358 Prof. 0. Lodge on tJw Controversy 



let the copper be cut across in the middle, so that an inde- 

 pendent E.M.F. can he introduced by wire electrodes and a 

 current maintained across the two junctions. " The Peltier 

 heat produced where the current, passes from iron to copper 

 is manifestly not the thermal equivalent of the work done. - " 

 [This sentence is ambiguous. It may mean the " work 

 done " on the whole circuit, in which case it is true ; it may 

 mean the "work done" precisely at the junction, in which 

 case I should say that the word " not " ought to be omitted. 

 The immediate context suggests the former interpretation; the 

 next succeeding context suggests the latter. The immediate 

 context runs as follows.] 



ie In fact, if the two junctions be at equal temperatures, 

 the amounts of Peltier heat produced and absorbed at the 

 two junctions will be equal, and the work done by the 

 independent E.M.F. will be spent solely in the frictional 

 generation of heat/'' [This is undeniable, though not appa- 

 rently to the point ; but the next section continues] 



" Many recent writers, overlooking . . . obvious prin- 

 ciples, have assumed that the Peltier evolution of heat is 

 the thermal equivalent of E.M.l<\ at the junction. And in 

 consequence much confusion, in respect to Volta's contact- 

 electricity and its relation to thermoelectric currents, has 

 clouded the views of teachers and students. We find over 

 and over again the statement that thermoelectric E.M.F. 

 is very much smaller than the Yolta contact E.M.F. of dry 

 metals. The truth is, Volta E.M.F. is found between metals 

 all of one temperature, and is reckoned in volts, or fractions 

 of a volt, without -reference to temperature." [Certainly.] 

 '' If it varies with temperature, its variations may be stated 

 in fractions of a volt per degree/' [By all means.] " On 

 the other hand, thermoelectric E.M.F. depends essentially on 

 difference of temperature " [the thermoelectric force of a 

 complete metallic circuit does, and not the thermoelectric 

 force at any particular junction ; that depends on the 

 temperature of the junction, and cannot be affected by the 

 temperature somewhere else] u and is essentially to be 

 reckoned per degree : as, for example, in fraction of a volt 

 per degree." [Not so ; an electromotive force of any kind 

 whatever must be reckoned in volts. Volts per degree is 

 not E.M.F., but is the so-called relative thermoelectric power 

 of two metals.] 



I have ventured thus to interpolate what I consider cor- 

 rective remarks in brackets into this readily accessible 

 quotation, because of a preceding sentence with which I 

 quite agree, viz. this : — " Peltier's admirable discovery . . , 



