Dictionary:
metal detector |
A device that senses the presence of metal, especially:
| Archaeology Dictionary: metal detector |
An electronic device used specifically for the detection of buried metal objects. Hand-held metal detectors began to be manufactured in large numbers during the mid 1970s and sold to create the basis of a hobby interest in finding old things. Human interest in discovering ancient things coupled with an extensive and lucrative market in ancient artefacts led to a great deal of damage to archaeological sites and to hostility between metal detector users and archaeologists. While a good deal of work is well conducted and the finds properly logged and reported there remains a criminal element who loot protected sites and sell what they find.
| Intelligence Encyclopedia: Metal Detectors |
Metal detectors use electromagnetic fields to detect the presence of metallic objects. They exist in a variety of walk-through, hand-held, 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 hunt for landmines, archaeological artifacts, and miscellaneous valuables; and for the detection of hidden or unwanted metallic objects in industry and construction. Metal detectors detect metallic objects, but do not image them. An x-ray baggage scanner, for example, is not classed as a metal detector because it images metallic objects rather than merely detecting their presence.
Metal detectors use electromagnetism in two fundamentally different ways, active and passive. (1) Active detection methods illuminate some detection space—the opening of a walk-through portal, for example, or the space directly in front of a hand-held unit—with a time-varying electromagnetic 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. (2) Passive detection methods do not illuminate the detection space, but take advantage of the fact that every unshielded detection space is already permeated by the Earth's natural magnetic field. Ferromagnetic objects moving through the detection space cause temporary, but detectable changes in this natural field. (Ferromagnetic objects are made of metals, such as iron, that are capable of being magnetized; many metals, such as aluminum, are conducting but not ferromagnetic, and cannot be detected by passive means.)
Walk-through metal detectors. Walk-through or portal detectors are common in airports, public buildings, and military installations. Their portals are bracketed with two large coils or loop-type antennae, one a source and the other a detector. Electromagnetic waves (in this case, low-frequency radio waves) are emitted by the source coil into the detection space. When the electromagnetic field of the transmitted wave impinges on a conducting object, it induces transient currents on the surface of the object; these currents, in turn, radiate electromagnetic waves. These secondary waves are sensed by the detector coil.
Hand-carried metal detectors. Metal detectors small 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, loose change, landmines). All such devices operate on variations of the same physical principle as the walk-through metal detector, that is, they emit time-varying electromagnetic 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 long been used to search for landmines; however, modern land mines are often made largely of plastic to avoid this cheap and obvious counter-measure. New technologies, especially neutron activation analysis and ground-penetrating radar, are being developed to search for nonmetallic landmines.
Gradiometer metal detectors. Gradiometer metal detectors are passive systems that exploit the effect 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 through a gradiometer metal detector's detection space, it causes a temporary disturbance in the earth's magnetic field, and this disturbance (if large enough) is detected. Gradiometer metal detectors are usually walk-through devices, but can also be mounted on a vehicle such as a police car, with the intent of detecting ferromagnetic weapons (e.g., guns) borne by persons approaching the vehicle. Gradiometer metal detectors are limited to the detection of ferromagnetic objects and so are not suitable for security situations where a would-be evader of the system is likely to have access to nonferromagnetic weapons.
Magnetic imaging portals. The magnetic imaging portal is a relatively new technology. Like traditional walk-through metal detectors, it illuminates its detection space with radio-frequency electromagnetic waves; however, it does so using a number of small antennas arranged ringlike around its portal, pointing inward. Each of these antennas transmits in turn to the antennas on the far side of the array; each antenna acts as a receiver whenever it is not transmitting. A complete scan of the detection space can take place in the time it takes a person to walk through the portal. Using computational techniques adapted from computed axial tomography (CAT) scanning, a crude image of the person (or other object) inside the portal is calculated and displayed. The magnetic imaging portal may for some purposes be classed as a metal detector rather than as an imaging system because it does not produce a detailed image of the metal object detected, but only reveals its location and approximate size.
Further Reading
Electronic
"Guide to the Technologies of Concealed Weapon and Contraband Imaging and Detection (NIJ Guide 602–00)." Institute of Justice, US Department of Justice. February 2001. <http://www.ojp.usdoj.gov/nij/pubs-sum/184432.htm> (April 23, 2003).
