534 Lord Kelvin [May 21, 



increases from the interior tlirougli tlie thin surface layer of a portion 

 of its surface affected by the crushing of the burnisher, more by 

 •32 volt than through any thin surface-layer of portions of its surface 

 left as polished and scratched by glass paper. 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 homogeneous interior of the zinc and its free 

 surface, by the burnishing. Pellat had found that polished metallic 

 surfaces, seemingly clean and free from visible contamination of any 

 kind, became more positive by rubbing them forcibly with emery 

 paper, zinc showing the greatest effect, which was '23 volt. Murray's 

 burnished surface of zinc actually fell ' 32 volt when scratched by 

 polishing on glass paper. 



§ 17. With two copper plates (a), (h) polished 

 on emery and each compared with standard 



gold, Murray found (a) - -11 volt. 



(6) _ -06 volt. 

 They were then burnished by rubbing them for- 

 cibly together, and again tested separately ; 



he found . ' (a) - -02 volt. 



(h) - -02 volt. 



Rises of Volta-potential of about the same amount were produced 

 by burnishing with a steel burnisher copper plates which had been 

 polished and scratched in various ways. Such experiments as those 

 of Murray with burnishing ought to be repeated with hammering or 

 crushing by a Bramah's press. Indeed Pellat * suggested that metals 

 treated bodily " par le laminage ou le martelage " (rolling or hammer- 

 ing) might probably show Volta-electric properties of the same kind as, 

 but more permanent than, those which he had found to be produced 

 by violent scratching with emery paper. 



§ 18. It is interesting to remark that Murray's most highly bur- 

 nished zinc differed from his emery-polished copper (a) by 1*13 



insulating fluids or solids, we find it convenient to use a mathematical function 

 of position called potential in the interior of each metal. This function must, for 

 the case of equilibrium, fulfil the condition that it is of uniform value through 

 each homogeneous portion of metal. Its value must, as a rule, change gradually 

 (or abruptly) with every gradual (or abrupt) change of quality of substance 

 occupying space. 



To illustrate the difficulty and complexity of expression with which I have 

 struggled, and to justify if possible my ungainly resulting sentence in the text, 

 consider the case of a crystal of pure metal : suppose, for example, an octahedron 

 with truncated corners, all natural faces and facets. In all probability Volta- 

 differences of potential would be found between the octahedronal and truncational 

 faces. "We might arbitrarily define the uniform interior potential as the potential 

 of the air either near an octahedronal face or near a truncational face ; or, still 

 arbitrarily, we might define it as some convenient mean or average related to 

 measurements of Volta-difierences of potential between the difierent faces. 



* Ann. de Chimie et de Physique, 1881, vol. xxiv. footnote on p. 83. 



