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Suppose I have a standard credit card with a magnetic stripe, lying

  1. right on top of a cellphone, or
  2. in a regular leather wallet that's lying right on top of a cellphone.

The cellphone is on (but not in a call), uses a 3G network, and is in a good reception area.

Will the credit card be demagnetized (to the point where it can't be read) in the above 2 scenarios? If so, how long will it typically take? An answer that shows all assumptions and calculations used would be especially useful!

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  • 3
    \$\begingroup\$ Note that a cell phone that is "not in a call" is still transmitting occasionally. \$\endgroup\$ – gbarry Sep 4 '12 at 19:14
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Mythbusters did this: http://en.wikipedia.org/wiki/MythBusters_%28season_1%29#Eelskin_Wallet

If I recall correctly, they tried rubbing cards with magnets and cellphones, but failed to cause any damage.

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  • \$\begingroup\$ Interesting, I'll try to find the episode... But rubbing a card with a cellphone may be very different from leaving a card on top of a cellphone for a few hours. \$\endgroup\$ – Eugene Osovetsky Oct 26 '10 at 15:20
  • \$\begingroup\$ They tried many things that did nothing. It was quite funny because they were writing the cards themselves and trying all kinds of things. \$\endgroup\$ – Kortuk Oct 26 '10 at 20:06
  • \$\begingroup\$ On the other hand, the vibrator in a cell phone is generally a tiny motor. Whether its field is strong enough to have an effect I have no idea, but I don't think I'll experiment with my own card! If someone tries this in a rigorously experiment, do let us know! \$\endgroup\$ – mickeyf_supports_Monica Sep 5 '12 at 3:44
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No, you're fine. However a GSM phone could knock out a 3.5" floppy disk - I learned that lesson the hard way a long time ago.

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The answer is “no.” Generally, to demagnetize a credit card, it would take a 1,000 gauss magnetic field. A cell phone's magnetic field intensity ranges from just 1.2 to 10 milligauss.

http://members.questline.com/Article.aspx?articleID=17783&accountID=1285&nl=10213

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I don't know about credit cards but last year I stayed at a hotel with key cards and I carried the card in my pocket with the cell phone and it disabled the key card. At least that is what the desk clerk said when I had to go down and get another one. It worked for a while before I did that so it wasn't the card. I have been careful since then.

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  • \$\begingroup\$ I can kill hotel cards by just looking at them. Swipe cards are a poxy solution to the problem. I think it's because they're being rewritten many times. I had one programmed at reception that failed by the time I got to my room door... \$\endgroup\$ – Al Bennett Sep 5 '12 at 15:29
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Magnetism tends to exist in "domains", which are little collections of magnetized molecules. These domains tend to stay stable until sufficient energy overcomes them. This means that the information in the magnetic stripe will resist being changed until exposed to a strong enough field. So, the answer to the "how long" question becomes, either "instantly", or "not at all".

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If you're VERY unlucky, and if you're talking about the magnetic stripe, yes, it could be rendered unreadable. But the radio is not the problem: speakers in the phone have permanent magnets. Vibration motors have permanent magnets, the case of the phone is really thin and does not shield at all magnetic fields. A credit card in the wrong place at the wrong moment and for enough time eventually will be ruined.

FWIW, it really happened to my mother, three times! Her pocket purse compartments were arranged in a way that put her phone motor exactly in the magnetic stripe center. Three ATM cards rendered useless in less than a month.

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