DAMIEN LOVEGROVE / SCIENCE PHOTO LIBRARY DAMIEN LOVEGROVE / SCIENCE PHOTO LIBRARY
Mini Disc. This is a type of audio compact disc on which music may be recorded as well as played back. The disc has a fine coating of a rare-earth metal alloy. A laser beam is focused onto its surface. When a magnetic field is applied, heat from the laser at a given point allows the alloy's magnetic state to be reversed at that spot. By quickly switching the magnetic field on and off, a digital pattern of magnetised spots may be built up. This may be played back as the magnetised spots change the polarisation of the laser beam reflected by the surface. The tiny disc (6.4cm diameter) uses data compression to store as much information as a conventional 12cm compact disc.
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