An electrical coil driven by an alternating current develops an alternating magnetic field. A segment of a wire-rope specimen is placed next to and on the axis of the coil, but perpendicular to the axis, so that the specimen lies in the field. Two sensor coils detect the field on the opposite side of the specimen from the field coil. The sensor coils are mutually conaxial, and paraxial with the specimen. The sensor coils are spaced apart very slightly, along the length of the specimen-one in each direction from the field-coil axis. Each sensor coil is asymmetric, in the shape of a "D," with its sensitive axis just within the "D" but nearer to the flat side; the specimen is placed just outside the "D" but also next to the flat side. Alternating current induced in the sensor coils are applied to the respective input terminals of a differential amplifier, producing an output signal in which the common signal components tend to cancel out. The residual signal is accordingly insensitive to uniform characteristics of the specimen, but sensitive to local variations such as localized defects. The alternating output of the differential amplifier is sampled at an adjustable phase point, selected to vary the display mode for the specimen material at hand.
