concerning Volta' s Contact Force. 371 



indirectly concerned with the zinc-copper junction and the 

 contact-force there located*. I should say that so great a 

 variation, caused by a slight change of surface, shows that 

 there is something already going on at the metal-air surface 

 to vary. I am not aware o£ anything that can be done to 

 a metal-metal junction that will affect the Volta force — except 

 of course heating it. Lord Kelvin, however, would say, I 

 suppose, something like this : — In a simple Volta electror 

 static arrangement there are three boundaries, zinc/copper, 

 copper/aether, aether/zinc, and there is a contact-force at 

 each. The Yolta effect observed is the sum of the three. 

 Its seat is mainly at the zinc-copper junction ; it is due 

 to the attraction of the two metals for each other, and its 

 value is calculable approximately from their energy of com- 

 bination. The effect of any contact-force there may be at the 

 other boundaries is to modify the observed magnitude of this 

 true and chief Yolta force. The Volta force also varies in a 

 subordinate manner with temperature, and hence there is a 

 minute reversible Peltier evolution of heat at the zinc-copper 

 junction whenever a current passes. [End of hypothetical 

 quotation^] 



Size of Atoms. 



The doctrine of Lord. Kelvin about the energy of attraction 

 of plates of molecular thinness approaching each other to 

 within molecular distance and so virtually forming an alloy, 

 though admirably ingenious, proves as it seems to me too 

 much, i.e. gives too sharp an upper limit to molecular dimen- 

 sions ; for, as he shows with great clearness in a table in his 

 Royal Institution discourse already often quoted, the estimate 

 of 10 -9 cm. for molecular thinness and distance would cause 

 the combining metals to rise to a temperature of 7900° C, 

 or more than the probable temperature of the sun ! Yet 



* The following quotation from Lord Kelvin's Royal Institution 

 Lecture may be made :— The result of the burnishing experiments " shows 

 that the potential in zinc (uniform throughout the homogeneous interior) 

 increases from the interior through the thin surface-layer of a portion of 

 its surface affected by the crushing of the burnisher, more by *o2 volt 

 than through any thin surface-layer of portions of its surface left as 

 polished and scratched by glass-paper." If I may say so, I should 

 express this fact in exactly the same way, viz. that the natural potential- 

 difference between clean scratched zinc and dry air can be increased one 

 third of a volt by burnishing its surface with a steel tool (a remarkable 

 fact and one not easy to explain). But I should not go on, as the next 

 sentence goes on :— " The difference of potentials of copper and zinc 

 across an interface of contact between them is only about 2| times the 

 difference of potential thus proved to be produced between" the homo- 

 geneous interior of the zinc and its free surface, bv the burnishing." 



