﻿Diamagnetic 
  and 
  Weakly 
  Magnetic 
  Substances. 
  441 
  

  

  taken 
  when 
  equilibrium 
  obtained. 
  The 
  slab 
  was 
  then 
  reversed, 
  

   top 
  for 
  bottom, 
  and 
  another 
  reading 
  obtained. 
  This 
  process 
  

   was 
  then 
  gone 
  through 
  with 
  all 
  the 
  five 
  or 
  six 
  substances 
  

   selected 
  for 
  a 
  particular 
  series 
  of 
  experiments, 
  another 
  deter- 
  

   mination 
  of 
  the 
  field 
  being 
  made 
  in 
  the 
  middle 
  of 
  the 
  series 
  

   and 
  still 
  another 
  at 
  the 
  end. 
  

  

  After 
  the 
  last 
  field 
  determination, 
  and 
  while 
  the 
  field 
  

   magnets 
  were 
  still 
  excited, 
  a 
  reading 
  of 
  the 
  balance 
  was 
  

   obtained 
  with 
  no 
  slab 
  in 
  the 
  clamp 
  of 
  the 
  suspension 
  apparatus. 
  

   This 
  reading 
  was 
  to 
  enable 
  us 
  to 
  make 
  correction 
  for 
  anv 
  

   magnetic 
  action 
  upon 
  the 
  suspension 
  apparatus 
  itself. 
  The 
  

   current 
  through 
  the 
  magnet 
  coils 
  was 
  then 
  cut 
  off. 
  

  

  The 
  wires 
  w 
  and 
  w' 
  were 
  hung 
  in 
  long 
  loops 
  from 
  fixed 
  

   supports 
  in 
  the 
  balance-case 
  to 
  a 
  little 
  rubber 
  support 
  firmly 
  

   attached 
  to 
  the 
  suspension 
  rod 
  P. 
  From 
  this, 
  being 
  well 
  

   insulated, 
  they 
  were 
  twined 
  about 
  the 
  rod 
  P 
  and 
  connected 
  

   with 
  W 
  and 
  W 
  7 
  at 
  the 
  nuts 
  d 
  and 
  d' 
  . 
  as 
  shown 
  in 
  fig. 
  4. 
  

   These 
  wires 
  would 
  vary 
  slightly 
  in 
  length 
  when 
  a 
  current 
  

   passed 
  and 
  correction 
  had 
  to 
  be 
  made 
  for 
  this. 
  Both 
  wires 
  

   w 
  and 
  w' 
  were 
  connected 
  with 
  one 
  nut 
  (d, 
  fig. 
  4) 
  and 
  the 
  

   same 
  current 
  used 
  in 
  making 
  the 
  field 
  determination 
  was 
  thus 
  

   sent 
  over 
  the 
  flexible 
  wires 
  in 
  the 
  balance-case, 
  without 
  going 
  

   near 
  the 
  field 
  magnets. 
  A 
  reading 
  of 
  the 
  balance 
  was 
  taken 
  

   when 
  the 
  current 
  had 
  been 
  properly 
  adjusted. 
  

  

  The 
  residual 
  field 
  was 
  then 
  eliminated 
  with 
  great 
  care. 
  

   This 
  was 
  done 
  by 
  reversing 
  over 
  and 
  over 
  again 
  a 
  small 
  

   current 
  through 
  the 
  magnet 
  coils 
  until 
  the 
  balance 
  gave 
  no 
  

   indication 
  of 
  a 
  field 
  when 
  a 
  current 
  was 
  sent 
  through 
  the 
  

   conductor 
  arranged 
  as 
  when 
  the 
  field 
  was 
  determined. 
  

  

  The 
  slabs 
  were 
  then, 
  each 
  in 
  turn, 
  clamped 
  in 
  the 
  clamp 
  of 
  

   the 
  suspension 
  apparatus 
  and 
  readings 
  of 
  the 
  balance 
  with 
  

   the 
  zero 
  field 
  taken. 
  These 
  readings, 
  with 
  those 
  taken 
  with 
  

   the 
  field 
  on 
  and 
  with 
  the 
  proper 
  corrections 
  mentioned 
  above, 
  

   are 
  sufficient 
  to 
  determine 
  the 
  mechanical 
  force 
  in 
  the 
  Z 
  

   direction. 
  This 
  force 
  we 
  have, 
  in 
  the 
  theory 
  of 
  the 
  method, 
  

   called 
  P. 
  

  

  A 
  scale 
  graduated 
  to 
  read 
  fifths 
  of 
  a 
  millimetre 
  was 
  used 
  

   to 
  determine 
  the 
  width 
  of 
  the 
  slab, 
  and 
  a 
  Brown 
  and 
  Sharpe 
  

   micrometer 
  screw-gauge 
  graduated 
  to 
  read 
  thousandths 
  of 
  a 
  

   centimetre 
  was 
  used 
  to 
  determine 
  the 
  thickness 
  of 
  the 
  slab. 
  

   The 
  product 
  of 
  this 
  width 
  and 
  thickness 
  gave 
  the 
  area 
  A. 
  In 
  

   making 
  these 
  width 
  and 
  thickness 
  measurements, 
  many 
  

   measurements 
  were 
  taken 
  on 
  a 
  given 
  slab 
  and 
  the 
  mean 
  of 
  

   these 
  used 
  in 
  the 
  calculations. 
  

  

  