A device for sensing the relative rotary position of first and second parts about a rotation axis, the device comprising a follower constrained to move on a first track fast with the first part and on a second track fast with the second part, the first track being linear and the second track comprising a plurality of circular arcs and at least one transition section connecting one of the circular arcs to another, the tracks being arranged so as to convert relative rotation of the parts into linear motion of the follower, wherein the second track is generally spiral, each circular arc is of constant radius about the rotation axis and the first track is perpendicular to the rotation axis.