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.

Abstract: A detector for measuring relative movement and/or displacement between two relatively movable members comprises a plurality of regularly shaped elements of magnetic material e.g. steel balls, periodically spaced along a path which is preferably linear, and a transducer which is movable relative to the elements in a direction parallel to said path. The transducer comprises a transmitter coil, or series of coils, disposed for producing a magnetic field parallel to said path, and at least two pick-up coils encasing the path and displaced along it. The pick-up coils sense the variation in the magnetic field due to the presence of the elements, so that the relative spacing of the pickup coils and the elements enable, on the production of the magnetic field, the generation by the pick-up coils of output signals denoting the relative movement between the members.