Credit card bag swiping trick

March 5, 2007 2:50 PM

Why did this credit card swiping trick work?

Recently I was purchasing gasoline, and went inside to pay. The clerk who swiped my card was having difficulty getting it to read (the reader was built into the keyboard). One of his co-workers proceeded to place my card into a white plastic bag (of the type with "thank you" printed repeatedly in red ink- you've probably seen them) and swiped it through the reader, successfully scanning my card and allowing me to conclude the transaction.

I mentioned this casually to a friend and he said that at a restaurant he once worked at, plastic bags were attached to the computers because they wouldn't read many cards if you didn't bag them first.

My question: why does this work?

posted by baphomet to work & money (24 comments total)

The Straight Dope appears to have the answer here.

Standard bank cards use the F2F (Aiken biphase) modulation scheme, in which flux reversals encoded in the . . . eh, too much information. Let's just say that when you swipe the card through the reader, the magnetized particles in its stripe generate a signal with "ticks" in it at intervals that the machine is able to interpret as digital ones or zeros. A scratch or other defect in the magnetic stripe can cause a spike (i.e., brief fluctuation) in the signal that a too-sensitive reader will interpret as a tick, meaning that the encoded data will fail the parity check (the numbers won't add up right) and the card won't work. Wrapping the card in plastic increases the distance between the read head and the magnetic stripe, thus reducing the strength and crispness of the signal and smoothing out anomalous fluctuations. Behold, the card works.
posted by OpinioNate at 2:56 PM on March 5, 2007 [2 favorites has favorites]

I second OpinioNate. I live in a city where I can always pay by interac, and find myself swiping my card a dozen times a day (I barely carry cash anymore). If your card is scratched up and the reader is having trouble, the bag trick almost always works.
posted by billy_the_punk at 3:13 PM on March 5, 2007

wow. i love the things i learn on mefi
posted by mittenedsex at 3:14 PM on March 5, 2007

I once had a clerk put a piece of clear tape on my card so that it would work. When she went to remove it, I asked her not to take it off. I just left it on for the life of the card. I always worked.
posted by nimsey lou at 3:19 PM on March 5, 2007

I can confirm that this trick works, and I'm very glad to now understand why. :)
posted by Malor at 3:20 PM on March 5, 2007

I have one grocery store that has to use the "bag treatment" more than half of the time. It is very odd but it works for me.
posted by JayRwv at 3:20 PM on March 5, 2007

So it's akin to hiding scratches in a photo by blurring it slightly? Cool!
posted by nebulawindphone at 3:32 PM on March 5, 2007

data point: I tried this with a brown plastic bag, and it didn't work. This was the only time I've tried it.
posted by philomathoholic at 3:37 PM on March 5, 2007 [1 favorite has favorites]

What works for me, at a picky ATM in my work buildin: Rubbing the magnetic stripe on jeans.
posted by GaelFC at 3:51 PM on March 5, 2007

I worked retail for years, and my trick was to use a piece of receipt paper folded over a finicky credit card. That also worked reliably well, and was less cumbersome than a bag.
posted by kimdog at 3:54 PM on March 5, 2007

Rubbing the magnetic stripe on jeans.

I initially read that as "Rubbing the magnetic stripe on Jesus."
posted by matildaben at 3:56 PM on March 5, 2007

"Rubbing the magnetic stripe on Jesus."

that's only if the plastic bag doesn't work and you don't have any other method of payment.
posted by jerseygirl at 4:11 PM on March 5, 2007

wow. i love the things i learn on mefi

Ditto.
posted by ericb at 5:01 PM on March 5, 2007

For those interested in some of the details, this PDF shows what the electrical waveform looks like. The card reader detects the spikes between magnetic bits. There are either 75 or 210 bits per inch on three separate tracks. The middle track holds your bank card data in 40 characters on a 2.5 inch strip.

