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A metal detector is a electronic instrument which detects the presence of metal nearby. Metal detectors are useful for finding metal inclusions hidden within objects, or metal objects buried underground. They often consist of a handheld unit with a sensor probe which can be swept over the ground or other objects. If the sensor comes near a piece of metal this is indicated by a changing tone in earphones, or a needle moving on an indicator. Usually the device gives some indication of distance; the closer the metal is, the higher the tone in the earphone or the higher the needle goes. Another common type are stationary "walk through" metal detectors used for security screening at access points in prisons, courthouses, and airports to detect concealed metal weapons on a person's body.
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 a 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.
The first industrial metal detectors were developed in the 1960s and were used extensively for mineral prospecting and other industrial applications. Uses include de-mining (the detection of land mines), the detection of weapons such as knives and guns (especially in airport security), 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.
Toward 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. Early machines were crude, used a lot of battery power, and worked only to a very limited degree. 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 metal detector worked correctly but the attempt was unsuccessful because the metal coil spring bed Garfield was lying on confused the detector.
The modern development of the metal detector began in the 1920s. 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 1925 he applied for, and was granted, the first patent for a metal detector. Although Gerhard Fisher was the first person granted a patent for a metal detector, the first to apply was Shirl Herr, a businessman from Crawfordsville, Indiana. His application for a hand-held Hidden-Metal Detector was filed in February 1924, but not patented until July 1928. Herr assisted Italian leader Benito Mussolini in recovering items remaining from the Emperor Caligula's galleys at the bottom of Lake Nemi, Italy, in August 1929. Herr's invention was used by Admiral Richard Byrd's Second Antarctic Expedition in 1933, when it was used to locate objects left behind by earlier explorers. It was effective up to a depth of eight feet. However, it was one Lieutenant Józef Stanisław Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland, during the early years of World War II, who refined the design into a practical Polish mine detector. They were heavy, ran on vacuum tubes, and needed separate battery packs.
The design invented by Kosacki was used extensively during the clearance of the German mine fields during the Second Battle of El Alamein when 500 units were shipped to Field Marshal Montgomery to clear the minefields of the retreating Germans, and later used during the Allied invasion of Sicily, the Allied invasion of Italy and the Invasion of Normandy. As it was a wartime research operation to create and refine it, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.
Many manufacturers of these new devices brought their own ideas to the market. White's Electronics of Oregon began in the 1950s 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 1950s and 1960s, metal detector manufacturers and designers made smaller lighter machines with improved circuitry, running on small battery packs. Companies sprang up all over the USA and Britain to supply the growing demand.
Modern top models are fully computerized, using integrated circuit 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.
The biggest technical change in detectors was the development of the induction-balance system. This system involved two coils that were electrically balanced. When metal was introduced to their vicinity, they would become unbalanced. What allowed detectors to discriminate between metals was the fact that every metal has a different phase response when exposed to alternating current. Scientists had long known of this fact by the time detectors were developed that could selectively detect desirable metals, while ignoring undesirable ones.
Even with discriminators, it was still a challenge to avoid undesirable metals, because some of them have similar phase responses e.g. tinfoil and gold, particularly in alloy form. Thus, improperly tuning out certain metals increased the risk of passing over a valuable find. Another disadvantage of discriminators was that they reduced the sensitivity of the machines.
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: two coils in a D shape, 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 different technique in metal detection called pulse induction. Unlike the beat frequency oscillator or the induction balance machines which both used a uniform alternating current at a low frequency, the pulse induction machine simply fired a high-voltage pulse of signal into the ground. In the absence of metal, the pulse 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 jewelry could now be located even under highly-mineralized black sand.
Metal detectors are widely used in archaeology with the first recorded use by military historian Don Rickey in 1958 who used one to detect the firing lines at Little Big Horn. However archaeologists oppose the use of metal detectors by "artifact seekers" or "site looters" whose activities disrupt archaeological sites.
|The examples and perspective in this section deal primarily with the United Kingdom and do not represent a worldwide view of the subject. (June 2013)|
In England and Wales metal detecting is legal provided that permission is granted by the landowner, and that the area is not a Scheduled Ancient Monument, a site of special scientific interest (SSSI), or covered by elements of the Countryside Stewardship Scheme.
