How can a plastic bag enhance the magnetic stripe on the back of a credit card?

by | Nov 16, 2009 04:08 PM ET

You Asked:

How can a plastic bag enhance the magnetic stripe on the back of a credit card? --- Annie, Reno, Nev.

Marshall Brain Answers:

You may have experienced the following: you walk into a store, the cashier tries to scan your credit card, the card doesn't work, so the cashier puts your card inside a plasic bag. When the plastic-wrapped card is scanned, it works fine.

Let's start by understanding what is happening when the cashier swipes the card. On the back of the card is a stripe of magnetic tape that is nearly identical to the tape in a cassette tape. The card reader is essentially a tape player that plays the stripe.

The stripe contains a small amount of data - things like the account number and expiration date as described on this page. In order to process your card, the card reader needs to get that data off the stripe without any errors.

The problem is that the stripes receive a fair amount of abuse in your pocket, as well as in the reader. Dirt, scratches and data smearing (where the bits of data on the stripe tend to bleed over into each other) can all make the card hard to read for the scanner.

When the cashier puts the card in a plastic bag, it creates a spacer. Now the card slides through the reader with a slight separation between the data stripe and the stripe-reading head. The separation weakens the signal (the strength of any magnetic field weakens with distance), but it also cleans it up, especially in the case of smearing, because the smeared data is no longer readable by the head due to the spacing.

This page talks about the construction of the stripe:

A magnetic stripe is the black or brown stripe that you see on your credit card, or maybe the back of your airline ticket or transit card. The stripe is made up of tiny magnetic particles in a resin. The particles are either applied directly to the card or made into a stripe on a plastic backing which is applied to the card.
The material used to make the particles defines the Coercivity (see below) of the stripe. Standard low coercivity stripes use iron oxide as the material to make the particles, high coercivity stripes are made from other materials like barium ferrite. These materials are mixed with a resin to form a uniform slurry which is then coated onto a substrate. In the case of a credit card or similar application the slurry is usually coated onto a wide plastic sheet and dried. The coating is very thin and the plastic allows the coating to be handled. It is then sliced into stripe widths and applied to the card during the card manufacturing process. The methods of application include lamination (where the stripe and backing is laminated into the card), hot-stamp (where a heated die is used to transfer the oxide stripe from the backing onto the card after the card is cut to size), and cold-peel (where the oxide stripe is peeled from the backing, and then laminated into the card). Each of the methods have their own advantages and are largely irrelevant to the user of the card.

This page describes the smearing process:

The magnetic stripe consists of magnetised particles embedded in a plastic binder with lubricant. It can be damaged in many ways, including exposure to a large external magnetic field, whether from a permanent magnet or a high-strength alternating current. Damage can also occur by a gradual "smearing" process, in which some magnetic particles are physically dragged from one part of the stripe to another. This transfers a few magnetised particles into zones of the stripe that previously had none, corrupting the data. This is by far the most common mode of failure, and this smearing process is probably under way on your card.

As that article points out, once the smearing has gotten bad enough to corrupt the reading process, it is time to call the credit card company and get the card replaced.

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