Reader Amanda writes, “I was a cashier for a few years and when the register wouldn’t read a credit card, we would wrap the card in a plastic bag and run it through again and it would work. What makes the register read the card when it’s wrapped with a plastic bag?"
The black stripe on the back side of the credit card is made up of a bunch of tiny magnetic particles bound in plastic. The particles are arranged in magnetic and non-magnetic “zones” to encode the data—like your account number, expiration date, etc.—that the card reader on the register needs to process the transaction. When you swipe the card, the card reader reads the information by detecting the changes between the zones. For the transaction to work, the info needs to get from the card to the reader to the computer without any errors.
The strip is pretty delicate, and the data on it can be corrupted by exposing it to a strong magnet or scratching it. Damage can also occur gradually with use and carrying the card around in your wallet. Over time, some of the magnetic particles can get dragged out of position in a process called smearing. If enough magnetic bits move into a non-magnetic space to create a weak signal, the data gets corrupted and the card reader gets an error.
With just a little bit of magnetic material in them, the contaminated non-magnetic zones still have a much lower magnetic strength than the parts that are supposed to magnetized. Increasing the distance between the card reader and the corrupted zones is enough to get the reader to read those weak parts as non-magnetized again. A plastic bag, usually at-hand at the cash register, makes a great spacer. Wrapped around the card, the bag basically mutes the corrupted parts of the magnetic stripe so the reader is presented with the data as its supposed to be.