XX. Experiments on contact electricity between non-conductors

It was observed that when a plate of copper was lifted from a plate of glass the copper was electrified, and also that when a plate of glass was lifted from a plate of wax the glass was electrified, care being taken to have as little friction as possible; it was afterwards found that the former experiment had already been made by Fechner (see Wiedemann’s ‘Gal-vanismus, page 21), who also tried lifting copper from sulphur and got the same effect; although the plates were lifted as carefully as possible, yet- it was not certain that friction had been entirely got rid of, so the following experiments were made to show that there is an electrical displacement when two non-conductors or a conductor and a non-conductor are put in contact without friction. The arrangement used was as follows:— Glass rods, AB, CD, EF, GH, were fixed in a wooden frame ACGE; round these rods silk threads, BF, DH, were wound; an aluminium needle carrying a mirror, M, was hung by a silk thread from a brass rod, T, fastened in the wooden frame; a wire from the needle dipped into a glass vessel, N, containing sulphuric acid; a small magnet was fastened to the back of the mirror, and a glass case wTas placed over the whole; outside the glass case were magnets, by means of which the position of the needle was regulated; a wire also from the outside dipped into the vessel N, and was used to charge the needle with electricity; positive electricity was got from an ordinary electrophorus, negative from an electrophorus in which the resin was replaced by a plate of glass which was excited by silk. If wax and glass were the substances experimented on; a cake, OQRP, was made, one half of which, OSQ, was glass, the other half, RPS, being wax; the junction of the wax and glass was parallel to OQ, the wax sticking fast to the glass: this cake was then placed on the silk threads under the needle, and it was found possible to bring the needle into such a position that when it was charged with positive electricity it was deflected from the glass part of the cake, when charged with negative it was attracted towards it. In order to get rid of any electricity which might have got on the cake in the making, the cake was made the day before it was placed on the threads, and the experiment was made at least a day, sometimes a week, after putting the cake on the threads; pieces of glass and sulphur which had been treated in as nearly as possible the same way as those of which the cakes were made were taken and placed separately on the threads, but no electricity could be detected on them.