April 14, 1 881] 



NA TURE 



567 



ON A METHOD OF MEASURING CONTACT 



ELECTRICITY^ 

 T N my reprint of papers on electrostatics and magnetism 

 (g 400, of date January, 1S62) I described briefly this 

 method, in connection with a new physical principle, for exhibit- 

 ing contact electricity by means of copper and zinc quadrants 

 substituted for the uniform bras< quadrants of my quadiant 

 electrometer. I had used the same method, but with movable 

 di;ks 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 indi- 

 cations, in an extensive series of experiments which 1 made in 

 the years 1S59-61. 



I was on the point of transmitting to the Royal S ociety a paper 

 which I had written describing these experiments, and which I still 

 have in manuscript, when I found a paper by Ilankel in Pog- 

 gendorff's Annalen forjanuirv, 1862, in which results altogether 

 ill accordance with my own were given, and I withheld my paper 

 till I might be able not merely to describe a new method, but if 

 possible add something to the valuable information regarding 

 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 1 82 1, showing the 

 phenomena of contact electricity to be independent of the sur- 

 rounding 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 Magnetism." As interest has 

 been recently revived in the sabject of contact electricity, the 

 following 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 Mr. 

 H. Pellat, described in the fournnl de Physique for May 18S0. 



The apparatus used in these experiments was designed to 

 secure the following conditions : — To support two circular disks 

 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 " Volta-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 steadying socket in 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 showni in the drawing, by which 

 the upper plate can be adjusted to parallelism to the lower plite. 

 The other apparatus used consisted of a quadrant electrometer, 

 and in my original experiments an ordinary Daniell's cell, in my 

 later one; a gravity Daniell's cell of the form wdiich I described 

 in Ptvc. R.S. 1871 (pp. 253-259) with a divider by which any 

 integral number ol per cents, froji o to 100 of the electi'omotive 

 force of the cell could be es-ablished between a ly two mutually 

 in mlated homogeneous metals in the apparatus- 



Connections. — The insulated plate was connected by a brass 

 wire passing through the cise of the Volta-condenser to the 

 electrode of the insulated pair of quadrants. The upper plate 

 was connected to the metal case 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 terminals of 

 the divider, which connected the poles of the cell, was connected 



to the case of the electrometer, and to the other terminal was 

 attached one of the contact wires, which was a length of insu- 

 lated copper wire having soldered to its outer end a short piece 

 of platinum. The other contact surface was a similar short 

 piece of platinum fixed to the insulated electrode of the electro- 

 meter. Hence it will be seen that metallic connection between 

 the two plates was effected by putting the divider at zero and 

 bringing into contact the two pieces of platinum wire. 



Order of Experiment. — The sliding piece of the divider was 

 put to ze-o, and contact made and broken and the upper plate 

 raised : then the deflection of the spot of light was observed. 

 These operations were repeated with the sliding piece at different 

 numbers on the divider scale until one was found at which the 

 make-break and separation caused no perceptible deflection. 

 The number thus found on the divider scale was the number of 

 per cents, which was equal 'o the contact electric difference of 

 the plates in the Volta-condenser. 



{Addendum, November 23, 1S80. — Since the commanicatic 

 of this paper to the British Association, I have found that a dry 

 platinum disk, kept for some time in dry hydrogen gas, and 

 then put into its position in dry atmospheric air in the 

 Volta-condenser, beomes positive to another platinum disk 



