Q & A: Magnetic strips

how does a magnetic strip work
- nikki (age 14)
sydney

A magnetic strip is a thin layer of magnetizable material which is placed on credit cards and driver's licenses and bank ATM cards and train tickets and hotel room keys and lots of other places. I've even seen paper telephone cards in Italy which use a magnetic strip on the back.

The magnetic strip is a lot like a tiny bit of magnetic recording tape, like that found in a music cassette or a videotape, except it is glued to the card. It is a plastic material with magentizable powder mixed in with it. The strip can be magnetized by exposing it to a strong magnetic field. Bits of it can be magnetized in different directions by a device which generates a strong magnetic field over a short distance. Read/write heads of a tape recorder do exactly this -- they have coils wrapped around special metals to make an electromagnet. Passing a current through these coils generates a magnetic field which can magnetize bits of the strip. The strip can have many many little North and South poles on it, and their numbers and locations are used to encode information (such as your credit card number or bank account number or driver's license number). For security reasons, your passwords or account balances should not be recorded on the strip because just about anyone with proper equipment can read the strips.

The strip is read with a similar (or even the same) device which wrote it. The process works in reverse -- by moving the strip back and forth over the read head, voltages are induced in the coils (Faraday's law describes how this works), and these can be amplified and recorded electronically, so that the information can be sent to a computer.

Sometimes magnetic strips do not work all that well. If you put them next to a magnet, they can get erased. Don't carry refrigerator magnets in your pocket next to credit cards and drivers' licenses! If the card gets bent or wrinkled they can be hard to read (the strip pretty much has to touch the read head of the reader, and if the card is bent it may not touch everywhere on the strip). Heating the card may make the strip lose its information, but most plastic cards will melt or deform first, causing problems. Don't leave them out in the sun or on a car's dashboard in the hot sun. They can also fail to work well when they are worn down from many uses.

Tom

(published on 10/22/2007)

I read the answer you gave about how magnetic strips on bank cards work, but I would like to know why, when a card won’t read sucessfully, putting it inside a plastic bag before swiping it will nearly always make it work. Also, rubbing the strip on a sweater, or making it wet, or putting a piece of paper or tape over the strip also will often make it work. Is it something to do with static electricity?
- mark
victoria, bc canada

The short answer is that I don't know, but here's some thoughts.

It's unlikely that static electricity is the key. The different procedures you name will change the static charges in very different ways, so it's hard to see why all would work if static were important.

Perhaps little rough spots on the surface interfere with the reading. Some of the procedures you mention could rub off bits of dirt and others could wash them off. Here's a wild guess about how plastic bags might work. These contain plasticizer- small molecules not well stuck to the polymer. The plasticizer can leak out- a problem for plastic food containers. It may serve to lubricate rough strips.

Here's what made me think of that. A type of copper high vacuum seal was shipped in little zip-lock bags. One of my colleagues worried that the copper might become coated with plasticizer, which could evaporate, slightly contaminating the vacuum. So he went to great lengths to clean the seals. Then they didn't work well at all. His guess that they became coated with plasticizer in the bag was right. It turned out, however, that the plasticizer was helping seal microscopic leaks at rough spots.

Mike W.


I had a problem with a magnetic strip card I had left out in the sun on my car's dashboard, and the plastic card warped and curled up on the edges. Running this card through a strip reader didn't work because the strip did not come in contact with the read head all the time. Sometimes it help if I used my hands to uncurl it a bit (very difficult to get it flat enough to read properly, and it proceeded to curl back up over time). One thing that could be happening above is that if a card is warped or bent, even just a tiny bit, it can lift off of the read-write head when read. If the card is more flexible, then it can bend back closer to its original shape when it is pulled through a narrow slot. Warming up the card a bit will make it more flexible. Many of the operations you describe above (especially rubbing it on a sweater) will warm the card up just enough to make it more flexible. I'm not sure how the plastic bag helps, but perhaps just handling the card with warm hands will help.

Tom

(republished on 07/13/06)