356 Prof. 0. Lodge on the Controversy 



existing at the junction whose temperature is T l5 and that 

 J<8) a is a measure of the E.M.F. existing along the gradient 

 of temperature in the metal A. 



Controversy also begins in respect of the function f(f) *. 

 There are those who decline to take an agnostic position with 

 respect to the absolute value, or second limit, of this function, 

 and who say that it represents absolutely the Yolta contact 

 force ; that is, the E.M.F. observed in the dielectric space 

 between two metals when a metallic circuit is not completed, 

 when there is only one metallic junction, and when a dielectric 

 space is substituted for the other junction. The value of the 

 E.M.F. thus observed is very large, say a thousand times as 

 big as any we have been so far dealing with ; it is not obser- 

 vable in any closed metallic circuit, it is only observable in a 

 dielectric portion of a circuit, and it has no apparent con- 

 nexion with thermoelectricity ; hence, those who assert that 

 this great E.M.F. is represented by /(£), that is, by a function 

 of the metallic junctions otherwise and solely known by its 

 differences and derivatives in thermoelectricity, should be 

 prepared with a definite proof of their statement. So far as 

 I know a proof has never even been attempted. To me it 

 appears that the introduction of a dielectric or an electrolyte 

 into the circuit, with its necessary two new junctions, each 

 with chemical potentiality, has entirely altered the whole 

 conditions ; and that no longer can it be said that the above 

 laws, appropriate to a closed metallic circuit, apply. I main- 

 tain that whether the assumption, that /(f), a function of the 

 metallic junctions alone, represents the Volta effect observed 

 in the air when a circuit is broken, is made tacitly or explicitly 

 it is made gratuitously and requires justification, which, so 

 far as I know, has not yet been forthcoming. 



To show that the unsupported statement just referred to 

 — whereby the Yolta effect, a potential gradient observed 

 in the air when a metallic circuit is broken, is introduced 

 into a thermoelectric function of metals alone, with which 

 it appears to me to have nothing to do — is responsibly 

 made, I might quote from several writers ; but there is no 

 need to quote more than Lord Kelvin's statements in his 

 Friday evening discourse to the Royal Institution for May 

 1897 (Reprinted in the Phil. Mag. July 1898) :— 



" § 19. Seebeck's great discovery of thermoelectricity 



(1821) was a very important illustration and extension of 



the twenty years' earlier discovery of the contact-electricity of 



dry metals by Volta. It proved independently of all dis- 



* Cf. Phil. Mag. March 1836, p. 267. 



