﻿Dr. 
  C. 
  Chree's 
  Notes 
  on 
  Thermometry, 
  307 
  

  

  careful 
  calibrations 
  by 
  Dr. 
  Thorpe 
  and 
  Prof. 
  Riicker 
  *, 
  Prof. 
  

   Rowland, 
  Mr. 
  Waldo 
  f, 
  and 
  others, 
  the 
  accuracy 
  actually 
  

   attained 
  is 
  usually 
  considerably 
  higher 
  than 
  this. 
  What 
  can 
  

   be 
  done 
  depends 
  very 
  much 
  on 
  the 
  natural 
  uniformity 
  of 
  the 
  

   bore. 
  If 
  the 
  preliminary 
  calibration 
  shows 
  the 
  tube 
  destined 
  

   for 
  a 
  Kew 
  standard 
  to 
  be 
  unduly 
  conical 
  or 
  erratic 
  in 
  bore 
  it 
  

   is 
  laid 
  aside. 
  When 
  calibration-tables 
  are 
  constructed, 
  a 
  tube 
  

   of 
  this 
  kind 
  may 
  seem 
  to 
  be 
  as 
  good 
  as 
  another 
  ; 
  but 
  I, 
  for 
  

   one, 
  would 
  be 
  sorry 
  to 
  pin 
  my 
  faith 
  to 
  it 
  even 
  when 
  calibrated 
  

   with 
  threads 
  of 
  1° 
  leno-th. 
  

  

  Pressure 
  Coefficients. 
  

  

  § 
  26. 
  The 
  application 
  of 
  increased 
  pressure 
  to 
  the 
  outside 
  

   of 
  a 
  thermometer, 
  whether 
  through 
  rise 
  of 
  barometric 
  pressure 
  

   or 
  immersion 
  in 
  a 
  liquid, 
  compresses 
  the 
  glass 
  and 
  reduces 
  its 
  

   internal 
  volume. 
  This 
  makes 
  the 
  mercury 
  rise 
  in 
  the 
  stem, 
  

   and, 
  supposing 
  the 
  increase 
  in 
  pressure 
  uniform 
  over 
  the 
  

   outer 
  glass 
  surface, 
  the 
  rise 
  bears 
  to 
  the 
  increase 
  in 
  pressure 
  

   a 
  constant 
  ratio, 
  known 
  as 
  the 
  u 
  external 
  pressure 
  coefficient." 
  

   The 
  unit 
  of 
  pressure 
  usually 
  employed 
  is 
  that 
  of 
  a 
  millimetre 
  

   column 
  of 
  mercury 
  at 
  0° 
  C, 
  the 
  rise 
  in 
  the 
  stem 
  being 
  measured 
  

   in 
  degrees. 
  Knowing 
  the 
  external 
  pressure 
  coefficient, 
  one 
  

   can 
  calculate 
  the 
  correction 
  necessary 
  to 
  reduce 
  the 
  readings 
  

   taken 
  with 
  the 
  thermometer 
  under 
  any 
  known 
  external 
  pressure 
  

   to 
  what 
  they 
  would 
  have 
  been 
  under 
  the 
  standard 
  pressure. 
  

  

  Again, 
  when 
  the 
  horizontal 
  position 
  is 
  adopted 
  as 
  the 
  standard 
  

   one 
  — 
  as 
  is 
  done 
  at 
  the 
  Bureau 
  International 
  and 
  the 
  Reichs- 
  

   anstalt 
  — 
  a 
  correction 
  is 
  necessary 
  when 
  the 
  thermometer 
  is 
  

   read 
  in 
  the 
  vertical 
  position, 
  to 
  allow 
  for 
  the 
  influence 
  of 
  the 
  

   increased 
  internal 
  pressure. 
  This 
  pressure 
  expands 
  the 
  bulb 
  

   and 
  compresses 
  its 
  contents, 
  both 
  effects 
  contributing 
  to 
  lower 
  

   the 
  reading. 
  If 
  we 
  treat 
  the 
  internal 
  pressure 
  as 
  uniformly 
  

   distributed, 
  and 
  proportional 
  to 
  the 
  length 
  of 
  the 
  mercury 
  

   column 
  measured 
  from 
  the 
  centre 
  of 
  the 
  bulb, 
  we 
  can, 
  by 
  

   observing 
  the 
  difference 
  in 
  the 
  readings 
  of 
  a 
  thermometer 
  

   when 
  vertical 
  and 
  when 
  horizontal 
  at 
  any 
  one 
  temperature, 
  

   calculate 
  an 
  " 
  internal 
  pressure 
  coefficient." 
  This 
  may 
  con- 
  

   veniently 
  be 
  the 
  ratio 
  of 
  the 
  observed 
  rise 
  of 
  reading 
  in 
  

   degrees 
  — 
  when 
  the 
  thermometer 
  is 
  transferred 
  from 
  the 
  ver- 
  

   tical 
  to 
  the 
  horizontal 
  position 
  — 
  to 
  the 
  length 
  of 
  the 
  mercury 
  

   column 
  measured 
  in 
  millimetres. 
  Knowing 
  this 
  internal 
  

   pressure 
  coefficient, 
  one 
  can 
  calculate 
  a 
  table 
  giving 
  the 
  cor- 
  

   rection 
  for 
  internal 
  pressure 
  to 
  be 
  applied 
  to 
  an}' 
  reading- 
  

   taken 
  with 
  the 
  thermometer 
  vertical. 
  

  

  * 
  B. 
  A. 
  Report, 
  1881 
  , 
  p. 
  541. 
  

  

  t 
  American 
  Journal 
  of 
  Science, 
  vol. 
  xxi. 
  1881. 
  

  

  