— LARRY GILMAN
| Wikipedia: Metal detector |
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It has been suggested that this article or section be merged with Inductive sensor. (Discuss) |
Metal detectors use electromagnetic induction to detect metal. Uses include de-mining (the detection of land mines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archaeology and treasure hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors.
In its simplest form, 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.
Towards the end of the 19th century, many scientists and engineers used their growing knowledge of electrical theory in an attempt to devise a machine which would pinpoint metal. The use of such a device to find ore-bearing rocks would give a huge advantage to any miner who employed it. The German physicist Heinrich Wilhelm Dove invented the induction balance system, which was incorporated into metal detectors a hundred years later. Early machines were crude, used a lot of battery power, and worked only to a very limited degree. Physicist Alexander Graham Bell used such a device to attempt to locate a bullet lodged in the chest of American President James Garfield in 1881; the attempt was unsuccessful because the metal bed Garfield was lying on confused the detector.[citation needed]
The modern development of the metal detector began in the 1930s. Gerhard Fisher had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fisher noticed that there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal, using a search coil resonating at a radio frequency. In 1937, he applied for, and was granted, the first patent for a metal detector. His designs were soon put to the test in a practical way, as they were used as mine detectors during World War II. They were heavy, ran on vacuum tubes, and needed separate battery packs, but they worked. After the war, there were plenty of surplus mine detectors on the market; they were bought up by relic hunters who used them for fun and profit. This helped to form metal detecting into a hobby.
Many manufacturers of these new devices brought their own ideas to the market. Whites Electronics of California began in the 50's by building a machine called the Oremaster Geiger Counter. Another leader in detector technology was Charles Garrett, who pioneered the BFO (Beat Frequency Oscillator) machine. With the invention and development of the transistor in the 50's and 60's, metal detector manufacturers and designers made smaller lighter machines with improved circuitry, running on small battery packs. The metal detector was reduced to a size that even a child could use - and use them they did. Fabulous finds were made; prehistoric gold ornaments, chests of Roman coins, jewelled daggers, arrow heads- all types of metal artifacts were coming out of the ground. Suddenly, there was a huge demand for those early electronic magic wands which might make a man rich overnight. Companies sprang up all over the USA and Britain who wished to supply the growing demand.
Larger portable metal detectors are used by archaeologists and treasure hunters to locate metallic items, such as jewelry, coins, bullets, and other various artifacts buried shallowly underground.
Technological changes were taking place at a rapid rate too, and very few of the smaller companies managed to stay in competition with the big outfits. GOLDAK, METROTECH, IGWT, TEC, and, quite recently, ARADO ceased production of hobby machines. Some devotees of metal detecting still treasure their Arado machines, which had a reputation for being difficult to set up, but were reputed to be the deepest-seeking hobby detectors ever made. The biggest technical change in detectors was the development of the induction-balance system, where two coils are set up in an electrical equilibrium to produce a 'null' or zero balance. Introducing metal to the vicinity of the coils caused them to unbalance, producing a change of tone in the machine's speaker. Scientists had long known that every metal has a specific response to stimulation by alternating current. Each metal produces a time lag or 'phase angle' in its induced current, in relation to the drive current. This meant that detectors could now be set up to ignore unwanted phase angles, and respond positively only to desired metals. But there was also a downside to the development of the 'discriminator' detectors. Introducing discrimination always had the effect of reducing the sensitivity of the machine, so it was less able to find deep objects. In addition, there was the fact that some desirable metals were quite near the area of unwanted metals, such as iron. Gold, particularly in alloy form, was quite close to tinfoil in the overall spectrum, so the discrimination control had to be used carefully. The price to be paid for setting up a detector to ignore iron and tinfoil was the possibility that, sooner or later, the user would scan over, and ignore, a valuable find - perhaps a diamond engagement ring on a beach.
Coil designers also tried out innovative designs. The original Induction Balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design; the two coils were made in a D shape, and were mounted back-to-back to form a circle. This system was widely used in the 1970s, and both concentric and D type (or Widescan as they became known) had their fans. Another development was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode. Later developments switched electronically between both modes. The development of the Induction Balance detector would ultimately result in the Motion detector, which constantly checked and balanced the background mineralization.
