Abstract: The device consists of a rigid pendulum (1) pivoted at its upper end (2) and upon which a ball (5) is retained at its lower end in a holder (6), so that the center of the ball lies on or close to the longitudinal axis of the pendulum. The holder itself is attached to a shoe (3) which pivots about a point (4) on a projection offset from the bottom end of the pendulum, such that the shoe moves in an arc which lies in a vertical plane perpendicular to the vertical plane in which the pendulum swings. Movement of the shoe about this pivot allows the ball holder (6) and thus the ball (5) to move in an approximately radial direction with respect to the pendulum pivot (that is, along the axis of the pendulum when the ball center is on that axis) and the degree of movement of the shoe about its pivot is limited by stops (7a, 7b) provided on the pendulum. The complete pendulum device is supported in a tripod framework or alternative rigid support.