Mr. F. S. Spiers on Contact Electricity. 71 



One of these is by Dr. J. T. Bottomley, F.R.S.*, and the 

 other by Dr. J. Erskine- Murray, F.R.S.E.f Dr. Bottomley 

 merely measured the Volta contact-difference between two 

 disks of zinc and copper in high vacua of air and hydrogen, 

 the pressure being less than 3^3 mm. of mercury, and showed 

 that on readmitting air to atmospheric pressure no variation 

 of the P.D. as great as 1 per cent, ensued. Seeing that 

 Dr. Bottomley did not so much as warm his vacuum-tube, 

 there is not much reason to suppose that the result would 

 have been otherwise than what it actually was. Dr. Erskine- 

 Murray, who quite realized the possible importance of the 

 condensed air-films in the production of the Volta phenomena, 

 attacked the problem rather more indirectly ; but his method 

 of removing the films by filing the surfaces of the metals 

 under melted paraffin-wax cannot, I think, be considered as 

 entirely satisfactory. In the first place it ignores the probable 

 electrolytic action of the wax, or of the moisture in the wax, 

 for apparently no precautions were taken to dry it ; and oven 

 if these effects be disregarded, I believe it improbable in the 

 highest degree that merely filing off the surface-layer of metal 

 is sufficient to remove these condensed air-sheets. Why, the 

 file itself is covered with a film of air ! 



§ 2. It seemed, therefore, that in spite of the enormous 

 quantity of experimental work that this controversy concerning 

 the contact effect had brought forth, there was still room for a 

 crucial and decisive experiment which should go to the very 

 root of the matter, and finally satisfy all conflicting parties. 

 It was with this ideal, possibly unattainable like all true ideals, 

 in view, that I began experiments in this field some two 

 years ago. 



§ 3. I will now proceed to describe in detail the various 

 forms of apparatus that were devised for these investigations, 

 and give a short account of the experiments that were made 

 with each. As a rule, only the outlines and results of each 

 experiment will be given ; consequently but a small proportion 

 of all the observations taken will be recorded here. 



§ 4. Some preliminary experiments were made with 

 the apparatus sketched in fig. 1, which was made for 

 Profs. Ayrton and Perry about twenty years ago. In this 

 they adopted the same principle as that used by them in the 

 exhaustive series of contact E.M.F. measurements they made 

 in Japan J. In the figure A and B represent the contact-couple, 

 in this case zinc and platinum. They are metallically con- 



* B. A. Report, 1885. 



t Proc. Roy. Soc. 1898, vol. lxiii. p. 113, 



X See Proc. Roy. Soc. 1877 & 1878 ; Phil. Trans. 1880. 



