The recorded information of bank cards and bankbooks are all stored on the magnetic stripe on the back, and the strong magnetic field generated by high-frequency electromagnetic waves when the mobile phone is working will magnetize the credit card, which will make the recorded information disordered, thus causing the bank card to be invalid. At the same time, some people put mobile phones, bank cards and other items together in the bag, which can easily cause the bank card to be "degaussed". In addition, if the magnetic strips of two bank cards overlap, they are likely to be demagnetized at the same time. In addition, the magnetic hidden buckle of the lady's bag, the card reader on the bus, and some medical examination equipment such as the nuclear magnetic vibrator will also cause the magnetization of the bank card. It is recommended to use a ferrule.