anti tampering means according to the present invention will depend to a great extent upon the nature of the product and the arrangement of the anti tampering means that is provided thereon the detector could advantageously be microprocessor based with an ability to learn by experience what is to be considered in any particular application to be a normal and what an abnormal condition for a hand held detector the magnetic field sensor s preferably might be provided at the end of a wand or the like arranged to be passed over the protected products having described the invention in the foregoing by reference to specific embodiments it is to be appreciated that the described embodiments are exemplary only and that modifications and variations thereto are possible without departure from the scope of the invention as set forth in the appended claims for example whereas in the foregoing the magnetic strips 7 in the embodiment of fig1 and the magnetic strips 24 in the embodiment of fig2 are shown as being parallel to each other each of the strips 7 and 24 could be accompanied by a further transversely oriented strip arranged to provide a characteristic detectable magnetic signature indicative that the closure is an authentic application of the present invention for example in a situation where breakage of the strips 7 24 was designed to produce a detectable stray field the additional transverse strips could be.