to limit the abrasion of the magnets 10 against the test material 2 u2014 the underside of the magnets 10 may make contact with the pipeline 2 or may be spaced therefrom by a very small gap the magnetiser 6 includes a housing 14 containing the magnets 10 and which allows convenient attachment of the magnetiser 6 by a first linkage 16 to a supporting structure 18 whereby the magnetiser 6 can be pulled along the test surface in a direction perpendicular to that of the array of magnets 10 the electrical winding assembly 8 comprises a set of c cores 20 the coils of which are interconnected with one another to form a single continuous winding 22 the cores 20 are mounted on a wear plate 24 of electrically insulating material adapted to engage the surface of the pipeline 2 the plate 24 which may be several millimetres thick whilst having negligible effect on the acoustic efficiency of the device when operating at frequencies useful for pipe inspection protects the cores 20 from the material surface the assembly 8 includes a housing 26 which screens the cores 20 against electromagnetic interference which is important when receiving acoustic signals and which enables the assembly 8 to be readily connected to the supporting structure 18 by means of a second linkage 28 the assembly 8 is thus positioned rearwards of the magnetiser 6 with the cores 20 in a row perpendicular to the anticipated direction.