494 REPORT — 1880. 



' TUESDAY, AUGUST Z\. 



The following Reports and Papers were read : — 



1. Report of the Committee for commencing Secular Experiments on the 

 Elasticity of Wires. — See Reports, p. 61. 



2. On the Elasticity of Wires. 

 By J. T. BoTTOMLET, M.A., F.B.S.E. 



3. Report of the Committee on the Specific Inductive Capacity of a good 

 Sprengel Vacuum. — See Reports, p. 197. 



4. On a method of measuring Contact Electricity. 

 By Professor Sir William Thomson, M.A., F.B.8. 



In my reprint of Papers on ' Electrostatics and Magnetism,' section 400 (of date 

 January 1862), I described briefly tliis method, in connection with a new physical 

 principle, for exhibiting contact electricity by means of copper! and zinc quadrants 

 substituted for the uniform brass quadrants of my quadrant electrometer. I had 

 used the same method, but with movable discs for the contact electricity, after the 

 method of Volta, and my own quadrant electrometer substituted for the gold-leaf 

 electroscope by which Volta himself obtained his electric indications, in an extended 

 series of experiments which I made in the years 1859-61. 



I was on the point of transmitting to the Royal Society a paper which I had 

 written describing these experiments, and which I still have in manuscript, when I 

 found a paper by Hankel in Poggendorif's ' Annalen ' for January 1862, in which 

 results altogether in accordance with my own were given, and I withheld my paper 

 till I mio'ht be able not merely to describe a new method, but, if possible, add some- 

 thino- to the available information regarding the properties of matter to be found in 

 Hankel's paper. I have made many experiments from time to time since 1861 by 

 the same method ; but have obtained results merely confirmatory of what had been 

 published by Pfaff in 1820 or 1821, showing the phenomena of contact electricity 

 to be independent of the surrounding gas, and agreeing in the main with the 

 numerical values of the contact differences of difierent metals which Hankel had 

 published ; and 1 have therefore hitherto published nothing except the slight state- 

 ments regarding contact electricity which appear iu my ' Electrostatics and Magne- 

 tism.' As interest has been recently revived in the subject of contact electricity, 

 the foUowin"- description of my method may possibly prove useful to experimenters. 

 The same method has been used to very good effect, but with a Bohnenberger 

 electroscope instead of my quadrant electrometer, in researches on contact electricity 

 by Monsieur II. Pellat, described in the ' Journal de Physique ' for May 1880. 



The apparatus used in these experiments was designed to secure the following 

 conditions : — To support two circular discs of metal about four inches in diameter 

 in such a way that the opposing surfaces should be exactly parallel to each other 

 and approximately horizontal ; and that the distance between them might be 

 varied at pleasure from a shortest distance of about ~ of an inch to about a 

 quarter or half an inch. The lower plate, which was the iusidated one, was fixed 

 in a glass stem rising from the centre of a cast-iron sole plate. The upper plate 

 was suspended by a chain to the lower end of a brass rod sliding through a steady- 

 ing socket iu the upper part of the case. A stout brass flange fixed to the lower 

 end of this rod bears three screws, one of which, S, is shown in the drawing, by 

 which the upper plate can be adjusted to parallelism to the lower plate. The other 

 apparatus used consisted of a quadrant electrometer and a gravity Daniell's cell of 

 the form which I described in ' Proc. R. S.' 1871 (pp. 253-259) with a divider by 

 which any integral number of per cents, from to 100 of the electromotive force 

 of the cell could be established between any two mutually insulated homogeneous 

 metals in the apparatus. 



