﻿Reduction 
  to 
  Air- 
  Temperature 
  of 
  Platinum- 
  Temperature. 
  525 
  

  

  The 
  mechanical 
  force 
  cannot 
  really 
  depend 
  upon 
  /x, 
  and 
  the 
  

   formula 
  which 
  leads 
  to 
  such 
  a 
  result 
  must 
  be 
  erroneous. 
  

  

  As 
  regards 
  the 
  problem 
  of 
  the 
  pressure 
  of 
  radiation, 
  I 
  

   conclude 
  that 
  in 
  this 
  case 
  also, 
  and 
  in 
  spite 
  of 
  the 
  formula, 
  

   the 
  permeability 
  of 
  the 
  reflector 
  is 
  without 
  effect, 
  and 
  that 
  the 
  

   consequences 
  deduced 
  by 
  Boltzmann 
  and 
  Wien 
  remain 
  un- 
  

   disturbed. 
  

  

  Another 
  investigation 
  to 
  which 
  perhaps 
  similar 
  conside- 
  

   rations 
  will 
  apply 
  is 
  that 
  of 
  the 
  mechanical 
  force 
  between 
  

   parallel 
  slabs 
  conveying 
  rapidly 
  alternating 
  electric 
  currents. 
  

   Prof. 
  J. 
  J. 
  Thomson's 
  conclusion 
  * 
  is 
  that 
  the 
  electromagnetic 
  

   repulsion 
  is 
  f* 
  times 
  the 
  electrostatic 
  attraction, 
  so 
  that 
  a 
  

   balance 
  will 
  occur 
  only 
  when 
  p,= 
  l. 
  It 
  seems 
  more 
  probable 
  

   that 
  the 
  factor 
  /x. 
  should 
  be 
  omitted, 
  and 
  that 
  balance 
  between 
  

   the 
  two 
  kinds 
  of 
  force 
  is 
  realized 
  in 
  every 
  case. 
  

  

  LV. 
  The 
  Reduction 
  to 
  normal 
  Air- 
  Temperatures 
  of 
  the 
  Pla- 
  

   tinum-Temperatures 
  in 
  the 
  Low 
  'Temperature 
  Researches 
  

   of 
  Professors 
  Dewar 
  and 
  Fleming. 
  By 
  J. 
  D. 
  Hamilton 
  

   Dickson, 
  M.A., 
  F.R.S.EA 
  

  

  TI^HE 
  measurement 
  of 
  temperature 
  by 
  means 
  of 
  platinum 
  

   _L 
  depends 
  upon 
  the 
  two 
  following 
  propositions 
  : 
  — 
  

   (1) 
  That 
  for 
  a 
  given 
  piece 
  of 
  pure 
  annealed 
  platinum-wire 
  the 
  

   temperature 
  is 
  a 
  single-valued 
  function 
  of 
  the 
  electric 
  resistance. 
  

   This 
  proposition 
  is 
  due 
  to 
  Prof. 
  Callendar 
  (1886), 
  and 
  has 
  

   been 
  fully 
  verified 
  by 
  many 
  subsequent 
  observers. 
  The 
  

   second 
  proposition 
  is 
  : 
  — 
  (2) 
  That 
  however 
  different 
  specimens 
  

   of 
  pure 
  annealed 
  platinum-wire 
  may 
  vary 
  among 
  themselves, 
  

   nevertheless 
  they 
  agree 
  in 
  giving 
  the 
  same 
  normal 
  air- 
  

   temperature 
  of 
  any 
  enclosure 
  in 
  which 
  they 
  may 
  be 
  

   simultaneously 
  placed. 
  This 
  proposition 
  might, 
  at 
  first 
  

   sight, 
  appear 
  as 
  a 
  logical 
  deduction 
  from 
  the 
  first; 
  but 
  a 
  

   little 
  consideration 
  will 
  show 
  that 
  the 
  two 
  propositions 
  are 
  

   equally 
  fundamental, 
  and 
  equally 
  necessarily 
  due 
  to 
  expe- 
  

   riment. 
  We 
  are 
  indebted 
  for 
  it 
  to 
  the 
  careful 
  researches 
  

   of 
  Mr. 
  E. 
  H. 
  Griffiths. 
  

  

  Theory 
  has 
  not 
  yet 
  provided 
  the 
  formula 
  referred 
  to 
  

   in 
  the 
  first 
  proposition 
  ; 
  meanwhile, 
  Prof. 
  Callendar 
  has 
  

   devised 
  a 
  double 
  formula 
  — 
  or, 
  rather, 
  a 
  formula 
  with 
  a 
  

   correction 
  — 
  which 
  amounts 
  to 
  the 
  expansion 
  of 
  the 
  electric 
  

   resistance 
  of 
  the 
  platinum- 
  wire 
  in 
  powers 
  of 
  the 
  temperature, 
  

   and 
  leads 
  to 
  a 
  somewhat 
  troublesome 
  reduction 
  before 
  finally 
  

  

  * 
  ' 
  Recent 
  Researches 
  in 
  Electricity 
  and 
  Magnetism,' 
  1893, 
  § 
  277. 
  

   t 
  Communicated 
  by 
  the 
  Author. 
  

  

  