Metal detector
Metal detectors use electromagnetic induction to detect metal.
Related Topics:
Electromagnetic induction - Metal
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In 1881, Alexander Graham Bell constructed a crude metal detector in an attempt to find an assassin's bullet in President James Garfield. Gerhard Fischar patented a portable version in 1931.
Related Topics:
Alexander Graham Bell - James Garfield - Gerhard Fischar
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Upright "archway" detectors are used at entrances to secured buildings, such as courthouses or airports, to detect metallic weapons which may be brought in. Small portable "wand" detectors are used by security staff to frisk persons for the same. Larger portable metal detectors are used by treasure hunters to locate metallic items, such as jewelry or coins, buried shallowly underground.
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There are three types of metal detectors: beat frequency oscillator, induction balance, and pulse induction.
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- In a beat frequency oscillator detector, a coil is used as an inductor in an oscillator, whose frequency changes when metal causes its inductance to change. Another oscillator produces a close frequency, and audible beats between them signal metal.
- In an induction balance detector, there are two coils, usually gibbous with about 10% overlap, and a sine wave is transmitted with one coil and received with the other. The coils are adjusted so that there is no signal in the receive coil when there is no metal nearby.
- In a pulse induction detector, a pulse is generated (usually by cutting off an inductor) and sent through a coil and the detector listens for echoes.
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| ► | Introduction |
| ► | Metal detectors and archaeology |
| ► | See also |
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Latest news on metal detector
Inside a London cab-driver's brain
FMRI scans have revealed the amazing workings of London's Black Cab drivers, who train for a decade to acquire "The Knowledge," an encylopedic ability to navigate London's streets: The hippocampus was only active when the taxi drivers initially planned their route, or if they had to completely change their destination during the course of the journey. The scientists saw activity in a different brain region when the drivers came across an unexpected situation - for example, a blocked-off junction. Another part of the brain helped taxi drivers to track how close they were to the endpoint of their journey; like a metal detector, its activity increased when they were closer to their goal. Changes also occurred in brain regions that are important in social behaviour. Taxi driving is not just about navigation: "Drivers do obsess occasionally about what their customers are thinking," said Dr Spiers. Taxi drivers 'have brain sat-nav' (Thanks, Ben!)...
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