﻿362 
  Mr. 
  N. 
  E. 
  Gilbert: 
  Experiments 
  upon 
  the 
  

  

  of 
  the 
  plane 
  of 
  polarization 
  of 
  light 
  in 
  a 
  magnetic 
  field 
  sug- 
  

   gested 
  that 
  the 
  particles 
  of 
  matter, 
  or 
  the 
  aether 
  in 
  connexion 
  

   with 
  them, 
  must 
  be 
  in 
  rotation. 
  As 
  the 
  result 
  of 
  the 
  theories 
  

   proposed 
  by 
  Ampere 
  and 
  Weber, 
  and 
  developed 
  by 
  Maxwell, 
  

   modern 
  theories 
  of 
  magnetism 
  are 
  based 
  on 
  some 
  kind 
  of 
  

   rotary 
  or 
  vortical 
  motion 
  in 
  the 
  yether 
  ; 
  and 
  if 
  a 
  piece 
  of 
  iron 
  

   is 
  magnetized, 
  we 
  imagine 
  that 
  the 
  molecules, 
  or 
  something 
  

   about 
  them, 
  rotate 
  also. 
  Maxwell 
  * 
  has 
  tried 
  to 
  detect 
  the 
  

   presence 
  of 
  any 
  such 
  rotation 
  in 
  an 
  electromagnet. 
  With 
  a 
  

   kind 
  of 
  gyroscope 
  he 
  showed 
  that, 
  if 
  it 
  exists, 
  the 
  angular 
  

   momentum 
  must 
  be 
  small 
  compared 
  with 
  any 
  quantities 
  which 
  

   we 
  can 
  measure. 
  An 
  attempt 
  was 
  made, 
  at 
  the 
  suggestion 
  

   of 
  Professor 
  Rowland, 
  by 
  Mr. 
  Paul 
  McJunkin 
  and 
  the 
  author, 
  

   to 
  determine 
  within 
  what 
  limits 
  it 
  is 
  possible 
  to 
  say 
  that 
  there 
  

   is 
  no 
  frictional 
  or 
  viscous 
  resistance 
  in 
  the 
  asther 
  connected 
  

   with 
  such 
  rotation. 
  

  

  Frictional 
  or 
  Viscous 
  Resistance 
  in 
  the 
  JEther. 
  — 
  The 
  existence 
  

   of 
  permanent 
  magnets 
  shows 
  that 
  any 
  retardation 
  due 
  to 
  any 
  

   kind 
  of 
  resistance 
  must 
  be 
  very 
  slight. 
  In 
  the 
  case 
  of 
  an 
  

   electromagnet, 
  any 
  energy 
  used 
  in 
  overcoming 
  such 
  resistance, 
  

   if 
  it 
  exists, 
  must 
  be 
  derived 
  from 
  the 
  exciting 
  current, 
  and 
  

   the 
  disappearance 
  of 
  such 
  energy 
  will 
  produce 
  an 
  apparent 
  

   resistance 
  added 
  to 
  that 
  of 
  the 
  wire. 
  An 
  attempt 
  was 
  there- 
  

   fore 
  made 
  to 
  determine 
  whether 
  a 
  wire 
  carrying 
  a 
  current 
  

   had 
  the 
  same 
  electrical 
  resistance 
  when 
  producing 
  a 
  magnetic 
  

   field 
  that 
  it 
  had 
  when 
  not 
  producing 
  it. 
  

  

  The 
  experiment 
  consisted 
  in 
  winding 
  two 
  coils 
  of 
  wire 
  

   together 
  on 
  an 
  iron 
  core 
  and 
  determining 
  whether 
  the 
  

   resistance 
  was 
  the 
  same 
  in 
  two 
  cases 
  : 
  — 
  

  

  (1) 
  When 
  the 
  current 
  was 
  so 
  passed 
  through 
  the 
  coils 
  that 
  

   both 
  produced 
  a 
  field 
  in 
  the 
  same 
  direction 
  ; 
  

  

  (2) 
  When 
  the 
  current 
  was 
  so 
  passed 
  that 
  the 
  fields 
  produced 
  

   counterbalanced 
  each 
  other. 
  

  

  The 
  great 
  difficulty 
  in 
  the 
  experiment 
  lay 
  in 
  the 
  necessity 
  

   of 
  measuring 
  the 
  resistance 
  of 
  a 
  coil 
  in 
  which 
  a 
  comparatively 
  

   large 
  current 
  was 
  flowing. 
  In 
  order 
  to 
  overcome 
  the 
  effect 
  

   of 
  changes 
  in 
  resistance 
  due 
  to 
  changes 
  in 
  temperature, 
  two 
  

   coils 
  were 
  wound, 
  as 
  nearly 
  as 
  possible 
  identical, 
  and 
  these 
  

   double 
  coils 
  were 
  used 
  for 
  the 
  four 
  arms 
  of 
  a 
  Wheatstone's 
  

   bridge 
  so 
  that 
  the 
  temperature 
  would 
  rise 
  in 
  all 
  four 
  arms 
  

   equally. 
  Each 
  coil 
  consisted 
  of 
  about 
  2500 
  turns 
  of 
  doubled 
  

   No. 
  30 
  copper 
  wire, 
  the 
  whole 
  inclosed 
  in 
  an 
  iron 
  case, 
  boiled 
  

   in 
  wax 
  for 
  five 
  hours 
  and 
  cooled 
  in 
  a 
  vacuum. 
  The 
  insulation 
  

   resistance 
  was 
  then 
  about 
  eleven 
  megohms. 
  Iron 
  cores 
  w 
  T 
  ere 
  

   used, 
  and 
  it 
  was 
  found 
  that 
  the 
  cases 
  effectually 
  protected 
  the 
  

   * 
  Maxwell, 
  Elect, 
  and 
  Magn., 
  art. 
  575. 
  

  

  