1891.] Mr Parker, On Contact- and Thermo- Electricity. 279 



Also let us suppose that when the battery is in action, the 

 chemical changes which take place are reversible, and the Peltier 

 effects and electromotive force the same as when the current 

 is infinitesimal. Then the heat absorbed at the junctions on 

 the passage of unit quantity can be easily proved to be 



a dE(. p a dD\ 



6 W (just as P = M ). 



Hence since the heat evolved in the homogeneous parts in the 

 same time is E, and the total heat evolved L, we have the result 

 of Helmholtz and Gibbs, 



L = E - e % < 15 >- 



It now only remains to look at the experimental evidence 

 relating to the contact theory. In so doing we must remember 

 that an electromotive force of contact is produced not only by 

 the contact of two conductors, but by the contact of a conductor 

 and a non-conductor, and also, but less easily, by the contact of 

 two non-conductors. Again, for the sake of simplicity we shall 

 often follow the custom of writing B\ A for D BA , and whenever 

 it is necessary to indicate the temperature, we shall use suffixes : 

 thus B/A e denotes that the two different bodies A, B, are at 

 the same temperature 6; B e jB eo , that two portions of the same 

 substance are at the different temperatures 6, 6 . 



It has been pointed out by Maxwell that in experiments 

 like those of Clifton and Pellat, in which two plates Z, C, of 

 different metals, are employed in the open air, we really measure 

 the sum A/Z+Z/C+ C/A, or D zc ,+ A/Z -A/C. In like manner, 

 when we employ two plates of the same metal Z but at different 

 temperatures 6, 0> we measure Z e /Z 9o + A/Z e — A/Z 9o . Now 

 hitherto the terms A/Z 9 — A/Z 9o have been alwaj^s omitted, and 

 the experiment has been supposed to prove that Z 9 /Z 9a is not 

 zero. But clearly we cannot assume that 



A/Z O -A/Z 0O = 0, or that A/Z 



is independent of the temperature ; and therefore the experiment 

 does not contradict our result that Z 9 /Z 9o = 0. 



Let us assume that the thermal effects measure the electro- 

 motive forces of contact ; and suppose that we repeat the ex- 

 periments of Clifton or Pellat with two plates Z, G, of different 

 metals, first at the temperature 6, and then at a slightly different 

 temperature. From the results obtained, we find the value of 