Items discovered which fall within the definition of treasure must be reported to the coroner or a place designated by the coroner for treasure. The voluntary reporting of finds which do not qualify as treasure to the Portable Antiquities Scheme or the UK Detector Finds Database is encouraged.
The situation in Scotland is very different. Under the Scots law principle of bona vacantia, the Crown has claim over any object of any material value 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 offence in Scotland.
|This section does not cite any references or sources. (November 2013)|
There are various types of hobby activities involving metal detectors:
|The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject. (June 2013)|
The metal detecting community and professional archaeologists have different ideas related to the recovery and preservation of historic finds and locations. Archaeologists claim that detector hobbyists take an artifact-centric approach, removing these from their context resulting in a permanent loss of historical information. Archaeological looting of places like Slack Farm in 1987 and Petersburg National Battlefield serve as evidence against allowing unsupervised metal detecting in historic locations.
Hobbyists often state that professional archaeologists' resource limitations results in the loss or damage of many artifacts by plows, development, erosion and livestock. The language and breadth of legislation regarding artifact collection is also an issue, as the Archaeological Resources Protection Act of 1979 excludes scattered coins, the main target of inland hobbyists. Many hobbyists take issue with the breadth of metal detecting bans, marking large swaths of property off-limits which are either well-documented already or unlikely to ever receive professional attention. Suggestions to certify or offer limited permits for detecting at historic sites have been attempted in some areas of the United States.
Recently, productive efforts for cooperation between professionals and metal detecting hobbyists have begun, including the Montpelier Archeology Project and Battlefield Restoration and Archaeological Volunteer Organization (BRAVO) and many more. In these programs, skilled detector hobbyists work with experienced professionals with common goals of accurate, efficient site discovery and excavation. Away from supervised sites, hobbyists using improved record keeping and employment of global positioning system, GIS, logbooks, photo scales and online databases may aid professionals in evaluating possible sites. When searching for a site, hobbyists can aid with electronic scanning, reducing the need for test holes. Some land managers, such as the Tennessee Valley Authority have cited a role for amateur archaeologists in protecting sensitive sites from illegal looting and metal detector hobbyists have aided in the location and preservation of many sites.
A series of aircraft hijackings led the United States in 1972 to adopt metal detector technology to screen airline passengers, initially using magnetometers that were originally designed for logging operations to detect spikes in trees. The Finnish company Outokumpu adapted mining metal detectors in the 1970s, still housed in a large cylindrical pipe, to make a commercial walk-through security detector. 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.
Industrial metal detectors are used in the pharmaceutical, food, beverage, textile, garment, plastics, chemicals, lumber, and packaging industries.
Contamination of food by metal shards from broken processing machinery during the manufacturing process is a major safety issue in the food industry. Metal detectors for this purpose are widely used and integrated into the production line.
Current practice at garment or apparel industry plants is to apply metal detecting after the garments are completely sewn and before garments are packed to check whether there is any metal contamination (needle, broken needle, etc.) in the garments. This needs to be done for safety reasons.
The industrial metal detector was developed by Bruce Kerr and David Hiscock in 1947. The founding company Goring Kerr pioneered the use and development of the first industrial metal detector. Mars Incorporated was one of the first customers of Goring Kerr using their Metlokate metal detector to inspect Mars bar.
The basic principle of operation for the common industrial metal detector is based on a 3 coil design. This design utilizes an AM (amplitude modulated) transmitting coil and two receiving coils one on either side of the transmitter. The design and physical configuration of the receiving coils are instrumental in the ability to detect very small metal contaminates of 1mm or smaller. Today modern metal detectors continue to utilize this configuration for the detection of tramp metal.
The coil configuration is such that it creates an opening whereby the product (food, plastics, pharmaceuticals, etc.) passes through the coils. This opening or aperture allows the product to enter and exit through the three coil system producing an equal but mirrored signal on the two receiving coils. The resulting signals are summed together effectively nullifying each other.
When a metal contaminant is introduced into the product an unequal disturbance is created. This then creates a very small electronic signal that is amplified through special electronics. The amplification produced then signals a mechanical device mounted to the conveyor system to remove the contaminated product from the production line. This process is completely automated and allows manufacturing to operate uninterrupted.