When lightning current is conducted to ground via the lightning downconductor, a magnetic field is created around the conductor. The strength of the magnetic field is proportional to the strength of the lightning current. By installing a card with a special magnetic code trail perpendicular on the conductor, the magnetic field generated by the lightning will delete the data in the area where its strength is higher than the resistance of the code trail. The maximum lightning current to which a card has been exposed can then be read by the card reader.