280 Mr Parker, On Contact- and Thermo- Electricity. [Nov. 23, 



which we will call f x {&). Next, if two plates of the same metal Z 



be employed but at slightly different temperatures, we obtain 



d 

 X z + -jn {AjZ) = a known quantity f 2 {&). Similarly we may get 



We then easily find 



^f - (Z z - X c ) =f x {ff) - {/, (0) -f 3 (6)}, 

 a result in which the influence of the air does not appear. 



If we accept the thermo-dynamical theory here developed, 

 these experiments give 



^ + d_ {A/z _ A/c}=fi(0)> 



±(A/Z)=f t (0), ^(AIG)=f 3 (0), 

 and therefore 



/, W - I A W - /. OT) - ^jf = l f 



which is the very result we obtained by assuming that the thermal 

 effects measure the electromotive forces of contact. Hence experi- 

 ments like those of Clifton and Pellat do not help us to decide 

 which of the two theories is correct. 



Again, on the assumption that P = D, it follows from the 

 experiment which gives D zo + A/Z — AJG, since P is very small, 

 that the electromotive forces of contact between the plates and 

 the air form great part of the phenomenon observed. It might 

 be supposed that this conclusion could be tested directly by ex- 

 periment by exhausting the air; but Pellat, by reducing the 

 pressure to 1 cm. or 2 cms. of mercury, found little difference in 

 the result. 



If we take the thermo-dynamical theory, it will follow that 

 in Pellat's experiment with two plates of the same metal at 

 different temperatures, the whole of the phenomenon observed is 

 due to the contact of the plates and air. In the case of two 

 plates of different metals at the same temperature, a large part 

 of the phenomenon observed will also probably be due to the 

 contact of the metals with the air. Taking into account the 



