﻿Diamagnetic 
  and 
  Weakly 
  Magnetic 
  Substances. 
  437 
  

  

  In 
  fig. 
  4 
  is 
  shown 
  the 
  suspension 
  arrangement 
  used 
  in 
  

   holding 
  the 
  slah. 
  P 
  is 
  the 
  suspension-wire 
  from 
  the 
  balance 
  

   beam 
  passing 
  through 
  the 
  vulcanite 
  cross-arm 
  c. 
  W 
  and 
  W 
  

   are 
  rigid 
  brass 
  wires 
  passing 
  easily 
  through 
  slots 
  in 
  c, 
  and 
  

   supported 
  by 
  means 
  of 
  nuts 
  d 
  and 
  d\ 
  the 
  upper 
  ends 
  of 
  W 
  

   and 
  W 
  being 
  threaded 
  ; 
  b 
  and 
  b' 
  are 
  small 
  vulcanite 
  blocks 
  

   to 
  which 
  the 
  rigid 
  wires 
  W 
  and 
  V\ 
  r/ 
  are 
  firmly 
  attached. 
  A 
  

   part 
  of 
  the 
  surface 
  / 
  of 
  block 
  b 
  is 
  made 
  conducting 
  and 
  in 
  

   metallic 
  connexion 
  with 
  W 
  ; 
  likewise 
  a 
  conducting 
  part 
  of 
  

   surface 
  f 
  is 
  in 
  metallic 
  connexion 
  with 
  W. 
  For 
  holding 
  

   the 
  slab 
  S, 
  a 
  clamp 
  operated 
  by 
  the 
  screw 
  s 
  is 
  used 
  ; 
  for 
  

   making 
  the 
  necessary 
  adjustments 
  in 
  the 
  positions 
  of 
  S, 
  the 
  

   nuts 
  e, 
  d, 
  d 
  f 
  serve. 
  The 
  reason 
  for 
  making 
  the 
  surfaces 
  / 
  

   and/' 
  partly 
  conducting 
  and 
  in 
  metallic 
  connexion 
  with 
  W 
  

   and 
  W 
  7 
  respectively 
  will 
  be 
  made 
  clear 
  when 
  the 
  method 
  

   used 
  in 
  making 
  the 
  field 
  determination 
  is 
  described. 
  In 
  

   circuit 
  with 
  the 
  magnet 
  coils 
  of 
  A 
  there 
  is 
  an 
  ammeter 
  which, 
  

   with 
  a 
  variable 
  resistance, 
  serves 
  to 
  keep 
  the 
  current 
  con- 
  

   stant 
  during 
  an 
  experiment. 
  In 
  the 
  same 
  circuit 
  there 
  is 
  

   also 
  a 
  commutator 
  which 
  serves 
  to 
  change 
  readily 
  the 
  direction 
  

   of 
  the 
  current 
  through 
  the 
  magnet 
  coils. 
  

  

  A 
  box, 
  opened 
  in 
  front 
  by 
  means 
  of 
  a 
  door, 
  serves 
  to 
  pro- 
  

   tect 
  the 
  suspension 
  apparatus 
  from 
  draughts. 
  

  

  An 
  idea 
  of 
  the 
  general 
  distribution 
  of 
  the 
  field 
  is 
  given 
  in 
  

   fig. 
  3. 
  

  

  The 
  slabs 
  used 
  in 
  the 
  experiments 
  were 
  rectangular 
  in 
  

   shape 
  and 
  of 
  approximate 
  dimensions 
  8 
  centim. 
  by 
  4*5 
  centim. 
  

   by 
  0*5 
  centim. 
  When 
  in 
  position 
  for 
  experimenting 
  the 
  long- 
  

   edges 
  of 
  the 
  slab 
  were 
  vertical. 
  Figs. 
  1, 
  2, 
  and 
  3 
  show 
  more 
  

   clearly 
  than 
  could 
  any 
  description 
  how 
  the 
  slabs 
  were 
  sus- 
  

   pended. 
  

  

  Theory 
  of 
  the 
  Method. 
  

  

  Considering 
  magnetic 
  forces 
  due 
  to 
  stresses 
  in 
  the 
  media, 
  

   it 
  can 
  then 
  be 
  shown* 
  that 
  the 
  following 
  equations 
  must 
  in 
  

   general 
  be 
  satisfied 
  for 
  isotropic 
  media 
  : 
  — 
  

  

  X 
  = 
  ^{2£L-33H}, 
  

  

  07T 
  

  

  Z,= 
  1-{29?N-$H}, 
  

   * 
  See, 
  for 
  instance, 
  Webster, 
  < 
  Electricity 
  and 
  Magnetism/ 
  p 
  387. 
  

  

  