﻿434 
  Mr. 
  A. 
  P. 
  Wills 
  on 
  the 
  Susceptibility 
  of 
  

  

  netic 
  phenomena 
  lies 
  in 
  making 
  the 
  inevitably 
  small 
  action 
  as 
  

   large 
  as 
  possible. 
  

  

  In 
  the 
  autumn 
  of 
  1895*, 
  acting 
  upon 
  the 
  suggestion 
  of 
  

   Prof. 
  A. 
  Gr. 
  Webster, 
  I 
  attempted 
  to 
  determine 
  the 
  suscepti- 
  

   bility 
  of 
  substances 
  by 
  suspending 
  an 
  ellipsoid 
  of 
  the 
  substance 
  

   in 
  a 
  homogeneous 
  magnetic 
  field, 
  and 
  observing 
  the 
  times 
  of 
  

   oscillation 
  with 
  a 
  known 
  field 
  and 
  with 
  no 
  field. 
  I 
  failed 
  in 
  

   this 
  object 
  owing 
  to 
  the 
  fact 
  that 
  I 
  was 
  unable 
  to 
  obtain 
  a 
  field 
  

   which 
  was 
  uniform 
  to 
  a 
  sufficient 
  degree 
  of 
  approximation, 
  

   the 
  effect 
  which 
  I 
  wished 
  to 
  observe 
  depending 
  upon 
  the 
  

   square 
  of 
  the 
  extremely 
  small 
  quantity 
  tc, 
  while 
  the 
  effect 
  

   depending 
  upon 
  the 
  non-uniformity 
  of 
  the 
  field 
  varies 
  with 
  

   the 
  first 
  power 
  of 
  k. 
  

  

  The 
  purpose 
  of 
  the 
  present 
  paper 
  is 
  to 
  describe 
  a 
  method 
  

   for 
  the 
  determination 
  of 
  k 
  for 
  bodies 
  in 
  which 
  it 
  is 
  extremely 
  

   small 
  and 
  either 
  positive 
  or 
  negative, 
  and 
  to 
  give 
  the 
  actual 
  

   results 
  obtained 
  from 
  experiments 
  upon 
  a 
  large 
  number 
  of 
  

   substances. 
  

  

  Description 
  of 
  the 
  Apparatus. 
  

  

  It 
  has 
  long 
  been 
  known 
  that 
  if 
  any 
  substance 
  be 
  introduced 
  

   into 
  an 
  inhomogeneous 
  magnetic 
  field 
  it 
  experiences 
  a 
  

   mechanical 
  force 
  which 
  tends 
  to 
  make 
  it 
  enter 
  a 
  stronger 
  or 
  

   weaker 
  part 
  of 
  the 
  field, 
  according 
  as 
  the 
  substance 
  is 
  mag- 
  

   netic 
  or 
  diamagnetic. 
  To 
  determine 
  this 
  force 
  the 
  amount 
  

   and 
  distribution 
  of 
  the 
  field 
  in 
  which 
  the 
  substance 
  is 
  placed 
  

   must 
  be 
  known. 
  

  

  I 
  shall 
  describe 
  the 
  apparatus 
  used 
  in 
  the 
  present 
  experi- 
  

   ments, 
  and 
  in 
  the 
  theory 
  of 
  the 
  method 
  shall 
  show 
  that 
  the 
  

   apparatus 
  as 
  here 
  described 
  is 
  adequate 
  to 
  allow 
  for 
  certain 
  

   simplifications 
  in 
  the 
  general 
  theory, 
  giving 
  the 
  force 
  acting 
  

   on 
  a 
  body 
  placed 
  in 
  an 
  inhomogeneous 
  magnetic 
  field. 
  As 
  a 
  

   result 
  of 
  these 
  simplifications 
  we 
  get 
  an 
  expression 
  for 
  the 
  

   mechanical 
  force 
  upon 
  the 
  body 
  in 
  a 
  convenient 
  direction, 
  

   which 
  involves 
  the 
  field-strength 
  at 
  the 
  two 
  limiting 
  surfaces 
  

   only, 
  in 
  that 
  direction. 
  A 
  suitable 
  form 
  for 
  the 
  experimental 
  

   bod} 
  7 
  is 
  that 
  of 
  a 
  thin 
  rectangular 
  slab. 
  Fig. 
  1 
  will 
  give 
  a 
  

   general 
  idea 
  of 
  the 
  arrangement 
  of 
  the 
  apparatus. 
  

  

  * 
  In 
  connexion 
  with 
  this 
  work 
  I 
  noticed 
  a 
  very 
  remarkable 
  behaviour 
  

   of 
  non-conductors 
  in 
  a 
  magnetic 
  field, 
  which 
  led 
  me 
  to 
  suspect 
  an 
  action 
  

   akin 
  to 
  hysteresis 
  in 
  iron. 
  As 
  my 
  direct 
  object 
  at 
  that 
  time 
  was 
  to 
  

   establish 
  a 
  method 
  for 
  tlie 
  determination 
  of 
  coefficients 
  of 
  susceptibility 
  

   I 
  did 
  not 
  push 
  my 
  experiments 
  far 
  enough 
  to 
  warrant 
  my 
  publishing 
  

   results. 
  Mr. 
  Duane 
  has 
  published 
  (Wied. 
  Ann. 
  Bd. 
  lviii. 
  p. 
  517, 
  1896) 
  

   an 
  account 
  of 
  similar 
  experiments 
  performed 
  by 
  him 
  in 
  the 
  physical 
  

   laboratory 
  at 
  Berlin, 
  which 
  seemed 
  to 
  indicate 
  hysteresis 
  in 
  non-con- 
  

   ductors, 
  but 
  in 
  which 
  later 
  research 
  showed 
  the 
  effect 
  to 
  be 
  due 
  to 
  traces 
  

   of 
  iron 
  (Wied. 
  Ann. 
  lxi. 
  p. 
  436, 
  1897). 
  

  

  