How do credit cards get demagnetized?

[Carina42]

I'm not sure if "demagnetized" is the correct term. Anyhow, I have two gas cards that I keep in my work vehicle in a little cubby hole. I can NEVER use the darn things to pay at the pump, I have to go inside & deal with an actual human so they can run it manually. Replacement cards, same deal. I keep a checkbook wallet in the truck much of the time too, with ATM & other credit cards, & they always work.

I never put these through scanners at airports, in the microwave, or through the washing machine, they're always in my truck, not near a heater vent or anything. I've heard several explanations, but none that really make sense.

Can anybody give me a definitive answer? I really don't mind one bit going in to pay, but I am curious.

Thanks!

[Jman]

Is that place close to any speakers? They can get demagnetized if they're close to any magnets. Your speakers would be enough to do it if they sat there long enough, or if they got close once. Check for some sort of magnetic material either in or near that area.

Jman

[Smeghead]

Hmmm. I wonder what kind of electrical wiring for the truck is near the glove box?

[Saltire]

You say they aren't kept near a heater vent, but maybe they're near the fan motor. That would have the same effect as speakers, and you probably can't tell from looking. It's hard to be sure where the actual motor lives without taking the dash apart.

Whatever it is, I wouldn't use that spot for your cards anymore.

[handy]

Carina42, just a guess, why not call the number on the credit card & ask them? Those people would know then you can come back & let us know.

[Jingo]

Your credit card can become non-functional for one of several reasons. The two most common are “demagnetization” and “frictional erosive degradation” (yes, that’s what my contact at First of Omaha Merchant Processing said). Demagnetization, he added, is more properly termed “electromagnetic particulate reassignment.” Sure.
The magnetic strip on your card is composed of a ferrous metal (different companies use different formulations) combined with a petrochemical (plastic) binder. Simply put, the strip is magnetized by a powerful field in a particular pattern that makes it unique.
The strip is relatively durable—BUT:

1. If it is rubbed enough, the strip material will wear away until there is not enough to hold a sufficient magnetic field to be readable by a reading device like a swipe reader. (Like me, you may be tempted to caress your status-symbol platinum card—don’t!) This seems pretty obvious. Repeated removal from and replacement into a tight card slot in a wallet will often cause this type of non-function.

2. A strong enough magnetic field, or prolonged exposure to a weaker field can change the magnetic properties of the strip. Though the strip is designed specifically to resist this, it ain’t perfect. A few of the things that typically change the strip’s field characteristics:
Proximity to the field generated by an electric motor (like the heater of your truck?)
The field generated by a CRT
The field generated by a loudspeaker’s magnet (like the speaker in your truck?)
And here’s a common one: The magnetic clasp on a purse or wallet—these are surprisingly common and often powerful. They aren’t usually immediate strip-alterers, but prolonged exposure will certainly do the trick. (The old yarn about eel skin is nonsense).

I do hope that this helps, and yes, Dave Barry, “Erosive Degradation” would be a good name for a rock band.
Thank you, and Goodnight

[whitetho]

Uncle Cecil reviewed this general topic in Return of the Straight Dope -- an online copy of the column (sans Slug's illustraion) is at Do eelskin wallets demagnetize bank teller cards? In the column, Cecil notes that magnetic clasps are often the problem, as Jingo pointed out.

[Zenster]

D@amn quick if you get a planar magnetron sputtering array anywhere near them. Just carrying one from a car to the bench wipe everything in my wallet clean.

[Carina42]

Handy...actually, I did call the customer "service" numbers. Amoco told me that if two cards were placed together, they would demagnetize. Since cards are stacked together in wallets usually without a problem, that didn't seem right. The nice lady at Total had no clue.

You other helpful folks...thanks! I'm thinking maybe the speaker in the door...I have big after market speakers about 18" away from where I keep the cards. Or maybe some motor or wiring in the dash. I think when I get new cards I'll keep them in the cubby hole between the seats.

Jingo: Frictional erosive degradation is something my boyfriend & I engage in often, and it sure feels like my particles are getting rearranged.

Now someone just has to explain magneton sputtering array to me, and I'll be clear as a bell on all this.