94 Lord Kelvin on 



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 u asserted that there was 

 no Volta effect in the slightly rarefied air then known as 

 vacuum "f. Pfaff experimented with varnished plates ; Pellat 

 arrived at the same conclusion with polished unvarnished 

 plates of zinc and copper. He found slight variations of the 

 Volta electromotive force due to the nature of the gas sur- 

 rounding the plates, and to differences of its pressure, of 

 which he says : " Ces variations sont tres faibles, par rapport 

 a la difference de potentiel totale Ces variations dans la diffe- 

 rence de potentiel sont toujours en retard sur les changements 

 de pression. Elles ne paraissent done pas dependre directement 

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

 la nature de la surface metallique, modifications qui mettent 

 un certain temps a se produire." The smallest pressures for 

 which Pellat made his experiments were from 3 to 4 or 5 

 centim. of mercury. 



§ 14. The same method was used by Mr. J. T. Bottomley 

 in an investigation by which he demonstrated with minute 

 accuracy the equality of the Volta-contact-difference measured 

 in a glass tube exhausted to less than g-J-g millim. of mercury J 

 (2-| millionths of an atmosphere), and immediately after 

 in the same tube filled with air to ordinary atmospheric 

 pressure ; and again exhausted and filled with hydrogen to 

 atmospheric pressure three times in succession ; and again 

 exhausted and filled to atmospheric pressure with oxygen. In 

 some cases the electrical test was repeated several times, 

 while the gas was entering slowly. The actual apparatus 

 which he used is before you, and in it I think you will see 

 with interest the little Volta-condenser, with plates of zinc 

 and copper a little larger than a shilling, the upper hung on 



* Ann. de Chim. 2nd series, vol xli. p. 238. 



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



\ A very high exhaustion had been maintained for two days, and 

 finally perfected by two and a half hours' working at the pump immediately 

 before the electric testing experiment. 



