Metal detectors use electric fields to see the presence of metallic objects. They exist in a variety of walk-through, hand held, and vehicle-mounted models and are used to search staff for concealed metallic objects at entrances to airfields, public colleges, courthouses, and other guarded spaces ; to seek for landmines, archaeological artifacts, and miscellaneous property ; and for the detection of concealed or unwanted metallic objects in industry and construction. Metal detectors notice metallic objects, but do not image them. An xray baggage scanner, for example, isn't classed as a metal detector as it photographs metallic objects instead of merely detecting their presence.
Metal detectors use electromagnetism in two essentially different ways, active and passive. ( one ) Active detection strategies illuminate some detection space-the opening of a walk-through portal, as an example, or the space directly in front of a hand-held unit-with a time-varying electro-magnetic field. Energy reflected from or passing through the detection space is affected by the presence of conductive material in that space ; the detector detects metal by measuring these effects. ( two ) Passive detection techniques don't illuminate the detection space, but take advantage of the proven fact that each unshielded detection space is already permeated by the Earth's natural magnetic field. Ferromagnetic objects moving through the detection space cause temporary, but noticeable changes in this natural field. ( Ferromagnetic objects are made of metals,eg iron, that are capable of being magnetized ; many metals, for example aluminum, are conducting but not ferromagnetic, and can't be uncovered by passive means. )
Walk-through metal detectors. Walk-through or portal detectors are often found in airfields, public buildings, and army installations. Their portals are bracketed with two big coils or loop-type antennae, one a source and the other a detector. Electromagnetic waves ( in this situation, low-frequency radio waves ) are emitted by the source coil into the detection space. When the electric field of the transmitted wave impinges on a conducting object, it induces passing currents on the surface of the object ; these currents, in turn, radiate electro-magnetic waves. These secondary waves are sensed by the detector coil.
Hand-carried metal detectors. Metal detectors tiny enough to be hand-held are often used at security checkpoints to localize metal objects whose presence has been detected by a walk-through system. Some units are engineered to be carried by a pedestrian scanning for metal objects in the ground ( e.g, nails, coins, landmines ). All such devices operate on permutations of the same physical principle as the walk-through metal detector, that is, they emit time-varying electric fields and listen for waves coming back from conducting objects. Some ground-search models further analyze the returned fields to distinguish diverse common metals from one another. Hand-carried metal detectors have long been used to look for landmines ; however [*COMMA] modern land mines are often made largely of plastic to avoid this cheap and obvious counter-measure.
Magnetic imaging portals. The magnetic imaging portal is a comparatively state-of-the-art technology. Like traditional walk-through metal detectors, it illuminates its detection space with radio-frequency electromagnetic waves ; [**] it does so using a number of tiny antennas prepared ringlike around its portal, pointing inward. Each of these antennas broadcasts in turn to the antennas on the far side of the array ; each antenna acts as a receiver whenever it isn't transmitting. A complete scan of the detection space can take place in the time it takes someone to stroll through the portal.
Gradiometer metal detectors. Gradiometer metal detectors are passive systems that exploit the effects of moving ferromagnetic objects on the earth's magnetic field. A gradiometer is an instrument that measures a gradient-the difference in magnitude between two points-in a magnetic field. When a ferromagnetic object moves thru a gradiometer metal detector's detection space, it causes a temporary disturbance in the earth's magnetic field, and this derangement ( if large enough ) is noted. Gradiometer metal detectors are usually walk-through devices, but can also be mounted on a vehicle such as a police vehicle, with the intention of detecting ferromagnetic weapons ( e.g, guns ) borne by people approaching the vehicle.
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