XXI. Account of the repetition of M. Arago's experiments on the magnetism manifested by various substances during the act of rotation

Abstract

1. The curious experiments of M. Arago described by M. Gay Lussac during his visit to London in the spring of the present year, in which plates of copper and other substances set in rapid rotation beneath a magnetized needle, caused it to deviate from its direction, and finally dragged it round with them, naturally excited much attention, and the investigation of their various circumstances, and of their connexion with the effects observed by Mr. Barlow in December, to be produced by the rotation of masses of iron, and described by him in a paper read to the Society, became an object of considerable interest. Accordingly, having erected at Mr. Babbage's house, in Devonshire-street, an apparatus for setting a copper plate in rotation about a vertical axis by the aid of a turning lathe, we proceeded to try its effect on a magnetized needle suspended over it. The first attempt failed from the use of too small a needle ; but this being replaced by a magnetic bar of considerable weight delicately suspended by a silk thread, we had the satisfaction of seeing it deviate several degrees from its point of rest in a direction corresponding with that of the rotation of the copper plate; and on employing instead of this bar, a very delicate azimuth compass, belonging to and the invention of Captain Kater, the influence of zinc, brass, and lead was similarly rendered sensible. 2. In this first trial, having neither the command of a very rapid rotation, nor of massive metallic discs, the deviation of the compass observed did not exceed 10 or 11 degrees. In order therefore to enlarge the visible effect, and at the same time disencumber ourselves of the limit set to it by the polarity of the needle, it occurred to us to reverse the experiment, and ascertain whether discs of copper or other nonmagnetic substances (in the usual acceptation of the word) might not be set in rotation if freely suspended over a revolving magnet. In order to make this experiment, we mounted a powerful compound horse-shoe magnet, capable of lifting 20 pounds, in such a manner as to receive a rapid rotation about its axis of symmetry placed vertically, the line joining the poles being horizontal and the poles upwards. A circular disc of copper, 6 inches in diameter and 0.05 inch thick, was suspended centrally over it by a silk thread without torsion, just capable of supporting it. A sheet of paper properly stretched was interposed, and no sooner was the magnet set in rotation than the copper commenced revolving in the same direction, at first slowly, but with a velocity gradually and steadily accelerating. The motion of the magnet being reversed, the velocity of the copper was gradually destroyed; it rested for an instant, and then immediately commenced revolving in the opposite direction, and so on alternately, as often as we pleased.