ABSTRACT
A tape recorder translates audio signals into a fl uctuating electromagnetic fi eld through a tape head, the small metal Brancusi-esque object you can see inside a cassette player or answering machine (see fi gure 9.1). Th e tape head’s undulating magnetism in turn aligns little tiny magnetic domains in the iron-like powder covering one surface of the recording tape, as if they were tiny compass needles. When the tape is played back the whole process reverses: the varying magnetic orientation retained by the mini-magnets on the tape now induces current fl ow inside the tape head which, when amplifi ed, resembles pretty closely what went into the tape recorder earlier-another instance of the reversibility of electromagnetism discussed in chapter 4. It’s not so diff erent from translating sound vibrations into grooves cut into a record’s surface, later followed by a needle whose wiggling is re-translated back into sound waves-only with tape it’s magnetic fl uctuations instead of shimmying grooves. Digital tape recordings, such as fl oppy disks or credit card stripes, are like cassette tape only simpler: the magnetic domains just fl op back and forth between two states, 0 and 1, instead of tracing the nuanced contour of an analog waveform.
