How Does a Metal Detector Work-

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					A Metal Detector is a device that responds to metal that may not be readily apparent.
The simplest form of a metal detector consists of an oscillator producing an
alternating current that passes through a coil producing an alternating magnetic field.
If a piece of electrically conductive metal is close to the coil, eddy currents will be
induced in the metal, and this produces an alternating magnetic field of its own. If
another coil is used to measure the magnetic field (acting as a magnetometer), the
change in the magnetic field due to the metallic object can be detected.
  According to Lenz's law of electromagnetism, when a conductor falls within a
certain range of an oscillating (alternating) magnetic field, it generates an oscillating
field of its own, which opposes the primary field. A magnetometer can pick up the
resulting changes in the overall field, signalling the nearby presence of a conductive
object, typically a piece of metal. The range of metal detectors varies from a few feet
for the smallest coils, to 10 feet (3 m) for 12 to 15-inch (30.5 to 38.1 cm) coils.
  The key to a functioning metal detector is the presence of eddy currents generated by
conductive objects in the environment. Just like pushing a paddle through a lake of
water can cause little vortices to appear on the surface, producing an oscillating field
in the environment causes electromagnetic vortices when the electrons in metal
generate their own oscillating field. Frequencies of 3 to 20 kHz are known to produce
the best results, and some more modern metal detectors even allow the operator to
change the frequency of the alternating field.
  A different, newer type of metal detector uses a technology called pulse induction.
This metal detector blasts the ground with a large electromagnetic pulse, and observes
the length of time it takes for the voltage to decrease to ambient levels. If there is a
conductive object under the ground, it will take a longer amount of time for the
voltage to decrease. It is a small effect, but modern sensors can pick it up well. This
technique has certain advantages over conventional metal detectors, such as the ability
to detect objects under highly mineralised "black sand."


The author is associated with IPFonline.
 http://www.ipfonline.com is India's No. 1 Industrial Product Finder. The portal is a
veritable storehouse of information on industrial products/services.

				
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posted:2/22/2011
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