Why does putting a card in a plastic bag help it read at checkout?

[cragar]

Sometimes when i swipe my card at walmart it doesn't read it , And the guy was like let's put it a plastic bag and then try it, and it works and I've heard a couple of other people doing this.
So when i slide my card the magnetic strip induces a voltage and reads the card. So why does putting it in a plastic bag work. Does this change the relative permittivity of the material.
Does the bag just add more di-electric material so the magnetic field interacts with the detector in the card reader like it should and not too strongly.
Any input will be much appreciated.

 

[Danger]

This is one of those things that I would have considered an "urban myth" if I had not experienced it myself. The cashier's explanation makes more sense than most that I've heard (and not just because she's a friend of mine). The readers collect crud from dirty cards when they're swiped. When enough has accumulated, it interferes with the reading process. The plastic (or cellophane tape, or whatever) added to the card acts as a "head cleaner" like you use in a VCR or DVD player. It scrapes the crap off of the heads prior to the mag stripe passing, and so allows reading. Further evidence of that is that once it has been done, subsequent purchasers don't have a problem.
As a side-note to that, my card would never work at the ATM terminal in one of my local gas stations. The cashier there said that it hardly ever worked, because it was constantly contaminated with petroleum fumes.

 

[cragar]

i see thanks for the response .

 

[MagnetDave]

Different analogy:

At 5:00AM I did a dance to make the sun rise. It didn't work. I sat down, and scratched my head and then tried it again at 6:30AM while wearing a headband, and, lo and behold, the sun rose. I therefore conclude that I was right to don a headband, and headbands make the sun rise. The popular use of headbands throughout the world prove my thesis.

Stated differently: the change you deliberately made is not necessarily the complete sum of changes made, therefore you can't assume the bag worked. The fact you can't think of a means by which it could work is probably indicative.

 

[Danger]

The fact you can't think of a means by which it could work is probably indicative.

Have you been paying attention? I just outlined a very plausible means by which it could work. I don't know for sure that it's the correct answer, but certainly consider it possible.