Apparatus is disclosed for generating a signal representative of a co-ordinate position of a movable indicating member, the member being constructed for interacting magnetically with sensing coils. The apparatus includes a plurality of sensing coils (2 to 11) for interacting magnetically with such a member, the coils being positioned substantially in or adjacent to a real or imaginary surface in or adjacent to which such an indicating member may be moved, the sensing coils being positioned to divide an area of interest in the surface into regions, each of the coils encircling part of the area of interest and having conductors in the area substantially along region boundaries only, each of the regions being within an arrangement of the sensing coils particular to that region. A further coil (1) is positioned substantially in or adjacent to the real or imaginary surface and surrounds the entire area of interest. There are a first one (2) and a second one (3) of the sensing coils which have conductors at substantially the same pitch but which coils are offset relative to each other. Processing means (16, D2 to D11, 17, G2 to G11 and L1 to L9) includes an exclusive-OR gate (15), an output of each of the first and second ones of the sensing coils being coupled only to a respective input of this gate. The processing means uses the output signal of the exclusive-OR gate and other signals derived from the sensing coils with reference to the signal in the further coil to produce a binary coded signal representative of a co-ordinate position of such a member.