Mr. J. Parker on Thermoelectric Phenomena. 



73 



Parallel and close to the plates {x, y) place equal plates of 

 any metal whatever, as iron, and connect these iron plates hv 

 long iron wires with a large distant mass of iron in the neutral 

 state. 



A 



K 



By slowly separating the plates x, and slowly bringing the 

 plates y nearer together, let unit charge be made to pass from 

 X to y, without altering the potential of either x or y. By 

 equation (8), the heat absorbed at the junction of the hot and 

 cold parts of the wire A will be 



and at the junction of the hot and cold parts of the metal B, 



Now let the wires A, B, A' be removed ; secondly, reduce the 

 potential of the plate y to equality with that of x ; thirdly, 

 make the unit charge return from y to x in a reversible 

 manner ; finally, bring the potential of y back to its original 

 value. All this may be accomplished without producing any 

 thermal effect. A complete reversible cycle of operations 

 having been performed, we have, by Carnot's principle, 



But we have already seen that 



s(t) = t|w-/«) (3' 



Hence (15) becomes 



K=h (16) 



Thus, if two portions of the same metal, at different absolute 

 temperatures t, t^ respectively, be in contact, and if V, Vq be 

 the potentials which they assume, then 



Y-Yo=mt'-fo'); (7) 



where k is a. constant which has the same value for all metals. 

 In other words, the difference of potential depends only on 

 the temperatures, and is the same for all metals. If, however, 

 unit charge be made to cross the junction, the heat that must 



