ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 519 



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

 sponding to one unit of electricity. 



xiv. In addition to this contact force, due 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 this 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. 1 



xv. The total contact force 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 disturbances, 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 the 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 the ' 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 strength of the current. 3 



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 roughly proportional 

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

 it. For instance, clean zinc is probably about 1*8 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 

 ■difference. 



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 



1 I here assume, what I suppose is recognised as true, that what is known as 

 frictional generation of electricity is really due to a contact force between the sub- 

 stances rubbed ; a force which is exceedingly great for insulators, see section (19). 

 Davy seems to have held this view, from a note on p. 50 of Ms Bakerian lecture in 

 1806, cited before. 



* These difficulties are, however, tremendous for most substances except metals. 

 -M. Bouty's is the only attempt I know of to examine junction energy between metals 

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

 that the statement says energy, not heat only. 



3 A current gains energy at any junction at which heat is absorbed, or chemical 

 •combination permitted, or any other form of energy destroyed, by the passage of 

 the current. The current gains the energy which has in the other form disappeared. 



A current loses energy at a point where it causes other forms of energy to make 

 their appearance ; e.g., generation of heat, decomposition of chemical compounds, &c. 



