On Contact Electricity of Metals. 113 



" On Contact Electricity of Metals." By J. ERSKiNE-MtJRRAY, 

 D.Sc., F.R.S.E., Heriot-Watt College. Communicated by 

 Lord Kelvin, G.C.V.O., F.R.S. Received August 4, 1897, 

 Read November 18, 1898. 



I. Introductory. 

 II. Method of Experiment. 



III. Effects of Different Methods of Cleansing the Metallic Surfaces. 

 IT. Thin Solid Films of Oxides, Iodides, &c. 

 V. Atmospheric or Time Effects. 

 VI. Very Thin Liquid Surface Films on Metals. 

 VII. Thick Liquid Films on Metals. 

 VIII. Films formed from Gases. 

 IX. Temperature Variations. 



X. Elimination of Metal- Air Potentials by Solid Non-conducting Films on the 

 Metallic Surfaces. 



I. Introductory. 



1. The experimental investigation described in this communica- 

 tion had as its primary object the elucidation and measurement of 

 the variations of Volta contact electricity of a pair of conductors, due 

 to changes in ihe state of that portion of the surface of each 

 conductor which was separated from the other conductor by an insulating 

 medium. 



2. The discovery of contact electricity of dry metals in air by 

 Yolta at the beginning of the present century, extended a quarter of 

 a century later by Pfaff to dry varnished metals in other gases, has 

 been confirmed by many subsequent experimenters. The reality of 

 the electrostatic force in air near an interface between copper and 

 zinc, inferred as an obvious consequence from it by Lord Kelvin, was 

 experimentally demonstrated by him in 1861*. In the next twenty 

 years many investigations were made, the more important being 

 those of Hankel, Gerland, Clifton, Ayrton and Perry, and von Zahn. 

 In 1881 a paper of great importance was published by M. Pellat,f 

 and as the present communication is, in some respects, only an 

 amplification and extension of his work, it may be of advantage to 

 give a short resume of it before going further. 



M. Pellat's most important results appear to be (1) his demonstra- 

 tion of the influence of the physical condition of the metallic surfaces 

 on their Yolta-potential ; thus he found that a sharply scratched 

 plate is positive to a more smoothly polished one of the same metal, 

 the metals being washed with alcohol after polishing in both cases 

 and allowed to dry before the measurement of their potential ; (2) 



* ' Electrostatics and Magnetism,' 400 ef seq. 

 f ' Ann. Chim. Phys.,' 1881. 

 VOL. LXIII. K 



