t 'ontact Electricity of Metals. 103 



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 aether in the neighbour- 

 hood of two opposed surfaces of different metals metallically 

 connected. 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 Philoso- 

 phical Society of Manchester, published in the l Proceedings ' 

 of the Society and in 'Electrostatics and Magnetism' (§ 400) 

 under the title of "A New Proof of Contact Electricity." 



§ 26. Yolta's second fundamental discovery also makes it 

 certain that movable pieces of two metals, metallically con- 

 nected, 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 affinity 

 between thin surface-layers of the two metals. And the work 

 done by it, w T hen 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 com- 

 bination 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 Y be the difference of potentials of the air or aether 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 foot- 

 note on § 16 above). The electric force in air or aether 

 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 



Hence 



V IV 



D =4 ^' P= 2^D A 



V*A 



87TD 2 



Hence the work done bv electric attraction in letting them 



