Abstract: A displacement following device comprises a slider (1) mounted on a slideway (2) for movement along a linear displacement path. A beam (4) mounted on the slider (1) is rotatable about an axis of rotation (5) at its midpoint. A pair of stops (6, 7) spaced apart along the displacement path are fixed to the slideway (2) on opposite sides of the beam (4) and project into the path of movement of the beam. A tension spring (8) has one end connected (at 10) to the slider (1) and its other end connected to a point (9) between the axis of rotation (5) and one end of the beam. The spring (8) biases the beam (4) about its axis of rotation (5) into engagement with the stops (6, 7) so as to bias the slider (1) relative to the slideway (2) into a datum position from which the slider is bidirectionally displaceable along the displacement path.