The peaks may be only 20 millivolts so it takes some sensitive hardware to detect. The ones and zeros are encoded by the distance between peaks -- a long distance is a zero and a half distance is a one. The decoder measures the time between peaks as you swipe the card and translates them to ones or zeros.

The tricky part is that the time between peaks depends on the speed that you swipe the card. So there are a string of zeros at the beginning of each track that the reader uses to measure the speed of your swipe and calibrate the bit time. Surprisingly, in practice, the bit timing from the beginning of your swipe to the end is generally very consistent. Some readers have sophisticated software and hardware that allows them to adjust the amplification of the signals on the fly by measuring the peaks, essentially doing the plastic bag thing for you. The hardware and software may also have filtering algorithms to remove spurious peaks. Most readers can read a card swiped in either direction. It is a simple thing for software to reverse the data. These differences in technical capabilities explains why your card may work at some stores and not others.
posted by JackFlash at 5:28 PM on March 5, 2007 [3 favorites has favorites]

data point: I tried this with a brown plastic bag, and it didn't work. This was the only time I've tried it.

I tried this once with a burlap sack, and nearly set the store on fire from all the friction.
posted by pdb at 6:03 PM on March 5, 2007

Thanks all, good stuff.

pdb: I tried it once with sandpaper and it was kind of a mess.
posted by baphomet at 6:15 PM on March 5, 2007

I should point out that this gets old after a while. I mean imagine waiting at every single checkout, watching the clerk try your card 5 times and then using the bag. Just call your bank/credit card company if this starts happening frequently and ask for a replacement. They usually send it in 2 business days at no charge.
posted by special-k at 7:01 PM on March 5, 2007

You know, I worked in retail as a cashier for 3 years in highschool, dutifully using this trick when appropriate, and never once bothered to look up how it worked, though I often wondered. Thank you for posting this!
posted by Alterscape at 7:07 PM on March 5, 2007

A friend of mine was paying for gas and the card wasn't working - the clerk grabbed it, licked it, swiped it, and it worked.
posted by Bud Dickman at 7:12 PM on March 5, 2007

wow. i love the things i learn on mefi

Ha. We use it at the bookstore all the time, but why it works has been an ongoing mystery. Thanks, AskMe. :)
posted by mediareport at 9:18 PM on March 5, 2007

So people slide them thru while inside the bag?! Never heard of or seen this though I worked retail for 18 years. We just used to the Scotch tape method--but remove the tape first; I always just thought it was thoroughly cleaning it and removing it of lint.

Further, the interesting thing about this bag trick is... how the hell do people know this? I assume some tech guy who works with the machines told one person and it spread but I can't believe how wide spread it seems to be. Weird.
posted by dobbs at 10:08 PM on March 5, 2007

I have also seen it down with the wax thermal receipt paper.

Dobbs: One possible way it can spread is that people who work in retail who know this trick go shopping, and when they have trouble with their own card, they tell the customer service rep there about the bag trick and it spreads like that.

For example:
My non-gold-chip having Mastercard works at stores in Europe, because for all intents and purposes, a Mastercard is a Mastercard. However, I usually have to explain that it must be swiped (as opposed to inserting the chip like a hotel keycard), and I have to sign one copy of the subsequent slip, because their Mastercards are in fact debit cards with a PIN setup and no signature. That's a "trick" I know with my Mastercard, and I'm sure the next North American MC cardholder will have an easier time because of it. After a while I'm sure it works its way into the informal training, those little tips and tricks the retail veterans pass on to the newbies.
posted by KevCed at 11:22 PM on March 5, 2007

When I worked at a grocery store, this was actually a part of our training. It's pretty standard op for any place with a card reader.
posted by GilloD at 8:13 AM on March 6, 2007

"because their Mastercards are in fact debit cards with a PIN setup and no signature."

No, they're not -- unless they're Maestro cards. Many European countries have chip-and-pin systems now for both debit and credit cards.
posted by macdara at 8:48 AM on March 6, 2007