This invention relates to detecting magnetic discontinuities below the surface of magnetisable material using magnetic flux leakage techniques. 
It is known from U.K. Patent Specification 2188156 that discontinuities such as cracks or pits below the surface of a specimen of magnetisable material can be detected by magnetising the material and sensing variations in leakage field near the surface of the specimen.  The apparatus disclosed in U.K. Patent Specification 2188156 utilises an array of flux leakage sensors which are moved over the material surface in close proximity thereto.  At successive locations in the direction of movement of the array, signals produced by each of the sensors in the array are recorded and subsequently the recorded values compared using appropriate signal processing apparatus, with an "average" of values recorded for each respective sensor at sensor locations successively before and successively after that particular location.  The difference between the recorded value and the "average" gives an indication of magnetic discontinuity at a particular sensor location.  Disadvantages with the apparatus and method disclosed in U.K. Patent 2188156 are two fold.  Firstly, powerful signal processing apparatus is required to analyse the sensor output signals and produce a "grid reference" model corresponding to the area tested so that locations where a discontinuity has been detected by the sensors can be accurately related to their actual position over the actual area tested.  Secondly, arising from the need to compare sensor signals at specific locations with signals from the same sensor at successive locations, and the results subsequently processed, the apparatus cannot be used to give a "real time" indication of discontinuities in the surface under test.