EI-ECTllOMOTIVE FORCES IN THE VOLTAIC CELL. 



519 



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roximiiteli' 

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 liiiation to 



occur, and observing the heat produced per amount of substance corre- 

 sponding^ to one unit of electricity. 



xiv. In addition to this contact force, duo to potential chemical action 

 or chemical strain, there is another -which is independent of chemical pro- 

 perties, but which seems to be greatest for badly conducting solids, and 

 which is in every case superposed upon the former contact force, the two 

 being observed together and called the Volta effect. Very little is known 

 about tliis latter force except in the case of metals ; and in these it varies 

 with temperature, and is small. In the case of non-metals it is often 

 much larger than the chemical contact force. ^ 



XV. The total contact foi'ce at any junction can be experimentally 

 determined by measuring the reversible energy developed or absorbed 

 there per unit quantity of electricity conveyed across the junction. 

 [Practical difficulties, caused by irreversible distui-bances, being sup- 

 posed overcome.]^ 



xvi. In a chain of any substances whatever, the resultant E.M.F. 

 between any two points is equal to the sum of tha true contact forces 

 acting across every section of the chain between the given points (neglect- 

 ing magnetic or impressed forces). 



xvii. In a closed chain the sum of tl^o ' Volta forces,' measured, 

 electrostatically in any (the same) medium, is equal to the sum of the true 

 contact forces ; whether each individual Volta force be equal to each 

 individual true force or not. See section (7). 



xviii. Wherever a current flows across a seat of E.M.F. there it must 

 gain or lose energy at a rate numerically equal to the E.M.F. multi- 

 plied by the sti'ength of the current.'' 



Development of the above and special application to metals. 



xix. A metal is not at the potential of the air touching it, but is 

 always slightly below that potential by an amount I'oughly proportional 

 to its heat of combustion, and calculable, at any rate approximately, from 

 it. For instance, clean zinc is probably about I'S volts below the air, 

 copper about "8 volt below, and so on. If an ordinary oxidising medium 

 be substituted for ' air ' in the above statement it makes but little 

 <afrurence. 



XX. Two metals put into contact reduce each other instantly to 

 practically the same potential, and consequently the most oxidisable one 

 receives from the other a positive charge, the effect of -which can be 

 observed electrostatically. 



xxi. There is a slight true contact force at the junction of two metals 



licre nssunic, what I .suppose is rcoo.ijniscd as truo, that wliat is known as 

 ■riiiliiinal LTi^noration oL' electricity is really due to a eiintaet force between the siib- 

 ^iiiiices rubbed; a force which is exccediiii^iy Lireat for insulators, see section (19). 

 I'avy seems to have held tiiis view, from a note on p. ."iO of his J!ak(;rian lecture in 

 l.^lH!, cited before. 



■ These difliculties are, however, tremendous for most substances execpt metals, 

 ^I. i')ir,ity'.s is the only attempt I know of to examine junction encrjr;\'b(?t\veen metals 

 anil solutions of their salts, which is the case next in simplicity to metals. Observe 

 tliat the statement says vnvrijii, not heat only. 



■' A current gains energy at any junction at whicii heat is absorbed, or chemical 

 coiiibjiiatidn peiiuitted, or any other foriu of enertry destroyed, by 1 lie i)assage of 

 till; rurreut. TIk; current gains the energy which has in the other form disapijeareil. 



A I'lirreiit loses energy at a point where it causes other forms of energy to mako 

 tlit'ir apjK'arance ; e.g., generation of heat, decomposition of chemical compounds, &c 



