Michelle Gain, of Corran School, asks :-

Can the information on a credit card be destroyed if the magnetic strips of two cards rub together?

Richard Blaikie, an electrical engineer at the University of Canterbury, responded.

The short answer to your question is no - well, not easily anyway. The magnetic field of one credit card isn't strong enough to erase the information on another card by simply rubbing them together. However, doing this may scratch the magnetic strips, which could erase the information.

Credit cards are rather cleverly made - the magnetic strip stores information such as the account number, the account type and the PIN in much the same way as music is stored on a cassette tape. When the card is swiped through an EFTPOS terminal or money machine this information is read and transmitted back to the bank's central computer to check that the card can be used. With everyday use the card will get scratched and damaged so this information is written onto the card many times. If one section is lost, another section will still have the vital details.

The information is written onto the magnetic strip using a strong electromagnet - many times stronger than the magnetic strip itself. It takes another strong magnet to change it or erase it - the magnetic field of another card isn't strong enough. It is therefore important to keep credit cards away from strong magnets. Many purses and handbags use magnetic clasps, which can be strong enough to erase the information on a credit card. In addition, if a card is stored loosely in a bag keys or other sharp objects may scratch it. They are best kept in a pouch or wallet away from strong magnets and sharp objects, so that the information on the magnetic strip will still be there when it's needed.