552 Lord Kelvin [May 21, 



matter to be found in Hankel's paper. I have made many experi- 

 ments from time to time since 1861 by the same method, but have 

 obtained results merely confirmatory of what had been published by 

 Pfatf 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 different metals 

 which Hankel had published ; and I have therefore hitherto published 

 nothing except the slight statements regarding contact electricity 

 which appear in my ' Electrostatics and Maguetism.' As interest has 

 been recently revived in the subject of contact electricity, the follow- 

 ing description of my method may possibly prove useful to experi- 

 menters. 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 Mr. H. 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, within a metallic sheath, 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 one- 

 fiftieth of an inch to about a quarter or half an inch. This part of 

 the apparatus I have called a " Yolta-condenser." The lower plate, 

 which was the insulated one, was fixed on 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 in the upper part of the sheath. An adjustable screw on 

 this stem prevents the upper plate from being let down to nearer than 

 about one-fiftieth of an inch, or whatever shortest distance may be 

 wanted in any particular 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 in my original experiments an ordinary Daniell's 

 cell, in my later ones a gravity Daniell's cell of the form which I 

 described in ' Proc. E.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 insu- 

 lated homogeneous metals in the apparatus. 



Connections. — The insulated plate was connected by a brass wire 

 passing through the case of the Volta-condenser to the electrode of 

 the insulated pair of quadrants. The upper plate was connected to 

 the metal sheath of the Volta-condenser, and to the metal case of the 

 electrometer, one pair of quadrants of which were also connected to 

 the case. One of the two terminals of the divider, connected to the 

 poles of the cell, was connected to the case of the electrometer. To 

 the third terminal (the bar carrying the slider) was attached one of 

 the contact wires, which was a length of copper wire having soldered 



