On electrical and magnetic rotations

Abstract

The author first recapitulates the manner in which he conceives time to influence the results of the magnetic phenomena observed by M. Arago, and which need not here be repeated, being in substance that given in a paper on the subject, in the Transactions of last year, stated in a more geometrical form. As the reasoning in this argument requires only that an attractive or repulsive force should be communicated from one body to another in a finite time, it occurred to him that electricity might be substituted for magnetism, and that rotations analogous to those observed by M. Arago might be produced by the use of electrified instead of magnetic bodies. He accordingly suspended by a fine silk thread a thin brass bar with circular ends over a disc of glass ; and the bar being electrified by contact with excited sealing-wax, the glass was made to revolve slowly, when the bar was observed to be dragged round in the same direction. The effect was decided; and all proper precautions were taken to avoid disturbing causes, such as currents of air, twist of the silk, &c. The effect was greatest with a slow velocity of rotation, about five turns in a minute. On substituting a stick of excited sealing-wax for the brass bar, the same effect was produced; but when the rotation of the plate was rapid, the stick remained nearly immoveable. The same effect was produced when the glass plate was covered with plates of copper, lead, or other metals cemented to it. A proper apparatus being constructed for the purpose of further experiments, an excited electrophorus was made to revolve under a flat needle of thin brass with circular ends, with various degrees of rapidity. The motions of the needles were irregular and complicated, but appear to the author capable of explanation, as well as the others on the same principle,—that electricity excited by induction is not instantly destroyed by removing the inducing body.