538 Lord Kelvin [May 21, 



Hence we see, looking to Fig. 10, if the movable copper plate C D is 

 allowed to move inwards (in the position shown in the diagram 

 it is pulled inwards by the Volta-electrifications of the opposed 

 surfaces of iron and copper), cold will be produced at the junction J, 

 all the metal being at one temperature to begin with ; and if we draw 

 out the copper plate C D, heat will be produced at J. The thermo- 

 dynamics of this action,* because it does not involve unequal tem- 

 peratures in different parts of the metals concerned, is a proper subject 

 for unqualified application of Carnot's law, and has nothing of the 

 unsatisfactoriness of the thermodynamics of thermoelectric currents, 

 which essentially involves dissipation of energy by conduction of heat 

 through metals at different temperatures in different parts. At 

 present we cannot enter further into thermodynamics than to remark 

 that when the plate C D is drawn out, the heat produced at J is not 

 the thermal equivalent of the work done by the drawing out of the 

 copper plate, but in all probability is very much less than the thermal 

 equivalent. Probably by far the greater part of the work spent in 

 drawing 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. Keturning to Fig. 9 ; suppose, by electrodes connected to 

 A B 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 junc- 

 tions will be equal, and the work done by the independent electro- 

 motive 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 evolution of heat is the 

 thermal equivalent of electromotive force at the junction. And in con- 

 sequence much confusion, in respect to Volta's contact electricity and 

 its relation to thermoelectric currents, has largely clouded the views 



* [March, 1898.] It has been given in a communication to the Eoyal Society 

 of Edinburgh entitled * The Thermodynamics of Volta-contact Electricity ' ; 

 Feb. 21, 1898. 



t Perhaps following Clerk Maxwell, or perhaps independently. At all events 

 we find the following in his splendid book of 1873 : " Hence J n 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. 



