﻿134 
  Mftlwd 
  of 
  Determining 
  the 
  Sensibility 
  of 
  a 
  Balance, 
  

  

  mirror. 
  The 
  double-suspension 
  mirror 
  is 
  fully 
  described 
  in 
  

   the 
  Phil. 
  Trans. 
  A, 
  1891, 
  p. 
  572. 
  It 
  is 
  of 
  course 
  not 
  

   essential 
  to 
  the 
  method, 
  but 
  was 
  chosen 
  because 
  of 
  the 
  great 
  

   magnification 
  of 
  the 
  deflexion 
  which 
  it 
  gives. 
  

  

  Let 
  us 
  suppose 
  that 
  the 
  value 
  of 
  the 
  scale-divisions 
  of 
  the 
  

   deflexion 
  is 
  to 
  be 
  determined 
  by 
  a 
  movement 
  of 
  the 
  rider 
  

   from 
  right 
  to 
  left. 
  The 
  two 
  micrometer-screws 
  are 
  with- 
  

   drawn 
  so 
  that 
  neither 
  is 
  in 
  contact 
  with 
  the 
  rider, 
  that 
  on 
  the 
  

   left 
  so 
  far 
  that 
  the 
  rider 
  will 
  not 
  touch 
  it 
  in 
  its 
  subsequent 
  

   travel. 
  The 
  beam 
  is 
  lowered 
  and 
  allowed 
  to 
  swdng. 
  Then 
  

   the 
  right-hand 
  screw 
  is 
  advanced 
  till 
  it 
  bears 
  against 
  the 
  end 
  

   of 
  the 
  rider 
  and 
  pushes 
  it 
  some 
  small 
  distance. 
  The 
  contact 
  

   is 
  seen 
  to 
  have 
  occurred 
  by 
  the 
  interference 
  with 
  freedom 
  of 
  

   swing, 
  as 
  watched 
  in 
  the 
  telescope. 
  The 
  micrometer 
  is 
  then 
  

   read. 
  Let 
  its 
  reading 
  be 
  Wi. 
  It 
  is 
  then 
  withdrawn 
  a 
  little 
  

   so 
  as 
  to 
  leave 
  the 
  rider 
  free, 
  and 
  the 
  centre 
  of 
  swing 
  Ci 
  is 
  

   determined 
  in 
  the 
  usual 
  way 
  from 
  three 
  successive 
  turning 
  

   points. 
  Then 
  the 
  micrometer 
  is 
  advanced 
  again 
  so 
  as 
  to 
  

   push 
  the 
  rod 
  a 
  little 
  further, 
  and 
  its 
  reading 
  m2 
  is 
  taken. 
  

   It 
  is 
  then 
  withdrawn 
  and 
  the 
  new 
  centre 
  of 
  swing 
  Cg 
  is 
  taken. 
  

   If 
  mi 
  — 
  m2 
  = 
  d, 
  Ci 
  — 
  C2 
  divisions 
  deflexion 
  are 
  due 
  to 
  an 
  

   addition 
  of 
  ^d/h 
  to 
  the 
  left 
  pan. 
  

  

  The 
  right-hand 
  micrometer 
  may 
  then 
  be 
  withdrawn 
  and 
  

   the 
  left-hand 
  micrometer 
  may 
  be 
  brought 
  into 
  action 
  in 
  a 
  

   similar 
  manner, 
  and 
  so 
  on, 
  the 
  two 
  screws 
  being 
  used 
  

   alternately. 
  

  

  The 
  balance-case 
  was 
  fixed 
  on 
  a 
  shelf 
  and 
  was 
  enclosed 
  

   in 
  a 
  tin-foiled 
  wood 
  box 
  with 
  wool 
  loosely 
  packed 
  between 
  

   box 
  and 
  case. 
  The 
  case 
  and 
  box 
  were 
  provided 
  with 
  plate- 
  

   glass 
  windows 
  to 
  view 
  the 
  mirror 
  and 
  the 
  micrometer 
  

   divisions. 
  The 
  following 
  abstract 
  of 
  some 
  determinations 
  

   of 
  sensibility 
  will 
  serve 
  to 
  show 
  what 
  accuracy 
  may 
  be 
  

   attained 
  : 
  — 
  

  

  Rider 
  German 
  silver 
  wire, 
  7*35 
  mgm. 
  

  

  Half 
  length 
  of 
  beam, 
  20-272 
  cm. 
  

   10 
  determinations 
  alternately 
  left 
  and 
  right. 
  

  

  Mean 
  travel 
  of 
  rider 
  ,.... 
  2*4850 
  mm. 
  

  

  Mean 
  deflexion 
  21*26 
  divisions. 
  

  

  Mean 
  value 
  for 
  20 
  divisions 
  ... 
  0*0848 
  mgm. 
  

   The 
  separate 
  determinations 
  range 
  between 
  

   20 
  divisions=0*0877 
  

   and 
  20 
  „ 
  =0*0824. 
  

  

  