﻿Theory 
  of 
  the 
  Thermoelectric 
  Forces. 
  783 
  

  

  potential-difference 
  has 
  never 
  been 
  measured, 
  but 
  it 
  ought 
  

   to 
  be 
  measurable 
  by 
  means 
  of 
  an 
  electrometric 
  method. 
  For 
  

   iron 
  /3„is 
  equal 
  to 
  -4-87; 
  if 
  we 
  put 
  T 
  2 
  = 
  373°, 
  T 
  = 
  273 
  we 
  

   find 
  V 
  1 
  -V 
  2 
  = 
  4*87 
  .6'46.10 
  4 
  .10- 
  8 
  = 
  0-0031 
  volt. 
  Since 
  

   the 
  actual 
  value 
  of 
  the 
  specific 
  heat 
  of 
  electricity 
  does 
  not 
  

   agree 
  with 
  the 
  theoretical 
  value, 
  we 
  cannot 
  predict 
  exactly 
  

   the 
  value 
  of 
  the 
  electric 
  force. 
  I£ 
  we 
  suppose 
  <r 
  a 
  to 
  be 
  

   independent 
  of 
  the 
  temperature, 
  and 
  if, 
  in 
  order 
  to 
  get 
  a 
  

   better 
  agreement 
  between 
  the 
  actual 
  and 
  the 
  theoretical 
  

   value 
  of 
  a 
  a 
  , 
  we 
  neglect 
  the 
  term 
  a/e, 
  we 
  find 
  for 
  a 
  bar 
  of 
  

   bismuth 
  whose 
  difference 
  of 
  temperature 
  is 
  100° 
  C, 
  a 
  

   potential-difference 
  of 
  0'00102 
  volt, 
  in 
  the 
  case 
  of 
  iron 
  

   0-00030 
  volt. 
  

  

  Though 
  the 
  forces 
  X 
  have 
  not 
  yet 
  been 
  detected 
  we 
  might 
  

   suspect 
  them 
  to 
  exist 
  not 
  only 
  in 
  metals 
  of 
  different 
  tempe- 
  

   ratures, 
  but 
  even 
  in 
  other 
  bodies, 
  whose 
  temperature 
  varies 
  

   from 
  point 
  to 
  point. 
  Thus 
  the 
  phenomena 
  known 
  under 
  the 
  

   name 
  of 
  pyroelectricity 
  may 
  to 
  a 
  certain 
  extent 
  be 
  ascribed 
  

   to 
  forces 
  of 
  the 
  same 
  origin 
  as 
  the 
  forces 
  X, 
  a 
  difference 
  of 
  

   temperature 
  being 
  connected 
  with 
  a 
  difference 
  of 
  electric 
  

   potential. 
  The 
  phenomena 
  of 
  pyroelectricity, 
  occurring 
  in 
  

   crystals 
  of 
  every 
  crystallographic 
  system, 
  depend, 
  however, 
  in 
  

   a 
  very 
  complicated 
  way 
  on 
  the 
  structure 
  of 
  the 
  crystal. 
  It 
  is 
  

   interesting 
  to 
  remark, 
  that 
  even 
  crystals 
  of 
  bismuth 
  exhibit 
  

   these 
  phenomena. 
  If 
  electric 
  poles 
  in 
  the 
  crystals 
  are 
  to 
  be 
  

   found 
  not 
  only 
  by 
  differences 
  of 
  temperature, 
  but 
  also 
  under 
  

   the 
  influence 
  of 
  pressure 
  and 
  of 
  rays 
  of 
  light, 
  these 
  latter 
  

   effects 
  may 
  be 
  reduced 
  to 
  the 
  former 
  ones. 
  

  

  § 
  8. 
  Conservation 
  of 
  Energy. 
  

  

  Let 
  us 
  consider 
  a 
  thermoelectric 
  circuit 
  made 
  up 
  of 
  two 
  

   metals 
  a 
  and 
  6, 
  whose 
  junctions 
  are 
  at 
  the 
  constant 
  tempera- 
  

  

  Fier. 
  4. 
  

  

  tures 
  T 
  and 
  T'. 
  Then 
  the 
  thermoelectromotive 
  force 
  E 
  will 
  

   give 
  rise 
  to 
  an 
  electric 
  current 
  i 
  ; 
  and 
  the 
  amount 
  of 
  heat 
  

   absorbed 
  in 
  unit 
  time 
  by 
  the 
  Peltier 
  and 
  Thomson 
  effects 
  

  

  