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Metal detectors use electronic fields to detect the presence of metallic objects. They exist in a selection of walk-through, handheld, and vehicle-mounted models and are used to search personnel for hidden metallic objects at entrances to airports, public schools, courthouses, and other guarded spaces ; to seek for landmines, archaeological artifacts, and miscellaneous valuables ; and for the detection of hidden or undesired metallic objects in industry and construction. Metal detectors sense metallic objects, but don't image them. An x-ray baggage scanner, for instance, isn't classed as a metal detector as it photographs metallic objects rather than merely detecting their presence.

Metal detectors use electromagnetism in two basically other ways, active and passive. ( 1 ) Active detection techniques illuminate some detection space-the opening of a walk-through portal, for instance, or the space right in front of a hand-held unit-with a time-varying electronic field. Energy reflected from or passing thru 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 methods don't illuminate the detection space, but take advantage of the proven fact that every unshielded detection space is permeated by the Earth's natural magnetic field. Ferromagnetic objects moving thru the detection space cause temporary, but discoverable changes in this natural field. ( Ferromagnetic objects are made from metals,eg iron, that are capable of being magnetized ; many metals, such as aluminum, are conducting but not ferromagnetic, and cannot be sensed by passive means. )

Walk-through metal detectors. Walk-through or portal detectors are often found in airports, public buildings, and military installations. Their portals are bracketed with 2 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 broadcast wave impinges on a conducting object, it causes passing currents on the surface of the object ; these currents, in turn, radiate electronic waves. These secondary waves are sensed by the detector coil.

Hand-carried metal detectors. Metal detectors little 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 designed to be carried by a pedestrian scanning for metal objects in the ground ( e.g, nails, coins, landmines ). All such devices operate on variations 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 various common metals from each other. Hand-carried metal detectors have for ages been used to go looking for landmines ; [**] modern land mines are frequently made principally of plastic to avoid this inexpensive and clear counter-measure.

Magnetic imaging portals. The magnetic imaging portal is a relatively state-of-the-art technology. Like conventional walk-through metal detectors, it illuminates its detection space with radio-frequency electromagnetic waves ; [**] it does so employing a number of little antennas arranged ringlike around its portal, pointing inward. Each of these antennas broadcasts to the antennas on the far side of the array ; each antenna acts as a receiver whenever it isn't transmitting. A total scan of the detection space can take place in the time it needs a person to walk thru the portal.

Gradiometer metal detectors. Gradiometer metal detectors are passive systems that exploit the consequences of moving ferromagnetic objects on the earth's magnetic field. A gradiometer is an instrument that measures a gradient-the difference in magnitude between 2 points-in a magnetic field. When a ferromagnetic object moves thru a gradiometer metal detector's detection space, it causes a temporary turmoil in the earth's magnetic field, and this disturbance ( if large enough ) is perceived. Gradiometer metal detectors are sometimes walk-through devices, but can also be mounted on a vehicle such as a police auto, with the intention of detecting ferromagnetic weapons ( e.g, guns ) borne by persons approaching the vehicle.


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Pen MIlkor is a free lance writer in the defence industry

I found a black rock with my metal detector and I have been trying to find out what it is? Any ideas?

I was doing some metal detecting along the shores of Lake Ontario when I was on vacation. The mental detector went crazy beeping on this rock. I have searched several sources and even asked the guy at the metal detector store. Does anyone know what this rock might be? It is solid, dull black and you can see many flecks of black crystal like bits in it. I unfortuntely can't get a good picture of it?

Answer
A picture of it might help a bit. The Ol' Sasquatch Ü