Question about unreadable credit cards

[BTox]

One for the physicists,electrical engineers and/or computer scientists - on occasion when I swipe my credit card at the supermarket, it comes up "unreadable". The kid at the register hands me a plastic bag, I put the card in the bag and swipe again, and it works. What's going on? Something to do with static electricity is my only guess, but I would think the plastic bag would exacerbate the problem.

 

[Goshawk]

http://money.howstuffworks.com/credit-card3.htm

If the ATM isn't accepting your card, your problem is probably either:

A dirty or scratched magstripe

An erased magstripe (The most common causes for erased magstripes are exposure to magnets, like the small ones used to hold notes and pictures on the refrigerator, and exposure to a store's electronic article surveillance (EAS) tag demagnetizer.)

The problem with your card is probably "dirt", which the baggie covers up.

 

[arcticpenguin]

I have never heard of this, but could come up with possibilities.

Perhaps the reading of the card is marginal, for whatever reason, and swiping it several times would result in a good read regardless of the presence of the bag (i.e. the bag use is superstitious)

Perhaps the thickness of the bag improves the spacing between the magnetic strip on your card and the read head.