KLECTROMOTIVE FOKCES IN THE VOLTAIC CELL. 



521 



but tbe 

 [t can bi) 

 ir known 

 , metaili« 



lium sur. 

 state of 



ference of 

 nevgies of 

 tbe " latter 

 rsy of tin; 

 icernt'd m 



c cneray is 

 nechaiiical 

 uivalent of 



t processes, 

 primary or 

 i(\ with the 

 bbe E.M.F. 

 rgy covre- 

 it is only 

 tit in calcu- 

 .ty,'p.l-i8.) 

 es in Avhich 

 depends on 

 3 imnierseJ, 



olta series, 

 T, the other 

 1 E.M.F. of 



les and add 



statement. 

 e attempted 

 »n cberaical 

 |ose giving ii 

 lose euertry- 

 Id a nnmher 

 [r a table of 

 lean be con- 

 latiu-e varies 



represented 

 aiiotlicr is 



M-i\ it will be 

 iter/l'ti'orthf 

 Involves both; 



linconstant or 



iudiciitP tho 



hetiiical datn 



It. NVc i.iive 



probably per saUum; at least, it is not known wliat is the effect of mixing 

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



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

 I will now give some Peltier series for a tew substances according to tho- 

 experiments of Professor Tait at ditTcrcnt Centigrade temperatures, 

 Expi'essing each number as a function of tlie temperature, wo are abla 

 to give an infinite immber of Peltier series in one table. Tho range ot' 

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

 400° 01* so, provided tho 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. Tho metals nsed by: 

 Tait were not chemically pure. 



True Contact E.M.F. or Fcltler Series. (Micro n-jlts.} 



Metals 



Iron . 



Hard riatimira 

 Soft riatinum , 

 Jlagncsium 

 Germ.T,n Silver , 

 Cadmium . 

 Zinc . 

 Silver 

 Lead . 

 Copper 

 Tin . 

 Alumininni 

 Palladinm . 

 Hypothetical ' 

 (Gaugain) 



^Icrcu 



To find the E.^I.F. of a junction at specified tcmpei'atui'e we have only 

 to subtract the ni;mbers in the above table, inserting the value of tho- 

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

 of 3'20 microvolts, acting from copper to zinc ; and a nnit current sent 

 across such a junction from copper to zinc, or from zinc to copper, absorb* 

 or generates heat at the rate of 820 microvolts, and the current gains or- 

 loses enei'gy at the same rate. Clerk Maxwell says that the force is ono 

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

 p. l-iO, 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 — G0° or — 80°. '^ 



' This row of numbers is little better than a jiness from some curves friven v.\ 

 Wiedemann's Elrl/fricilat. A more ]irobable deduction from some quite new experi- 

 ments of C. L. Weber (Wied. Ami. November 1884) gives, for Mercury, 1181 + 5-08* 

 + ()0.jf-= (cf. note to section 27). 



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

 a junction; but it is not always the same way as the oirrent Hows, by any means^ 

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

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

 iron circuit with ono 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 tho metal of 

 higher to the metal of lower thermo-electric value : th.is is nfit necessarily true. The- 

 safe statement is to say that tho electromotive force acts from liigli to low thcrmo- 

 tlectric value, at either junction. 



t 



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