﻿Method 
  of 
  Graduating 
  a 
  Thermometer. 
  233 
  

  

  the 
  case 
  of 
  a 
  perfect 
  gas 
  ; 
  and 
  this 
  approximate 
  value 
  for 
  / 
  

   may 
  be 
  used 
  in 
  the 
  term 
  involving 
  the 
  Joule 
  -Thomson 
  effect 
  

   on 
  the 
  right-hand 
  side 
  of 
  the 
  equation. 
  We 
  then 
  get 
  

  

  pv 
  = 
  Ct 
  ^ 
  h 
  pB. 
  

  

  This 
  equation 
  represents 
  our 
  second 
  approximation 
  to 
  the 
  

   correct 
  formula. 
  We 
  may 
  also 
  write 
  it 
  

  

  p(v-B) 
  = 
  Ct-^p. 
  

  

  1 
  zv 
  

  

  Imagine 
  now 
  that 
  we 
  keep 
  v 
  constant, 
  and 
  use 
  the 
  equation 
  

   to 
  determine 
  t 
  when 
  p 
  is 
  measured. 
  Let 
  the 
  suffix 
  refer 
  

   to 
  the 
  freezing-point, 
  and 
  the 
  suffix 
  1 
  to 
  the 
  boiling-point 
  as 
  

   before. 
  We 
  then 
  have 
  

  

  OA 
  

  

  By 
  subtraction 
  

  

  or 
  

  

  HA 
  

  

  (;?i 
  _ 
  Po)( 
  ,_B) 
  = 
  C(Wo), 
  

   (P-Po) 
  (v-B) 
  = 
  C(t-t 
  Q 
  ), 
  

  

  P—Po 
  _ 
  t 
  — 
  U 
  m 
  

   Pi—Po 
  " 
  h—to' 
  

  

  Pi-Po 
  

  

  To 
  the 
  degree 
  of 
  approximation 
  to 
  which 
  we 
  are 
  working, 
  

   therefore, 
  there 
  is 
  no 
  thermodynamic 
  correction 
  needed 
  for 
  a 
  

   constant-volume 
  gas 
  thermometer. 
  There 
  may 
  be 
  a 
  correc- 
  

   tion 
  involving 
  squares 
  of 
  small 
  quantities, 
  which 
  would 
  

   appear 
  on 
  a 
  nearer 
  approximation. 
  Such 
  a 
  correction, 
  

   however, 
  would 
  not 
  be 
  worth 
  taking 
  into 
  account 
  in 
  the 
  

   case 
  of 
  a 
  thermometer 
  constructed 
  with 
  air 
  or 
  hydrogen, 
  as 
  

   the 
  unavoidable 
  errors 
  of 
  experiment 
  would 
  certainly 
  be 
  

   much 
  larger 
  than 
  the 
  correction. 
  It 
  is 
  satisfactory 
  to 
  know 
  

   that 
  for 
  all 
  practical 
  purposes 
  absolute 
  temperature 
  is 
  to 
  be 
  

   obtained 
  with 
  very 
  great 
  accuracy 
  from 
  Regnault's 
  thermo- 
  

   metric 
  system 
  by 
  simply 
  adding 
  the 
  value 
  of 
  t 
  to 
  his 
  numbers 
  

   for 
  temperature 
  on 
  the 
  centigrade 
  scale. 
  

  

  This 
  result 
  differs 
  from 
  that 
  obtained 
  by 
  Rowland 
  *, 
  who 
  

   employed 
  Lord 
  Kelvin's 
  law 
  of 
  the 
  inverse 
  square 
  of 
  the 
  

  

  * 
  Proceedings 
  Amer. 
  Acad, 
  Arts 
  & 
  Sciences, 
  xv. 
  (n. 
  s. 
  \ii.) 
  p. 
  114, 
  

  

  