Within every square inch of magnetic stripe on the back of an ID card, there are 200 million (tiny) bar magnets.
Left to themselves, bar magnets “line up”, with ends that point north facing the south-pointing ends of adjoining magnets. Together the bar magnets form one long line, pointing at magnetic North Pole.
ID card encoders can select individual bar magnets within this line and reverse their polarity. The altered magnet’s north pole then faces the north pole of the magnet next to it; its south pole faces the south pole of the magnet on its other side.
The like poles of magnets repel each other. This repulsion creates a magnetic field. The heads of magnetic stripe readers detect these fields as an electric voltage – where two north ends face each other, a positive voltage; where two south ends face, a negative one. Readers convert the analog signal of this electric current into the binary language that computers read.
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