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Metal detector

Metal detector

A metal detector (metal detector) is an electronic device that allows you to detect metal objects in a neutral or weakly conductive environment due to their conductivity. The metal detector detects metal in soil, water, walls, wood, under clothes and luggage, in food products, in the human and animal body

A metal detector is an instrument that detects the presence of metal nearby. Metal detectors are useful for finding metal inclusions hidden within objects, or metal object 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 (see § Security screening below) used 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 (inductive sensor) 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 detecting 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.It also used by army to find underground bomb.

History and development: The modern development of the metal detector began in the 1920s. Gerhard Fischer had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fischer noticed 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 Fischer 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. These units were still quite heavy, as they ran on vacuum tubes, and needed separate battery packs.

The design invented by Kosacki was used extensively 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 the creation and refinement of the device was a wartime military research operation, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.

Beat Frequency Induction

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 United States and Britain to supply the growing demand. Beat Frequency Induction requires movement of the detector coil; akin to how swinging a conductor near a magnet induces an electric current; except the pulse is electric EMF and not magnetic EMF[further explanation needed].

Refinements

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.

State of the art metal detectors have further incorporated extensive wireless technologies for the earphones, connect to Wi-Fi networks and Bluetooth devices. Some also utilize built in GPS locator technology to keep track of searching location and the location of items found. Some connect to smartphone applications to further extend functionality.

Discriminators

The biggest technical change in detectors was the development of a tunable induction system. This system involved two coils that are electro-magnetically tuned. One coil act as an RF transmitter the other as a receiver; in some cases these can be tuned to between 3 and 100 kHz. When metal is in their vicinity, a signal is detected owing to eddy currents induced in the metal. What allowed detectors to discriminate between metals was the fact that every metal has a different phase response when exposed to alternating current; longer waves (low frequency) penetrate ground deeper, and select for high conductivity targets like silver, and copper; than shorter waves (higher frequency) which, while less ground penetrating, select for low conductivity targets like iron. Unfortunately, high frequency is also sensitive to ground mineralisation interference. This selectivity or discrimination allowed detectors to be 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.

New coil designs

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.

Pulse induction

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 (PI) machine simply magnetized the ground with a relatively powerful, momentary current through a search coil. In the absence of metal, the field 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 eddy current would be induced in the metal, and the time for sensed current decay 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 mostly impervious to the effects of mineralization, and rings and other jewelry could now be located even under highly mineralized black sand. The addition of computer control and digital signal processing have further improved pulse induction sensors.

One particular advantage of using a pulse induction detector includes the ability to ignore the minerals contained within heavily mineralized soil; in some cases the heavy mineral content may even help the PI detector function better.[citation needed] Where a VLF detector is affected negatively by soil mineralization, a PI unit is not.

Uses

Larger portable metal detectors are used by archaeologists and treasure hunters to locate metallic items, such as jewelry, coins, clothes buttons and other accessories, bullets, and other various artifacts buried beneath the surface.

Archaeology

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 problem with use of metal detectors in archaeological sites or hobbyist who find objects of archeological interest is that the context that the object was found in is lost and no detailed survey of its surroundings is made. Outside of known sites the significance of objects may not be apparent to a metal detector hobbyist.

England and Wales

In England and Wales metal detecting is legal provided that the landowner has granted permission 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.

The Treasure Act 1996 governs whether or not items that have been discovered are defined as treasure. Finders of items that the Act defines as treasure must report their finds to the local coroner. If they discover items that are not defined as treasure but that are of cultural or historical interest, finders can voluntarily report them to the Portable Antiquities Scheme and the UK Detector Finds Database.

France

The sale of metal detectors is allowed in France. The first use of metal detectors in France which led to archaeological discoveries occurred in 1958: people living in the city of Graincourt-lès-Havrincourt who were seeking copper from World War I bombshell with military mine detector found a Roman silver treasure. The French law on metal detecting is ambiguous because it refers only to the objective pursued by the user of a metal detector. The first law to regulate the use of metal detectors was Law No. 89-900 of 18 December 1989. This last is resumed without any change in Article L. 542-1 of the code of the heritage, which states that "no person may use the equipment for the detection of metal objects, for the purpose of research monuments and items of interest prehistory, history, art and archeology without having previously obtained an administrative authorization issued based on the applicant's qualification and the nature and method of research. " Outside the research of archaeological objects, using a metal detector does not require specific authorization, except that of the owner of the land. We often read, from some archaeologists, that the use of a metal detector is itself prohibited without official authorization. This is false. To realize this, one must look to the legislative intent in enacting the Law No. 89-900 of 18 December 1989. Asked about Law No. 89-900 of 18 December 1989 by the member of parliament mister Calloud, Jack Lang, Minister of Culture at the time, replied by letter the following: "The new law does not prohibit the use of metal detectors but only regulates the use. If the purpose of such use is the search for archaeological remains, prior authorization is required from my services. Apart from this case, the law ask to be reported to the appropriate authorities an accidental discovery of archaeological remains." The entire letter of Jack Lang was published in 1990 in a French metal detection magazine, and then, to be visible on internet, scanned with permission of the author of the magazine on a French metal detection website.

Scotland

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.

United States

The sale of metal detectors is allowed in the United States. People can use metal detectors in public places (parks, beaches, etc.) and on private property with the permission of the owner of the site. In the United States, cooperation between archeologists hunting for the location of colonial-era Native American villages and hobbyists has been productive.

Timeline

Further Resources

Title
Author
Link
Type
Date

Top 5 - Best Metal Detectors (2022)

Web

January 22, 2022

XP DEUS II FMF® Advantages | XP Metal Detectors

Web

December 14, 2021

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