522 



REroiiT — 1884. 



Hitherto wo have supposed tho circuit to bo all ab one tempcratnre ; 

 but if diffbrcTit ])arts aro at diHereiit temperatures, wo shall liavo to use 

 ii yet further series, viz., a Thomson series, for tho lO.M.F, acting' in any- 

 one substance with a ditterenco of temperature between its ends, or tlio 

 Ibrce acting at a junction of two pieces of the same metal at different 

 temperatures. Tliis series can be deduced from tho preccdinnf, using only 

 tlio coefhcient of t'^, and multiplying it by tho difference of the squares of 

 iho ahsnlnic temperature of tho two ends of the piece of metal. Such a 

 series then stands thus: — 



Thoinson Series, or E.M.F. in a mdal vhose niJs diffcf in temperature. 



(Mirruvults.) 



Iron . . . •0487 (/,-/„) {274+ U/,-[-/..)| 

 German silver . •O.'il'i (/, -/,) (-74+.\(<, +/2)j- 

 Zinc . . . ~-()-2^i^it^-t^)[274-\-l(t,+f.,)} 



and so on. 



Whether a series of this sort can be made to include any non-metallic 

 conductors also hag not yet l)een discovered. ]M. Uouty's experiments 

 provisionally indicate the very intei-esting fact that- SirW. Thomson's 

 general thermodynamic laws of the tliermoelectric circuit apply pei'fectly 

 to circuits which include some electrolytes as well as metals. 



Now the meaning of statement Xo. xxiv. is as follows : regard zinc and 

 copper in contact as a circuit completed by air or by water, as the case 

 may be, and let tho temperature be uniform, and say 10°', to reckon up 

 the total E.M.F. we must look in the proper Volta series for Zn/air (or 

 Zn/water), which we find 18 say; fof Zn/Cu, which we don't find, or 

 find zero ; for Cu/air, which we find '8. Tlien we must look in the 10° 

 Peltier series for Zn/air or Zn/water, which at present we shall not find 

 there for want of data (possibly we have no right to put them there if 

 we had data) ; for Zn/Cu, which we find about 320 microvolts ; and for 

 Cu/air, which again Ave don't find. Add them all up with their jiroper 

 ' sijxns, and we have the total E.^I.F. of the circuit. 



Again, consider the case of a Daniell cell at a given temperature pro- 

 ducing a current ; wo sliall have to look in each series for Zu/ZnSO„ 

 for ZnS0.,/CuS04, for CuSO.,/Cu, and for Cu/Zn, and add them all up. 

 It is true that these tables of numbers have practically yet to be made, 

 for at present they include so few substances : that does not affect the 

 question of the existence and independence of these two kinds of series. 



It is, of course, a question how far all E.M.F. of contact may be found to 

 depend on chemical tendency. For instance, when bismuth and antimony 

 arc put into contact, does the E ]\I.F. developed measure the alloying 

 aflinity of these two metals ? When sodium is dropped into mercury, 

 docs the heat produced represent the thermoelectric power of a sodium- 

 mercury junction ? When metal touches glass, does the tremendous 

 E.M.F. developed represent a tendency of the metal to combine with the 

 glass ? These arc questions for experiment to decide ; but to me it does 

 not seem probable that it will re{)ly iu the affirmative. 



We know that Sir W. Thomson, niul Davy before him, considered the 

 apparent contact force at tlu; junctii)n of zinc and copper to be duo to the 

 chemical aflinity between these two metals, and to be measured by the heat 

 of formation of brass ; but this we have seen strong I'easons for disbelieving. 

 It sounds more pi'obable that the real contact foi'ce at a junction of bismuth 

 and antimony should be due to the chemical aflinity between these metals; 



