Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Credit card magnetic strip

  1. Feb 21, 2006 #1
    I have a credit card that I use quite more often then others, and obviously the magnetic strip on the back of the card has worn down quite a bit. Often, when someone slides the card it doesn't read properly and the cashier person has to manual type in my card number. Someone told me if you put the credit card in a plastic bag, such as a shopping bag from Walmart, and slide the card while its in the bag, it will read. Now when my card doesn't read, I tell the cashier to put in it in a plastic bag and it always works. What about the card being in a plastic bag makes the magnetic strip work? Thanks in advanced for any replies, my Physics professor couldn't figure it out.
  2. jcsd
  3. Feb 21, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    I think the problem is that scratches on the surface of the magnetic strip produce spikes in the reader which causes the checksum to fail. Placing a plastic bag over the card (strip) prevents those spurious readings.
  4. Feb 21, 2006 #3


    User Avatar
    Gold Member

    Applying cellophane (Scotch, etc.) tape to the stripe can be effective as well, and a bit handier.
  5. Feb 21, 2006 #4
    I would opt to GET A NEW CARD. It's simple, free and takes about a week or 2. As a side note, men are more prone to damaging credit cards than women due to where men generally keep them... in their wallet behind their butt, which can crack and damage a card.
    Other solutions might remedy an emergency condition, but, GET REAL. You need a new card. ITS FREE.

    Sometimes I will use http://www.creditcounselorhut.com [Broken] if I want some paid advice.
    Last edited by a moderator: May 2, 2017
  6. Feb 21, 2006 #5
    K well, the question wasn't whether or not I needed a new card. I'm fully aware it's free and easily ordered and a task that can be done over the phone in 3 minutes. I was asking about the reasoning behind it working with the plastic bag. Thanks for your side note.
  7. Feb 22, 2006 #6
    for anyone who was truly interested in the answer.. i found it on http://www.straightdope.com/columns/040116.html

    Dear Cecil:

    I've heard of this before, but didn't believe it. Now I've seen it for myself, and my
    skeptical mind needs a good explanation. A lady in front of me at the supermarket swiped
    her credit card but it didn't work. She tried again, didn't work. The cashier tried,
    didn't work. So far, everything was proceeding scientifically. Then the cashier says, "My
    friend who works at a bank taught me this." What's he do? He puts the credit card in a
    plastic shopping bag, pulls the plastic tight around the card, and swipes it. Now the
    part I don't get: It worked. --Tim, Newton, Massachusetts

    Cecil replies:

    At least the cashier's friend didn't tell him to bury the card at a crossroads at
    midnight. Wacky though the plastic-bag technique seems, several of Cecil's engineering
    buddies admit they've seen it work. Here's the deal. 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. Worth a try, anyway--God forbid the clerk should
    have to punch in the numbers by hand.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook