Contact Electricity of Metals. 97 



gen eons 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 '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 *H times the difference of potential thus proved 

 to he 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 polish- 

 ing on glass-paper. 



§ 17. With two copper plates (a), (b) 

 polished on emery and each 

 compared with standard gold, 

 Murray found (a) — "11 volt. 



(j) _-06 volt. 

 They were then burnished by 

 rubbing them forcibly to- 

 gether, and again tested sepa- 

 rated : he found .... (a)— '02 volt. 



(&)— -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 hammering) 

 might probably show Volta- electric properties of the same 



dually (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 comers, all natural faces and facets. In 

 all probability Volta-differences of potential would be found between the 

 octahedronal and truncation al faces. We might arbitrarily define the 

 uniform interior potential as the potential of the air either near an octa- 

 hedronal face or near a truncational face ; or, still arbitrarily, we might 

 define it as some convenient mean or average related to measurements of 

 Volta-differences of potential between the different faces. 



* Ann. de Chimie et de Fhysique, 1881, vol. xxiv. footr.ote on p. 83. 



Phil. Mag. S. 5. Vol. 46. No. 278. July 1898. H 



