A small quantity of the magnetisable liq. (2') is enclosed in a hollow torus (3') of circular cross-section in a vertical plane. At its lowest point the torus (3') passes through the centre of the bob of a pendulum (1) suspended from a point (M) at the centre of the torus. A ring magnet (9) surrounding the bob confines the magnetic liq. to a limited region of the torus (3'), the remainder of which is filled with a different liq. (10) whose viscosity exerts a force on the magnetic liq. opposing any movement of the pendulum (1) in either direction.
