1897.] on Contact Electricity of Metals. 631 



of the range is insufficient, add a second cell ; if it is still insufficient, 

 add a third cell ; if still insufficient, add a fourth.* 



§ 12. By this method I made an extended series of experiments in 

 the years 1859-61, as stated in a short paper communicated to Sec- 

 tion A of the British Association at its Swansea meeting in August 



1880, which with additions published in ' Nature ' for April 14, 1881, 

 is appended to the present article. 



§ 13. Quite independently, "f Mr. H. Pellat found the same method, 

 and made admirable use of it in a series of experiments described in 

 theses presented to the Faculty of Sciences in Paris in 1881, J of 

 which the results, accurate to a degree of minuteness unknown in 

 previously published researches on the electrical effects of dry contacts 

 between metals, constitute in many respects the most important and 

 most interesting extension of our knowledge of contact electricity 

 since the times of Volta and Pfaff. One of his results (I shall have 

 to speak of others later) was that Pfaff was right in 1829 § when he 

 described experiments in which he found no difference in the Volta- 

 contact-electromotive force between zinc and copper, whether tested in 

 dry or damp air, oxygen, nitrogen, hydrogen, carburetted hydrogen, or 

 carbonic acid, so long as no visible chemical action occurred ; and that 

 De la Eive was not right when he " asserted that there was no Volta 

 effect in the slightly rarefied air then known as vacuum." || Pfaff ex- 

 perimented with varnished plates ; Pellat arrived at the same con- 

 clusion with polished unvarnished plates of zinc and copper. He 

 found slight variations of the Volta electromotive force due to the 

 nature of the gas surrounding the plates, and to differences of its 

 pressure, of which he says : " Ces variations sent tres faibles, par 

 rapport a la difference de potentiel totale. . . . Ces variations dans 

 la difference de potentiel sont toujours en retard sur les change- 

 ments de pression. Elles ne paraissent done pas dependre directement 

 de celle-ci, mais bien des modifications qui en resultent dans la nature 



* The only case hitherto tested by any experimenter, so far as known to me, 

 in which more than two Daniell cells would be required for the compensation, is 

 bright metallic sodium, guarded against oxide by glass, in Mr. Erskine Murray's 

 experiments (§ 18 below), showing volta-difference of 3-56 volts from his standard 

 gold plate. For direct test this would require four Daniell cells on the potential 

 divider. The greatest volta-difference of potentials observed by Pellat was 1 • 08 

 volts, for which a Daniell's cell would rather more than suffice. About 1862 I found 

 considerably more than the electromotive force of a single Daniell's element 

 required to compensate the Volta electromotive force between polished zinc and 

 copper oxidised by heat to a dark purple or slate colour. 



t Ann. de Chimie et de Physique, vol. xxiv. 1881, p. 20, footnote. 



t ' Theses presente'es a la Faculte des Sciences de Paris, pour obtenir le Grade 

 de Docteur-es-Sciences Physiques/ par M. H. Pellat, Professeur de Physique au 

 Lycee Louis le Grand, No. 461, juin 22, 1881. See also 'Journal de Physique,' 



1881, xvi. p. 68, and May 1880, ' Diffe'rence de potentiel des couches e'lectriques 

 qui recouvrent deux metaux en contact.' 



§ Ann. de Chim., 2 series, vol. xli. p. 236. 

 11 Lodge, Brit. Assoc, Report, 1884, pp, 477-8. 



