CONTACT OF DIELECTRICS. l8l 



The electrical energy must be less than that which corresponds 



to the alloy. As this is known to within about - of its value, we 



5 



may determine by the preceding equation a lower limit for the 

 distance e of the electrical layers. If their thickness were less, the 

 metals would have lost their properties, as they would no longer be 

 capable of acquiring their characteristic difference of potential on 

 contact ; we may consider that the molecular constitution of these 

 bodies would be thus modified, and that the thickness thus calcu- 

 lated is of the same order as the mean distance of the atoms. 

 Sir W. Thomson found thus, for copper and for brass, 



e = 



3-io 8 ' 



which corresponds to about - - of the wave length of green light. 



If we could separate the plates without a partial recombination 

 of the opposite layers of electricity, each of them would be found at 

 an extremely high potential. This potential depends on the dimen- 

 sions of the plates, for the capacity of the system is proportional to 

 the surface of contact, while the capacity of each of the plates 

 separately is proportional to its linear dimensions. Assuming that 



the distance of the electrical layer is -^ of a millimetre, Helmholtz 



has shown that if a disc of zinc 10 centimetres in radius were in 

 contact with a disc of copper in connection with the earth, the 

 potential of the zinc when carried to a great distance would be 

 39.I0 6 times as great as the original potential due to contact. 

 With Sir W. Thomson's numbers, the final potential would be 30 

 times as great. 



192. CONTACT OF DIELECTRICS. Volta's first law appears to 

 apply also to the contact either of metals with dielectrics, or of 

 dielectrics with each other. In these two cases, however, the de- 

 termination of the electromotive forces of contact presents great 

 difficulties. 



A single point of contact between two conductors is sufficient to 

 set up equilibrium of the potentials ; the charges only depend then 

 on the capacity of the system, at the moment at which their sepa- 

 ration is completed. With bad conductors there is equilibrium at 

 the points of actual contact only, and only these become charged 

 with electricity. The total charge will then vary greatly with the 



