With reference to the figure, a position detector comprises a r ow of magnetic balls (1a) contained in a tube (1) in contact with one another and the tube (1) being mounted on a first member (2) of a machine. A transducer (5) and pickup coils (6) are mounted on a second member (7) of the machine. The first and second members (2, 7) are movable relative to one another. The transducer (5) transmits a periodic magnetic field and the pickup coils (6) sense phase displaced variations produced in the magnetic field as a result of relative movement between the first and second members (2, 7). The ratio of the distance between the line of point contact of the balls (1a) and the pickup coils (6) to the diameter of the balls (1a) is in the range of 1.25:1.37 in order to optimise the accuracy of measurement of the detector.
