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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