The tape head’s undulating magnetism in turn aligns tiny magnetic domains in the iron-like powder covering one surface of the recording tape, as if they were midget compass needles. When the tape is played back, this process reverses: the varying magnetic orientation retained by the mini-magnets on the tape induces tiny currents inside the tape head that, when greatly amplified, resemble pretty closely what went into the tape recorder —another instance of the reversibility of electromagnetism. It’s not so different from translating sound vibrations into grooves cut into a record’s surface, to be traced later by a needle whose wiggling is transformed back into sound waves—only with tape it’s magnetic fluctuations instead of shimmying grooves. Card readers consists of a tape head inside a housing that guides the card smoothly past it, along with circuitry needed to decode the digital data.