Contact Electricity of Metals* 95 



a spiral wire by a long hook carrying also a small globe of 

 soft iron. Thus you see by aid of an external magnet I can 

 lift and lower the upper plate without moving the vacuum 

 tube, which, during the experiments, was kept in connexion 

 with a Sprengel pump and phosphoric acid drying-tubes. 

 Mr. Bottomley sums up thus : — " The result of my investiga- 

 tion, so far as it has gone, is that the Yolta contact effect, so 

 long as the plates are clean, is exactly the same in common 

 air, in a high vacuum, in hydrogen at small and full pressure, 

 and in oxygen. My apparatus, and the method of working 

 during these experiments, was so sensitive that I should 

 certainly have detected a variation of 1 per cent, in the value 

 of the Volta contact effect, if such a variation had presented 

 itself"*. 



§ 15. With the same method further researches have been 

 carried on by Mr. Erskine Murray, and important and 

 interesting results obtained, within the last four years, in the 

 Physical Laboratories of the Universities of Glasgow and 

 Cambridge. He promises a. paper for early communication 

 to the Royal Society, and, from a partial copy of it which he 

 has already given me, I am able to tell you of some of his 

 results. Taking generally as standard a gilt brass disc which 

 he found among the apparatus remaining from my experi- 

 ments of 1859-61, he measured Volta-differences from it in 

 terms of the modern standard one volt. These differences are 

 what we may call the Volta-potentials of the different metallic 

 surfaces, or surfaces of metallic oxides, iodides, &c, or 

 metallic surfaces altered by cohesion to them of gases or 

 vapours, or residues of liquids which had been used for 

 washing them ; if for simplicity we agree to call the Volta- 

 potential of the gold, zero. As a rule he began each experi- 

 ment by polishing the metal plate to be tested on clean glass- 

 paper or emery-cloth, and then measured its difference of 

 potential from the standard gold plate. After that the plate 

 was subjected to some particular treatment, such as filing or 

 burnishing ; or polishing on leather or paper ; or washing 

 with water, or alcohol, or turpentine, and leaving it wet or 

 drying it ; or heating it in air, or exposing it to steam or 

 oxygen, or fumes of iodine or sulphuretted hydrogen ; or 

 simply leaving it for some time under the influence of the 

 atmosphere. The plate as altered by any of these processes 

 was then measured for potential against the standard gold. 

 Very interesting and instructive results were found ; only of 

 one can I speak at present. Burnishing by rubbing it firmly 

 with a rounded steel tool, or by rubbing two plates of the 

 * Brit. Assoc. Report, 1885, pp 901-3. 



