Abstract: A depth measuring device is used for measuring the depth of insulation on an electric conductor travelling along a predetermined path. The device includes a cylindrical magnetizable core (11) which generates a magnetic field having an axis directed to intersect the path. The core (11) is magnetized by an electric coil (12) coupled to the core (11) and the current drawn by the coil (12) will vary with the distance of the conductor from the core (11) and so provides an indication of the distance of the conductor from the core (11). A laser beam (14) is directed along the axis of the core so as to lie coaxial with the magnetic field. A detector (15) detects light reflected from the outer surface of the insulation and so provides an indication of the distance of the insulation from the core (11). An electrical system subtracts the two measured distances from each other to provide an indication of the insulation thickness.