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Metal detectors detect metal by using
electromagnetic induction.
Alexander Graham Bell In 1881, assembled a basic metal detector in an
effort to locate an assassin's bullet in President James Garfield. In
1931, Gerhard Fischar patented the portable version of the metal
detector.
Since 9/11, the increased use of the archway or upright metal detectors
have been used at entrances to secured buildings, airports, and in some
cases schools, as well as public auditoriums, concerts or sporting
events. The purpose of these is to detect any metallic weapon that could
be smuggled into the building. Small hand held wand detectors are used
by security or screening officers to scan a person, mainly an individual
in which the archway detector had sounded an alarm. Security isn't the
only use of metal detectors. Today, large portable metal detectors are
being used by thousands of professional and amateur treasure hunters?
world wide
to find such items as jewelry, coins, and other metal treasures
shallowly buried in the ground.
Beat frequency oscillator, induction balance, and pulse induction are
the three kinds of metal detectors available today.
Beat frequency oscillator detector:
This detector uses a coil in an oscillator as an inductor. Metal causes
its inductance to change changing the frequency. Another kind of
oscillator creates a close frequency. Audible beats between the
frequencies indicate metal.
Induction balance detector:
Have two coils, which are normally convex having approximately a 10%
overlap. A sine wave, a wave formed from a single constant frequency and
amplitude, is transmitted from one coil them received by the other.
Metal has to be near in order for the signal to be transmitted and
received.
Pulse induction detector:
Just as the name indicates, this system generates a pulse normally by
shutting off an inductor, then sent through a coil. The detector listens
for echoes.
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