It doesn't need a 'very powerful magnet'. In fact I'm surprised how easy it is to make the magnetic strip unreadable. The way I (unintentionally) do it is simpl

It doesn't need a 'very powerful magnet'. In fact I'm surprised how easy it is to make the magnetic strip unreadable. The way I (unintentionally) do it is simply to keep the cards together in the wallet. Their individual magnetic stripes destroy each other. All the cards are unreadable now, except for the newest card. And no, there aren't any other magnetic or electro-whatever device nearby.