At the same time, developers were looking at using a completely different type of technology in metal detectors. This was the process known as Pulse Induction. Unlike the Beat Frequency Oscillator or the Induction Balance machines which both used a uniform alternating current at a low radio frequency, the pulse induction machine simply fired a high-voltage pulse of signal into the ground. In the absence of metal, the 'spike' decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured. However, if metal was present when the machine fired, a small current would flow in the metal, and the time for the voltage to drop to zero would be increased. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and identify the presence of metal at a reasonable distance. These new machines had one major advantage: they were completely impervious to the effects of mineralization, and rings and other jewelery could now be located even under highly-mineralized 'black sand'. They had one major disadvantage too: there was no way to incorporate discrimination into a Pulse induction detector. At least, that was the perceived wisdom of scientists and engineers until Eric Foster, who had run Location Technology in Ireland for many years, started a new company in Britain and produced the Goldscan, the first Pulse Induction detector which had the apparent ability to differentiate between metals. This was a new type of 'junk eliminator' circuit, which relied on the size of the target as well as its metallic response to give a control that would show positive for a gold ring and negative for a copper coin. Its ability to differentiate between non-ferrous metals was not an exact science, but gave unparalleled depth on mineralized soil or sand. Pulse Induction detectors are now widely used in the construction industry; the Whites PI-150 is an industrial machine which can detect large objects to 10 feet, using a 12 or 15 inch coil.
Modern top models are fully computerized, using microchip technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, etc., and hold these parameters in memory for future use. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.
New genres of metal detector have made their appearance. BB (Beat Balance) and CCO (Coil Coupled Operation) were unveiled by the electronics press in 2004. Both were invented by electronics writer and designer Thomas Scarborough and combine unprecedented simplicity with good sensitivity.
In England and Wales metal detecting is legal provided permission is granted by the landowner, and the area is not a Scheduled Ancient Monument or covered by elements of the Countryside Stewardship Scheme. Voluntary reporting of finds to the Portable Antiquities Scheme or the UK Detector Finds Database is encouraged. These schemes have their critics, however, including some archaeologists and metal detectorists. The situation in Scotland is very different. Under the Scots law principle of bona vacantia[1], the Crown has claim over any object of any material where the original owner cannot be traced. There is also no 300 year limit to Scottish finds. Any artifact found, whether by metal detector survey or from an archaeological excavation, must be reported to the Crown through the Treasure Trove Advisory Panel at the National Museums of Scotland. The Panel then determines what will happen to the artifacts. Reporting is not voluntary, and failure to report the discovery of historic artifacts is a criminal offense in Scotland.
Archeology is beginning to recognize the contribution responsible metal detecting provides in adding to the knowledge of our past. One example is utilizing the skilled use of the metal detector to examine wide areas such as battlefield sites where surface scatters of metal objects may be all that survives. This has recently been demonstrated during archaeological work conducted at Antietam National Battlefield in the United States.[citation needed]
Many people use consumer metal detectors to look for coins on the beach. Most metal detectors are good to detect metal only within a foot or so below the ground. The detection depth depends on the type of metal detector, type of metal in the buried object, size of buried object, type of metals in the ground, and other objects in the ground.
There are five major types of hobbyist activities involving metal detectors:
The first industrial metal detectors were developed in the 1960s and were used extensively for mining and other industrial applications. A series of aircraft hijackings led the Finnish company Outokumpu to adapt mining metal detectors, still housed in a large cylindrical pipe, to the purpose of screening airline passengers as they walked through. The development of these systems continued in a spin off company and systems branded as Metor Metal Detectors evolved in the form of the rectangular gantry now standard in airports. In common with the developments in other uses of metal detectors both alternating current and pulse systems are used, and the design of the coils and the electronics has moved forward to improve the discrimination of these systems. In 1995 systems such as the Metor 200 appeared with the ability to indicate the approximate height of the metal object above the ground, enabling security personnel to more rapidly locate the source of the signal. Smaller hand held metal detectors are also used to locate a metal object on a person more precisely.
Contamination of food by metal shards from broken processing machinery during manufacture is a major safety issue in the food industry. Metal detectors for this purpose are widely used and integrated in the production line.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Archaeology Dictionary. The Concise Oxford Dictionary of Archaeology. Copyright © 2002, 2003 by Oxford University Press. All rights reserved. Read more |
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| Intelligence Encyclopedia. Encyclopedia of Espionage, Intelligence, and Security. Copyright © 2004 by The Gale Group, Inc. All rights reserved. Read more |
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