the Roberts- Austen Recording Pyrometer, 75 



tributed along each of the wires A and B, due to the difference 

 between the temperatures of different parts of the same 

 wire (the Thomson E.M.F.). 



A 



The observed E.M.F. is, in reality, the algebraic sum of 

 the difference between the E.M.F.'s at H and C, and the 

 difference between the E.M.F.'s in A and B. 



Lord Kelvin has shown that it is possible, by means of the 

 second law of thermo-dynamics, to distinguish between some 

 of these constituents of the observed E.M.F., for the thermo- 

 couple may be regarded as a reversible engine. 



Let a be the " specific heat of electricity " in the metal A, 

 that is, the amount of heat (measured in ergs) absorbed by 

 the passage of one unit of electricity (or by unit current in 

 unit time) through a rise in temperature of one degree ; and 

 let <t' be the specific heat of electricity in B. 



Let H be the coefficient of the Peltier effect, that is, the 

 amount of heat absorbed at the junction by unit current in 

 unit time passing from A to B. Then if T x be the tempera- 

 ture of the cold junction, and T 2 that of the hot junction, 

 measured from the absolute zero, the heat absorbed in the 

 wires A and B in unit time by the passage of a current I 

 will be I(cr — o-')(T 3 — T x ) if the temperatures T 2 and 1\ are 

 nearly equal, so that the values of a and a' may be taken for 

 a mean-temperature T. 



It then follows, from the second law of thermo-dynamics, 

 that 



IH 2 _IH 1 ^ I(cr-^XT 2 -T 1 )_ A 

 T 2 Ti T ' 



J^T 2 

 T 



it follows that H = T— ,, 



CLL 



and E = Tg+rf,(T)-C (A) 



dE 

 In this equation T^ is the E.M.F. at the hot junction, C is 



