Abstract: A detector for measuring relative movement and/or displacement between two relatively movable members comprises a linear array of steel balls, or other suitable longitudinal element of which the magnetic permeability varies cylically in a longitudinal direction, mounted on one of the members, and a transducer on the other member. The transducer is arranged to produce a magnetic field through the balls, preferably across the line thereof, and includes at least two pick-up coils spaced along the line of balls for sensing the variation in the magnetic field due to the presence of the balls, so that the relative spacing of the pick-up coils and the balls enable, on production of the magnetic field, the generation by the pick-up coils of output signals denoting the relative movement between the two members.