522 keport— 1884. 



Hitherto we have supposed the circuit to be all at one temperature ; 

 but if different parts are at different temperatures, we shall have to use 

 a yet further series, viz., a Thomson series, for the E.M.F. acting in any 

 one substance with a difference of temperature between its ends, or the 

 force acting at a juuction of two pieces of the same metal at different 

 temperatures. This series can be deduced from the preceding, using only 

 the coefficient of i 2 , and multiplying it by the difference of the squares of 

 the absolute temperature of the two ends of the piece of metal. Such a 

 series then stands thus : — 



Thomson Series, or E.M.F. in a metal ivhose ends differ in temperature. 



(Microvolts.') 



Iron . . . ■■0487-(t 1 -<,){274+i(*,+*,)} 

 German silver . "0512 (t l -U) f274+J(*i + *«)J 

 Zinc . . . -•024(i,-< 2 ){274+f(i,+* 2 )} 



and so on. 



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

 •conductors also has not yet been discovered. M. Bouty's experiments 

 provisionally indicate the very interesting fact that Sir W. Thomson's 

 general thermodynamic laws of the thermoelectric circuit apply perfectly 

 to circuits which include some electrolytes as well as metals. 



Now the meaning of statement No. 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 the temperature be uniform, and say 30° ; 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 1'8 say; for Zn/Cu, which we don't find, or 

 find zero ; for Cu/air, which we find "8. Then 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 we don't find. Add them all up with their proper 

 signs, and we have the total E.M.F. of the circuit. 



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

 ducing a current ; we shall have to look in each series for Zu/ZnS0 4 , 

 for ZnS0 4 /CuS0 4 , for CuS0 4 /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 a£Z 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.M.F. developed measure the alloying 

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

 does 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 are questions for experiment to decide ; but to me it does 

 not seem probable that it will reply in the affirmative. 



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

 apparent contact force at the junction of zinc and copper to be due to the 

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

 of formation of brass ; but this we have seen strong reasons for disbelieving. 

 It sounds more probable that the real contact force at a junction of bismuth 

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



