﻿Determin'nuj 
  the 
  Sensihility 
  of 
  a 
  Balance. 
  

  

  m 
  

  

  we 
  shall 
  call 
  it 
  " 
  the 
  rider." 
  Its 
  weight 
  is 
  determined 
  before 
  

   use 
  as 
  accurately 
  as 
  possible 
  by 
  weighing 
  on 
  an 
  assay 
  balance. 
  

   The 
  sensibihty 
  is 
  determined 
  by 
  moving 
  the 
  rider 
  either 
  to 
  

   right 
  or 
  left 
  a 
  measured 
  distance. 
  If 
  this 
  distance 
  is 
  d^ 
  

   if 
  the 
  half 
  length 
  of 
  beam 
  is 
  h, 
  and 
  if 
  the 
  weight 
  of 
  the 
  rider 
  

   is 
  R, 
  the 
  movement 
  is 
  equivalent 
  to 
  an 
  addition 
  of 
  weight 
  to 
  

   one 
  pan, 
  ^djh. 
  

  

  In 
  order 
  to 
  move 
  the 
  rider 
  a 
  definite 
  distance 
  a 
  stout 
  hori- 
  

   zontal 
  rod 
  (fig. 
  2) 
  passes 
  through 
  the 
  balance-case 
  from 
  side 
  

   to 
  side 
  without 
  contact 
  with 
  the 
  case, 
  and 
  is 
  supported 
  at 
  its 
  

   ends 
  outside, 
  and 
  independent 
  of, 
  the 
  case. 
  It 
  is 
  parallel 
  to 
  

   the 
  beam 
  and 
  a 
  little 
  lower 
  than 
  the 
  V 
  frame. 
  On 
  the 
  rod 
  

   are 
  fixed 
  horizontally 
  two 
  Brown 
  & 
  Sharp 
  micrometer-screws 
  

   divided 
  to 
  '01 
  mm. 
  and 
  allowino: 
  an 
  estimate 
  of 
  *001 
  mm. 
  

  

  Fig. 
  2. 
  

  

  Arrangement 
  of 
  right-hand 
  micrometer-screw. 
  

  

  Their 
  axes 
  are 
  in 
  one 
  line 
  coinciding 
  with 
  the 
  axis 
  of 
  the 
  

   rider, 
  and 
  they 
  are 
  fixed 
  so 
  that 
  one 
  can 
  bear 
  against 
  one 
  end 
  

   and 
  the 
  other 
  against 
  the 
  other 
  end 
  of 
  the 
  rider. 
  Their 
  ends 
  

   are 
  plane 
  and 
  the 
  ends 
  of 
  the 
  rider 
  are 
  bluntly 
  pointed. 
  

   Each 
  micrometer 
  screw-head 
  has 
  a 
  cross 
  piece 
  fixed 
  on 
  it, 
  

   and 
  a 
  fork 
  which 
  can 
  be 
  rotated 
  about 
  an 
  axis 
  in 
  the 
  con- 
  

   tinuation 
  of 
  the 
  axis 
  of 
  the 
  screw 
  by 
  a 
  pulley 
  outside 
  the 
  

   case, 
  can 
  engage 
  with 
  the 
  cross 
  piece, 
  and 
  so 
  advance 
  or 
  

   withdraw 
  the 
  screw. 
  The 
  pulley 
  is 
  worked 
  by 
  an 
  endless 
  

   string 
  passing 
  to 
  a 
  pulley 
  at 
  the 
  side 
  of 
  the 
  observer, 
  who 
  is 
  

   about 
  2 
  metres 
  in 
  tront 
  of 
  the 
  balance. 
  The 
  micrometer 
  

   divisions 
  are 
  illuminated 
  and 
  each 
  micrometer 
  is 
  viewed 
  by 
  

   its 
  own 
  telescope. 
  The 
  position 
  of 
  the 
  balance-beam 
  is 
  read 
  

   by 
  a 
  double-suspension 
  mirror, 
  telescope 
  and 
  scale. 
  The 
  scale 
  

   is 
  divided 
  to 
  millimetres 
  and 
  is 
  about 
  3 
  metres 
  from 
  the 
  

  

  