﻿Magnetic 
  Field 
  of 
  tivo 
  Electromagnets 
  in 
  Rotation. 
  989 
  

  

  the 
  mirror, 
  and 
  over 
  the 
  hole 
  a 
  converging 
  spectacle-lens 
  of 
  

   4 
  metres 
  focal 
  length 
  was 
  fastened 
  with 
  wax. 
  

  

  The 
  optical 
  observations 
  were 
  made 
  by 
  setting 
  with 
  a 
  

   micrometer 
  eyepiece 
  upon 
  the 
  centre 
  of 
  the 
  image, 
  formed 
  

   by 
  the 
  mirror 
  and 
  lens, 
  of 
  one 
  of 
  the 
  terminal 
  portions 
  of 
  

   the 
  incandescent 
  filament 
  of 
  an 
  electric 
  candle. 
  The 
  micro- 
  

   meter 
  which 
  carried 
  the 
  lens 
  and 
  cross-hairs 
  of 
  the 
  eyepiece 
  

   read 
  directly 
  to 
  0*01 
  mm. 
  The 
  distance 
  from 
  eyepiece 
  to 
  

   mirror 
  was 
  greater 
  than 
  4 
  metres 
  (see 
  below). 
  

  

  Observations 
  were 
  made 
  with 
  the 
  magnet, 
  always 
  half-way 
  

   between 
  the 
  pole-faces, 
  in 
  three 
  different 
  positions 
  : 
  a 
  little 
  

   below 
  the 
  top 
  of 
  the 
  poles, 
  a 
  little 
  above 
  the 
  bottom, 
  and 
  at 
  

   the 
  centre 
  of 
  the 
  field. 
  In 
  each 
  case 
  readings 
  were 
  made 
  in 
  

   sets 
  of 
  four 
  ; 
  the 
  first 
  and 
  fourth 
  being 
  made 
  with 
  the 
  

   magnet 
  on 
  the 
  right 
  rotating 
  clockwise 
  as 
  seen 
  from 
  its 
  right 
  

   end, 
  the 
  second 
  and 
  third 
  with 
  the 
  motions 
  reversed. 
  The 
  

   difference, 
  D, 
  obtained 
  by 
  subtracting 
  the 
  mean 
  of 
  the 
  second 
  

   and 
  third 
  from 
  the 
  mean 
  of 
  the 
  first 
  and 
  fourth, 
  was 
  the 
  

   immediate 
  quantity 
  sought. 
  

  

  By 
  taking 
  observations 
  in 
  the 
  manner 
  indicated 
  it 
  was 
  

   sought 
  to 
  eliminate 
  errors 
  arising 
  from 
  the 
  drift 
  of 
  the 
  zero, 
  

   the 
  relative 
  displacement 
  of 
  the 
  magnets 
  produced 
  by 
  the 
  

   pull 
  of 
  the 
  belts 
  and 
  the 
  rotation, 
  and 
  the 
  difference 
  between 
  

   the 
  widths 
  of 
  the 
  image 
  when 
  at 
  rest 
  and 
  in 
  motion. 
  

  

  Some 
  of 
  the 
  most 
  reliable 
  results 
  are 
  given 
  in 
  Table 
  I. 
  

  

  Table 
  I. 
  

  

  

  

  Position 
  of 
  

   magnet. 
  

  

  Number 
  

   of 
  sets. 
  

  

  D. 
  

  

  Distance 
  from 
  

   eyepiece 
  to 
  mirror. 
  

  

  Above 
  

  

  5 
  

  

  mm. 
  

  

  -0-08 
  

  

  mm. 
  

  

  ±o-oi 
  

  

  m. 
  

   4-1 
  

  

  Above 
  

  

  5 
  

  

  -0-09 
  

  

  ±0-02 
  

  

  44 
  

  

  Centre 
  

  

  5 
  

  

  -0 
  05 
  

  

  ±0-01 
  

  

  4-4 
  

  

  Below 
  

  

  5 
  

  

  +0-11 
  

  

  ±0-04 
  

  

  4-4 
  

  

  Above 
  

  

  5 
  

  

  -004 
  

  

  ±0-03 
  

  

  4-4 
  

  

  Below 
  

  

  5 
  

  

  +0-07 
  

  

  ±0-02 
  

  

  4-4 
  

  

  The 
  observations 
  in 
  the 
  first 
  four 
  lines 
  were 
  obtained 
  

   consecutively, 
  as 
  were 
  those 
  in 
  the 
  last 
  two. 
  The 
  + 
  sign 
  

   indicates 
  a 
  twist 
  of 
  the 
  lines 
  in 
  a 
  clockwise 
  direction 
  as 
  seen 
  

   from 
  above. 
  The 
  quantity 
  following 
  the 
  + 
  is 
  the 
  average 
  

   absolute 
  departure 
  of 
  the 
  sets 
  in 
  each 
  group 
  from 
  the 
  mean. 
  

  

  