When my bank debit card doesn鈥檛 work when swiped, some petrol stations seem to have an ingenious and effective solution. They wrap the card in a thin, clear plastic bag and swipe it again 鈥� and it works. Why?
The card reader has a small induction coil that detects a succession of magnetic and non-magnetic zones in the card鈥檚 magnetic stripe. When the card is pulled through the reader, each magnetised zone makes a small electrical pulse as it passes the coil. The zones are arranged to encode the data needed to complete a transaction 鈥� along with, in countries such as the UK, the client鈥檚 PIN.
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 鈥渟mearing鈥� 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.
Advertisement
However, the 鈥渃ontaminated鈥� zones have a lower magnetic field than the properly magnetised zones. Magnetic field intensity is governed by an inverse-square law, so if the card normally passes within 0.01 millimetres of the reader and you double this, the signal picked up by the reader will fall to a quarter of its normal level. Just a small increase in distance will reduce the weak signal from a contaminated zone 鈥� which is supposed to be blank 鈥� to a level that the reader is likely to register as a zero. The extra distance can be created using any non-magnetic spacer, such as a plastic bag or sticky tape.
Readers also vary in their sensitivity. Some will read the card properly, and others will need the bag or tape. This situation will progress after an initial failure until the remedy stops working. Once you have experienced the first occurrence, arrange for a card to be reissued.
Bill Jackson, Toronto, Ontario, Canada