102 Lord Kelvin on 



ing out the plate against the electric attraction goes to storing 

 up electrostatic energy, and but a small part of it is spent on 

 heat produced at J ; or on excess (positive or negative) of 

 this Peltier heat above quasi-Peltier (positive or negative) 

 absorptions of heat in the surface-layers of the opposed 

 surfaces when experiencing changes of electrification. 



§ 23. Returning to fig. 9 ; suppose, by electrodes con- 

 nected to AB and an independent electromotive force, a 

 current is kept flowing from copper to iron through one 

 junction, and from iron to copper through the other ; the 

 Peltier heat produced where the current passes from iron to 

 copper is manifestly not the thermal equivalent of the work 

 done. In fact, if the two junctions be at equal temperatures, 

 the amounts of Peltier heat produced and absorbed at the two 

 junctions will be equal, and the work done by the independent 

 electromotive force will be spent solely in the frictional 

 generation of heat. 



§ 24. Many recent writers *, overlooking the obvious 

 principles of § § 22, 23, have assumed that the Peltier evolu- 

 tion of heat is the thermal equivalent of electromotive force 

 at the junction. And in consequence much confusion, in 

 respect to Volta's contact electricity and its relation to thermo- 

 electric currents, has largely clouded the views of teachers 

 and students. We find over and over again the statement 

 that thermoelectric electromotive force is very much smaller 

 than the Volta-contact electromotive force of dry metals. 

 The truth is, Volta-electromotive force is found between 

 metals all of one temperature, and is reckoned in volts, 

 or fractions of a volt, without reference to temperature. If 

 it varies with temperature, its variations may be stated in 

 fractions of a volt per degree. On the other hand, thermo- 

 electric electromotive force depends essentially on difference 

 of temperature, and is essentially to be reckoned per degree', 

 as for example, in fraction of a volt per degree. 



§ 25. Volta's second fundamental discovery, that is, his 

 discovery (§ 5 above) that vitreous and resinous electricity 

 flow away from zinc and copper to insulated metals connected 



* Perhaps following Clerk Maxwell, or perhaps independently. At all 

 events we find the following' in his splendid book of 1873 : " Hence Jn 

 represents the electromotive contact force at the junction acting in the 



positive direction Hence the assumption that the potential of a 



metal is to be measured by that of the air in contact with it must be 

 erroneous, and the greater part of Volta's electromotive force must be 

 sought for, not at the junction of the two metals, but at one or both 

 of the surfaces which separate the metals from the air or other medium 

 which forms the third element of the circuit." — ' Treatise on Electricity 

 and Magnetism,' vol. i. § 249. 



