized 
  circular 
  cylindrical 
  magnet. 
  

   When 
  the 
  magnet 
  was 
  rotated 
  about 
  its 
  geometrical 
  axis 
  he 
  

   did 
  not 
  get 
  the 
  effect 
  observed 
  by 
  Hoppe 
  ; 
  but 
  the 
  effect 
  

   appeared 
  when 
  the 
  magnet 
  was 
  rotated 
  excentrically 
  about 
  

   a 
  line 
  parallel 
  to 
  its 
  axis. 
  Hence 
  he 
  concluded 
  that 
  Hoppe's 
  

   results 
  were 
  due 
  to 
  unsymmetrical 
  magnetization 
  of 
  his 
  

   magnets 
  and 
  that 
  his 
  interpretation 
  of 
  them 
  was 
  incorrect. 
  

  

  There 
  being 
  available 
  some 
  apparatus 
  which, 
  though 
  con- 
  

   structed 
  primarily 
  for 
  another 
  investigation 
  §, 
  was 
  capable, 
  

   when 
  slightly 
  modified, 
  of 
  giving 
  information 
  on 
  the 
  

   problem 
  examined 
  by 
  Hoppe 
  and 
  Valentiner 
  of 
  a 
  much 
  

   more 
  exact 
  character 
  than 
  could 
  be 
  obtained 
  by 
  their 
  

   methods, 
  I 
  thought 
  it 
  worth 
  while, 
  a 
  number 
  of 
  years 
  ago, 
  

   to 
  make 
  some 
  experiments 
  on 
  the 
  subject. 
  These 
  experi- 
  

   ments 
  have 
  recently 
  been 
  repeated 
  and 
  extended 
  under 
  much 
  

   more 
  favourable 
  conditions, 
  again 
  with 
  apparatus 
  mostly 
  

   designed 
  primarily 
  for 
  other 
  work. 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

   t 
  Phys. 
  Zeit. 
  v. 
  p. 
  650 
  (1904). 
  

   X 
  Phys. 
  Zeit. 
  vi. 
  p. 
  10 
  (1905). 
  

  

  § 
  Phys. 
  Rev. 
  xxxv. 
  p. 
  323 
  (1912) 
  ; 
  Science, 
  xxxvii. 
  pp. 
  113, 
  560 
  (1913); 
  

   Phys. 
  Zeit. 
  xiii. 
  p. 
  803 
  (1912), 
  and 
  xiv. 
  p. 
  251 
  (1913). 
  

  

  3X2 
  

  

  