﻿366 
  Mr. 
  N. 
  E. 
  Gilbert 
  : 
  Experiments 
  upon 
  the 
  

  

  wheel 
  was 
  rotated. 
  Connexion 
  was 
  made 
  to 
  a 
  delicate 
  

   galvanometer 
  by 
  bringing 
  out 
  the 
  ends 
  of 
  the 
  coil 
  of 
  wire 
  at 
  

   the 
  centre 
  of 
  the 
  axis, 
  one 
  on 
  either 
  side 
  of 
  the 
  wheel. 
  

  

  Several 
  lines 
  of 
  thought 
  might 
  lead 
  us 
  to 
  expect 
  to 
  find 
  a 
  

   current 
  in 
  such 
  a 
  circuit 
  when 
  the 
  wheel 
  was 
  rotated. 
  If 
  we 
  

   consider 
  an 
  electric 
  current 
  to 
  be 
  a 
  continuous 
  giving 
  wav 
  of 
  

   the 
  aether 
  under 
  the 
  action 
  of 
  the 
  electric 
  stresses, 
  which, 
  in 
  a 
  

   dielectric, 
  give 
  rise 
  to 
  electric 
  displacement, 
  then 
  we 
  may 
  

   think 
  of 
  a 
  moving 
  stream 
  of 
  sether 
  as 
  constituting 
  an 
  electric 
  

   current, 
  and 
  we 
  might 
  expect 
  that 
  a 
  wire, 
  moving 
  relative 
  to 
  

   the 
  aether, 
  would 
  have 
  a 
  current 
  generated 
  in 
  it. 
  Moreover, 
  

   we 
  are 
  familiar 
  with 
  several 
  phenomena 
  in 
  which 
  the 
  behaviour 
  

   of 
  positive 
  electricity 
  is 
  different 
  from 
  that 
  of 
  negative, 
  such 
  

   as 
  the 
  discharge 
  of 
  negative, 
  but 
  not 
  of 
  positive, 
  by 
  ultra- 
  

   violet 
  light, 
  and 
  the 
  complete 
  dissimilarity 
  between 
  the 
  phe- 
  

   nomena 
  at 
  the 
  electrodes 
  of 
  a 
  Crookes' 
  tube 
  when 
  a 
  discharge 
  

   is 
  passing 
  through 
  the 
  tube. 
  In 
  practically 
  all 
  of 
  these 
  cases 
  

   the 
  positive 
  electricity 
  has 
  been 
  shown 
  to 
  be 
  more 
  sluggish 
  

   in 
  its 
  action 
  than 
  the 
  negative. 
  J. 
  J. 
  Thomson 
  * 
  has 
  

   imagined 
  that 
  ordinary 
  metallic 
  conduction 
  may 
  be 
  only 
  a 
  

   kind 
  of 
  electrolytic 
  action, 
  in 
  which 
  case 
  we 
  might 
  expect 
  

   the 
  positive 
  electricity 
  to 
  move 
  more 
  slowly 
  or 
  to 
  lag 
  behind 
  

   the 
  negative 
  in 
  a 
  moving 
  conductor. 
  Such 
  a 
  lag 
  would 
  

   constitute 
  the 
  current 
  for 
  which 
  we 
  are 
  looking. 
  

  

  But 
  the 
  consideration 
  which, 
  above 
  all 
  others, 
  has 
  led 
  us 
  

   to 
  look 
  for 
  such 
  a 
  current 
  is 
  the 
  fact 
  that 
  it 
  would 
  give 
  us 
  at 
  

   once 
  a 
  simple 
  explanation 
  of 
  the 
  cause 
  of 
  the 
  earth's 
  mag- 
  

   netism. 
  If 
  we 
  may 
  assume 
  that 
  a 
  moving 
  body 
  has 
  generated 
  

   in 
  it, 
  by 
  virtue 
  of 
  its 
  motion, 
  an 
  electromotive 
  force, 
  then 
  a 
  

   rotating 
  body 
  like 
  the 
  earth, 
  which 
  presents 
  a 
  closed 
  circuit, 
  

   will 
  contain 
  a 
  current 
  capable 
  of 
  producing 
  a 
  magnetic 
  field 
  

   identical 
  with 
  that 
  which 
  a 
  constant 
  current, 
  produced 
  by 
  an 
  

   E.M.F. 
  equal 
  to 
  ^ 
  E.M.F. 
  around 
  the 
  circuit, 
  would 
  produce 
  

   in 
  a 
  stationary 
  earth. 
  Dr. 
  Schmidt 
  |, 
  by 
  an 
  extension 
  of 
  

   Gauss' 
  harmonic 
  analysis, 
  has 
  recently 
  arrived 
  at 
  the 
  conclusion 
  

   that 
  97*5 
  p. 
  cent, 
  of 
  the 
  terrestrial 
  magnetism 
  is 
  due 
  to 
  causes 
  

   within 
  the 
  earth, 
  while 
  Schuster 
  J 
  believes 
  that 
  not 
  more 
  

   than 
  5 
  p. 
  cent, 
  can 
  possibly 
  be 
  attributed 
  to 
  outside 
  causes. 
  This 
  

   fraction 
  is 
  so 
  small 
  that 
  in 
  the 
  present 
  investigation 
  it 
  may 
  

   be 
  neglected 
  entirely. 
  

  

  The 
  earliest 
  explanations 
  of 
  this 
  phenomenon, 
  which 
  depend 
  

   upon 
  the 
  existence 
  of 
  permanent 
  magnetism 
  within 
  the 
  earth, 
  

  

  * 
  Recent 
  Researches, 
  Par. 
  34. 
  Hajiports 
  presenUs 
  an 
  Conyvcs 
  Inter- 
  

   national 
  de 
  Physique 
  (Paris, 
  1900), 
  p. 
  138. 
  

   t 
  Ciel 
  et 
  Terre, 
  Dec. 
  16, 
  1900. 
  

   % 
  B. 
  A. 
  Report, 
  1898, 
  p. 
  745. 
  

  

  