Quote:
Originally Posted by HarryT
If you want to divert a magnetic field (you can't block it, but you can divert it) you need to use a ferromagnetic material: ie something containing iron, nickel, or cobalt. A thin steel plate would work, but copper or aluminium ("tin") foil will not.
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On the other hand, anything thick enough, even paper or styrofoam, can reduce the effect not by weakening or diverting the field, but by simply increasing the distance. As far as I know, magnetic effects are quite sensitive to distance (and anyone who plays with magnets can experience that), so a tiny increase in the distance between the cover magnet and the reader can make a difference.