The effect of the presence of the mass of copper is to damp the vibrations of the needle. Each swing takes the same time as before, but the amplitudes are lessened; the motion dying down, as though there were some invisible friction at work. Arago remarked that it gave evidence of the presence of a force which only existed whilst there was relative motion between the magnet-needle and the mass of copper. He gave the phenomenon the name of magnetism of rotation. In 1825 he published a further experiment, in which, arguing from the principle of action and reaction, he produced a reaction on a stationary needle by motion of a copper disk (Fig. 1). Suspending a compass-needle in a glass jar closed at the bottom by a sheet of paper or of glass, he held it over a rotating disk of copper. If the latter turns slowly the needle is simply deviated out of the magnetic meridian, tending to turn in the sense of rotation of the disk, as though invisibly dragged by it. With quicker rotation the deviation is greater. If the rotation is so swift that the needle is dragged over 90° a continuous rotation ensues. Arago found, however, that the force was not simply tangential. Suspending a needle vertically from the beam of a balance over the revolving disk he found that it was repelled when the disk was revolved. The pole which hung nearest the disk was also acted upon by radial forces tending, if the pole was near the edge of the disk, to force it radially outward, but if the pole was nearer the center, tending to force it radially inward.