As has so often occurred in other branches of science, the discovery of the magnetic rotations was made nearly simultaneously by several persons, for all of whom priority has been claimed. About 1824, Gambey the celebrated instrument-maker of Paris, had made the casual observation that a compass-needle, when disturbed and set oscillating around its pivot, comes to rest sooner if the bottom of the compass-box is of copper than if it is of wood or other material. Barlow and Marsh, at Woolwich, had at the same time been observing the effect on a magnetic needle of rotating in its neighborhood a sphere of iron. Arago the astronomer, who is said to have learned of the phenomenon from Gambey, but who is also said to have independently discovered it in 1822, when working with Humboldt at magnetic determinations, was beyond question the first to publish an account of the observation, which he did verbally before the Académie des Sciences of Paris, on November 22, 1824. He hung a compass-needle within rings of different materials, pushed the needle aside to about 45°, and counted the number of oscillations made by the needle before the angle of swing decreased to 10°. In a wooden ring the number was 145, in a thin copper ring 66, and in a stout copper ring it was only 33.
