1897.] on Contact Electricity of Metals. 539 



of teachers and students. We find over and over again the statement 

 that thermoelectric electromotive force is very much smaller than the 

 Volta-contact electromotive force of dry metals. The truth is, Yolta- 

 electromotive force is found between metals all of one temperature, and 

 is reckoned in volts, or fractions of a volt, without reference to tem- 

 perature. If it varies with temperature, its variations may be stated 

 in fractions of a volt per degree. On the other hand, thermoelectric 

 electromotive force depends essentially on difference of temperature, 

 and is essentially to be reckoned per degree ; as for example, in fraction 

 of a volt per degree. 



§ 25. Volta's second fundamental discovery, that is, his discovery 

 (§ 5 above) that vitreous and resinous electricity flow away from zinc 

 and copper to insulated metals connected with them (for example, the 

 two electrodes of an insulated electrometer) when the two metals are 

 separated after having been in metallic contact, makes it quite certain 

 that there must be electric force in the air or ether in the neighbour- 

 hood of two opposed surfaces of different metals metallically con- 

 nected. This conclusion I verified about thirty-six years ago by 

 experiments described in a letter to Joule, of January 21, 1862, 

 which he communicated to the Literary and Philosophical Society 

 of Manchester, published in the Proceedings of the Society and in 

 ' Electrostatics and Magnetism ' (§ 400) under the title of *' A New 

 Proof of Contact-electricity." 



§ 26. Volta's second fundamental discovery also makes it certain 

 that movable pieces of two metals, metallically connected, attract one 

 another, except in the particular case when their free surfaces are 

 Volta-electrically neutral to one another. This force, properly 

 viewed, is a resultant of chemical af&nity between thin surface layers 

 of the two metals. And the work done by it, when they are allowed 

 to approach through any distance towards contact between any parts 

 of the surfaces, is the dynamical equivalent of the portion of their 

 heat of combination due to the approach towards complete chemical 

 combination constituted by the diminution of distance between the 

 two bodies. To fix the ideas, let the metals be two plane parallel 

 plates of zinc and copper, with distance between them small in 

 comparison with their diameters, and let us calculate the amount of 

 the attractive force between them at any distance. Let V be the 

 difference of potentials of the air or ether very near the two metallic 

 frontiers, but at distances from these frontiers amounting at least to 

 several times the distance from molecule to nearest molecule in either 

 metal (see footnote on § 16 above). The electric force in air or 

 ether between these surfaces will be V/D, if D denotes the distance 

 between them. Hence (our molecular microscopic binocular set 

 aside) if p is the electric density of either of the opposed surfaces, 

 A the area of either of the two, and P the attraction between them, 

 we have 



