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I've on occasion seen when a cashier has trouble getting a credit card to read, will wrap it in a thin plastic grocery bag, and try swiping it again. Much to my surprise, it seemed to work.

Since the card uses a magnetic strip, I can't see how the plastic would help. Is there any reason it should, or is this a case where it worked, but not because of the plastic?

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migrated from Mar 22 '11 at 21:10

This question came from our site for scientific skepticism.

up vote 22 down vote accepted

The best explanation I have seen for this is that the plastic bag increases the distance between the read head and the magnetic strip. As the magnetic strip is used, small particles break off and become embedded elsewhere on the stripe. These have very little magnetic force, but enough that they throw off today's more sensitive readers, even though they're embedded in a larger area of stronger magnetism.

By increasing the distance between the read head and the mag stripe, even though it's just the thickness of the plastic bag, the smaller "noisy" bits impart a smaller field on the sensor than the larger areas of magnetic material surrounding them. It increases the signal to noise ratio by placing a low pass filter between the strip and the read head.

In other words, it's similar to putting on earmuffs so you can only hear the bass line of a piece of music, muffling the midrange, and almost muting the high range frequencies.

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I was ready to verdict this question as a myth. Your answer is convincing. +1 – Georg Mar 22 '11 at 21:36
So why aren't cards laminated? – Tom Gullen Mar 23 '11 at 12:51
@Tom They are, but the lamination thickness is measured on the order of thousands of an inch, and even if it were thicker it would eventually wear out. – Adam Davis Mar 23 '11 at 12:56
@Peter Alternately, one would use an oscilloscope to quantify the signal directly off the read head in the failing scanner vs the signal on a good scanner with a variety of cards, failing, not failing, covered, uncovered. We should be able to gather a lot of good data regarding what kind of signal the bad cards present to the two different readers, what the bag changes about that signal, and discern what the mechanism is that causes this to work. I strongly suspect that the "better" readers have a better signal processing chain that performs the same function as the bag. – Adam Davis Mar 23 '11 at 13:03
@Adam I suppose so. Card manufacturers must have process engineers, perhaps even physicists, but I wonder whether part of the specification for cards is "will wear out after two to three years of normal use for the sake of our bottom line". On the other side, the credit card company wants this cost center under some sort of control, and doesn't want customers to hate their cards failing too often. Your response to Tom pushed me over the fence anyhow. I'd been holding out on the +1, like a dork, but your Answer is obviously a useful starting point. Send Adam your credit cards! – Peter Morgan Mar 23 '11 at 13:09

protected by Qmechanic Jun 2 '14 at 14:50

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