Electromotive Forces in a Voltaic Cell. 337 



These are, as far as we want them : — (1) If a circuit consist of 

 one metal only, the electromotive force around the circuit is 

 nil however the temperature may vary in different parts ; this 

 of course neglecting the thermoelectric effects of stress and 

 magnetism discovered by Sir W. Thomson. (2) If the 

 circuit consist of two metals with the junctions at different 

 temperatures t l9 t 2} then the electromotive force round the 

 circuit is the difference of a function of t 2 and of the same 

 function of t x . According to Prof. Tait the function is 



(t -+- 1 \ t 2 



T — 2 -^- l Y or, as we may write it, A + B* 2 - C-|- — 



\ A + Bti — C-|- 1 ; the series may perhaps extend further, but, 



according to Tait's experiments, the first three terms are all 



that are needed. 



Now, but for the second law of thermodynamics we should 



Gt 2 

 naturally assume that A + Bt 2 ^- was the difference of 



Gt 2 

 potentials at the junction of temperature £ 2 , and A + B/ x ^i- 



at the junctions of temperature t x ; we should further assume 



that what the unit of electricity did was to take energy 



Gt 2 

 A + Bt 2 -i~ out of the region immediately around the hot 



junction with disappearance of that amount of heat, and to 



Gt 2 

 take energy A + B^ -^ into the region immediately 



surrounding the cold junction, with liberation of that amount 

 of heat. Now apply the second law of thermodynamics in 



the form 2-=0,and we have 



<H) i0 ^=°> 



whence it follows that A=0, which may be, and that = 0, 

 which is contrary to experiment. The current then must do 

 something else than has been supposed, and the hypotheses 

 differ in expression at least as to what that something else is. 

 The fact to be expressed is simply this : when a current 

 passes in an unequally heated metal, there is a reversible 

 transference of heat from one part of the metal to another, 

 whereby heat is withdrawn from or given to an element of 

 the substance when a current passes through it between 

 points differing in temperature, and is given to or withdrawn 

 from that element if the current be reversed. Sir W. 

 Phil. Mag. S. 5. Vol. 20. No. 125. Oct. 1885. 2 B 



