Reed Tachometer (3A60.50)
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A set of metal reeds of descending natural frequencies is attached to a gyroscope. The gyroscope is slightly off balance so that it vibrates as it spins, and as its rotational frequency passes through the frequencies of the reeds each reed vibrates in turn.
Use one of the motorized gyroscope starters and watch the various reeds vibrate as it slows down.
Modern methods of measuring rotation speed include stroboscopes or digital tachometers with lasers:
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