﻿440 
  Mr. 
  A. 
  P. 
  Wills 
  on 
  the 
  Susceptibility 
  of 
  

  

  vertical 
  force 
  F 
  upon 
  the 
  conductor 
  is 
  determined 
  by 
  means 
  

   of 
  the 
  balance. 
  The 
  field 
  is 
  given 
  by 
  the 
  equation 
  

  

  where 
  I 
  is 
  the 
  current 
  in 
  amperes, 
  / 
  is 
  the 
  length 
  of 
  the 
  wire 
  in 
  

   centimetres, 
  F 
  is 
  the 
  force 
  in 
  grains 
  measured 
  by 
  the 
  balance. 
  

  

  A 
  rectangular 
  slab 
  of 
  plaster-of-paris 
  was 
  made 
  with 
  nearly 
  

   the 
  dimensions 
  of 
  the 
  experimental 
  slabs. 
  On 
  three 
  edges 
  of 
  

   this 
  slab 
  (the 
  bottom 
  edge 
  and 
  the 
  two 
  side 
  edges) 
  were 
  

   pasted 
  thin 
  strips 
  of 
  tinfoil 
  just 
  as 
  wide 
  as 
  the 
  slab 
  was 
  thick. 
  

   Now 
  when 
  this 
  slab 
  is 
  clamped 
  in 
  the 
  clamp 
  of 
  the 
  suspension 
  

   apparatus, 
  so 
  that 
  the 
  tinfoil 
  of 
  the 
  side 
  edges 
  is 
  in 
  contact 
  

   with 
  the 
  metallic 
  part 
  of 
  the 
  surfaces/ 
  and 
  /', 
  we 
  shall 
  have 
  

   metallic 
  connexion 
  between 
  W 
  and 
  W 
  / 
  by 
  way 
  of 
  the 
  tin- 
  

   foil 
  along 
  the 
  sides 
  and 
  across 
  the 
  bottom 
  of 
  the 
  slab. 
  When 
  

   a 
  field 
  determination 
  is 
  to 
  be 
  made 
  the 
  slab 
  is 
  adjusted 
  with 
  

   reference 
  to 
  the 
  poles 
  in 
  just 
  the 
  same 
  manner 
  as 
  are 
  the 
  

   experimental 
  slabs. 
  Then 
  the 
  tinfoil 
  strip 
  across 
  the 
  bottom 
  

   of 
  the 
  slab 
  is 
  a 
  conductor 
  in 
  the 
  field 
  we 
  wish 
  to 
  determine. 
  

   Since 
  the 
  corners 
  turned 
  by 
  the 
  tinfoil 
  are 
  very 
  sharp 
  the 
  

   length 
  of 
  this 
  conductor 
  can 
  be 
  determined 
  with 
  great 
  

   accuracy. 
  

  

  For 
  the 
  determination 
  of 
  the 
  current 
  I 
  a 
  Weston 
  millivolt- 
  

   meter 
  was 
  used 
  in 
  connexion 
  with 
  a 
  german-silver 
  shunt. 
  

   The 
  millivoltmeter 
  was 
  carefully 
  calibrated 
  by 
  means 
  of 
  a 
  

   Eayleigh 
  current-weigher 
  constructed 
  by 
  Mr. 
  S. 
  N. 
  Taylor 
  

   in 
  connexion 
  with 
  his 
  work 
  on 
  Clark 
  and 
  Weston 
  cells. 
  

  

  Method 
  of 
  Experimentation. 
  

  

  The 
  substance 
  for 
  a 
  series 
  of 
  experiments 
  having 
  been 
  

   selected, 
  the 
  field 
  magnets 
  were 
  excited 
  with 
  the 
  proper 
  

   current 
  regulated 
  by 
  a 
  suitable 
  resistance. 
  An 
  assistant 
  kept 
  

   the 
  needle 
  of 
  an 
  ammeter 
  in 
  circuit 
  with 
  the 
  magnet 
  coils 
  

   always 
  over 
  a 
  selected 
  line 
  of 
  the 
  scale. 
  The 
  plaster-of-paris 
  

   slab 
  described 
  above 
  was 
  then 
  introduced 
  into 
  the 
  proper 
  

   position. 
  Equilibrium 
  was 
  obtained 
  when 
  no 
  current 
  passed 
  

   through 
  the 
  conductor 
  in 
  the 
  field 
  and 
  a 
  reading 
  of 
  the 
  balance 
  

   taken. 
  A 
  current 
  measured 
  by 
  the 
  Weston 
  millivoltmeter 
  

   was 
  then 
  sent 
  through 
  the 
  conductor 
  and 
  a 
  new 
  reading- 
  

   taken 
  when 
  equilibrium 
  had 
  been 
  again 
  secured. 
  The 
  differ- 
  

   ence 
  in 
  these 
  two 
  readings 
  gave 
  the 
  force 
  in 
  grams 
  F, 
  due 
  to 
  

   the 
  field, 
  upon 
  the 
  current 
  in 
  the 
  conductor. 
  Then 
  the 
  

   plaster 
  slab 
  was 
  removed 
  and 
  a 
  slab 
  of 
  one 
  of 
  the 
  substances 
  

   to 
  be 
  investigated 
  introduced, 
  and 
  a 
  reading 
  of 
  the 
  balance 
  

  

  