Contact Electricity of Metals, 93 



shortest distance between the plates sufficiently to prevent 

 bridges of shred or dust between them, and so to give good 

 insulation. The smaller we make the shortest distance with 

 perfect enough insulation, the more sensitive is the apparatus 

 for the measurement of contact electricity performed as 

 follows : — 



§ 11. Run the slider to zero; make and keep made the 

 contact at P till the spot of light settles at the metallic zero ; 

 break contact at P, and lift the upper plate slightly. (If you 

 lift it too far, the spot of light may fly out of range.) If the 

 spot of light moves in the direction showing positive elec- 

 tricity on the insulated quadrants (as it does if the lower 

 plate is zinc and the upper copper), connect the cell to make 

 the slider negative (as shown in fig. 7). Repeat the experi- 

 ment with the slider at different points on the scale, until you 

 find that, with contact P broken, lifting the upper plate 

 causes no motion of the spot of light. If the compensating 

 action with the slider at the top 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 experi- 

 ments in the years 1859-61, as stated in a short paper 

 communicated to Section 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, M. H. Pellat found the same 

 method, and made admirable use of it in a series of experi- 

 ments described in theses presented to the Faculty of 

 Sciences in Paris in 1881 J, of which the results, accurate to 



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

 to ine, 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), showiug volta- 

 difference of 356 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 Yolta electromotive force between 

 polished zinc and copper oxidized by heat to a dark purple or slate 

 colour. 



t Ann de Chimieet de Physique, vol. xxiv. (1881) p. 20, footnote. 



\ Theses i^resentees a la Facidte 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, aud May 1880, "Difference de 

 potentiel des couches electriques qui recouvrent deux metaux en contact." 



