ELECTROMOTIVE FORCES IN THE "VOLTAIC CELL. 



m 



probably per solium; at least, it is not known what is tbe effect of mixing 

 media, and so passing gradually from one to the next. 



We have given several Volta series ; and, for the sake of completeness,, 

 I -will now give some Peltier series for a few substances according to the 

 experiments of Professor Tait at different Centigrade temperatures. 

 Expressing each number as a function of the temperature, we are able 

 to give an infinite number of Peltier series in one table. The range off 

 temperature over which this table may be interpreted is from— 18° to- 

 400° or so, provided the metals do not begin to melt. Non-metallic sub- 

 stances have not yet been introduced into such series : much experimental 

 work remains to be done before they can be. The metals used by 

 Tait were not chemical ly pure. 



To find the E.M.F. of a junction at specified temperature we have only 

 to subtract the numbers in the above table, inserting the value of the 

 temperature. Thus a junction of zinc and copper at 10° has an E.M.F. 

 of 320 microvclts, acting from copper to zinc ; and a unit current sent 

 across such a junction from copper to zinc, or from zinc to copper, absorbs 

 or generates heat at the rate of 320 microvolts, and the current gains or 

 loses energy at the same rate. Clerk Maxwell says that the force is one 

 microvolt, and that it acts from zinc to copper ('Elementary Electricity/' 

 p. 149, note); but he only means, I suppose, that the E.M.F. of a zinc 

 copper circuit with one junction a degree hotter than the other is a 

 microvolt, and is such as to drive the current from zinc to copper across 

 the cooler junction; at least this is true above — 60° or — 80°. 2 



1 This row of numbers is little better than a guess from some curves given in 

 Wiedemann's ElcMrititat. A more probable deduction from some quite new experi- 

 ments of C. L. Weber (Wied. Ann. November 1884) gives, for Mercury, HSl + 5'68i 

 + 005* 2 (cf. note to section 27). 



- It is always easy to tell from thermo-electric data which way the force acts aiJ 

 a junction ; but it is not always the same way as the current flows, by any means-. 

 A current, excited by differences of temperature in a simple metallic circuit, may be 

 urged against the force at both junctions. This is the case, for instance, in a copper- 

 iron circuit with one junction above 275° and the other below it by a greater amount. 

 It is customary to say that the current flows across a hot junction from the metal of 

 higher to the metal of lower thermo-electric value : this is not necessarily true. The 

 safe statement is to say that the electromotive force acts from high to low thermo- 

 electric value, at either junction. 



