360 Prof. 0. Lodge on the Controversy 



And in case anything that I can say can make the matter 

 clearer, I would reassert that since E.M.F. is work per nnit 

 charge conveyed, not only for the whole circuit but for any 

 portion of a circuit, and since a unit charge conveyed across 

 a junction develops an amount of heat ±11, it follows that 

 the physical E.M.F. located at that junction is measured bv 

 and is equal to + JII (assisting the current in one direction 

 and thereby consuming heat or developing cold, opposing it 

 in the other direction and therefore developing extra heat) ; 

 unless some other mode of accounting for the appearance or 

 non-appearance of the reversible heat is devised, or unless 

 the reversible energy takes some other form than heat. 



Wherever energy appears or disappears in a circuit, there, 

 in that place, must be located an E.M.F. It may be, so to 

 -peak, the frictional E.M.F. appropriate to Ohm's law in a 

 homogeneous conductor; it may be the chemical E.M.F. at 

 a fluid or semifluid junction discovered by Yolta : or it may 

 be the contact-force at a solid junction, of which tbe integral 

 round a whole circuit of non-uniform temperature was dis- 

 covered by Seebeck. the locality of part of that E.M.F. by 

 Peltier, and the localitv and existence of the rest of that 

 E.M.F. by Lord Kelvin. 



At a chemical junction the reversible energy need not all 

 of it or much of it take the form of heat, and in that case the 

 major part of the E.M.F. may be rightly called chemical, not 

 physical : but at a metallic junction, as in a homogeneous 

 conductor, heat is usually the only form of energy expected 

 or observed, and in that case wherever the E.M.F. is located 

 there the heat or cold must appear. The ordinary way out of 

 this conclusion is to suppose that electricity is a fluid with a 

 real specific heat whose value changes from metal to metal, so 

 that heat can be evolved or absorbed at a junction by a sudden 

 change in the heat-capacity of the electric fluid flowing across 



the same as tlie Yolta effect, or suggests that the Yolta effect has any- 

 thing to do with the matter ; except on the hypothesis that the air layer 

 intervening when a metallic junction is broken i? utterly inert and 

 impotent. 



It is no argument in favour of the impotence of the air layer to say 

 that it produces no apparent effect when it bathes the metal uniformly 

 all round : the question is what it does when uniformity all round does 

 not exist. Balanced forces are not the same thing as no force, although 

 they produce the same zero accelerative effect. Xor does the absence"of 

 acceleration prove the absence of all force. A body subject to balanced 

 forces and obeying the first law of motion is a frequent spectacle — a 

 railway train or a snowflake for hostance — but a body subject to no forces 

 is decidedly rare. 



