Metal detector
Overview
 
A metal detector is a device which responds to metal that may not be readily apparent.

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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. 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.
Encyclopedia
A metal detector is a device which responds to metal that may not be readily apparent.

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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. 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
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...

), 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 mining and other industrial applications. Uses include de-mining (the detection of land mine
Land mine
A land mine is usually a weight-triggered explosive device which is intended to damage a target—either human or inanimate—by means of a blast and/or fragment impact....

s), the detection of weapons such as knives and guns, especially in airport security
Airport security
Airport security refers to the techniques and methods used in protecting airports and aircraft from crime.Large numbers of people pass through airports. This presents potential targets for terrorism and other forms of crime due to the number of people located in a particular location...

, geophysical prospecting
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...

, archaeology and treasure hunting
Treasure hunting
Treasure hunting is the physical search for treasure which has been a notable human activity for millennia. -In modern times:In recent times, the early stages of the development of archaeology included a significant aspect of treasure hunt; Heinrich Schliemann's excavations at Troy, and later at...

. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars
Rebar
A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...

 in concrete and pipes and wires buried in walls and floors.

History and development

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. The German physicist Heinrich Wilhelm Dove
Heinrich Wilhelm Dove
Heinrich Wilhelm Dove was a Prussian physicist and meteorologist. He was the father of Alfred and Richard Dove.-Early Years:...

 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. Alexander Graham Bell
Alexander Graham Bell
Alexander Graham Bell was an eminent scientist, inventor, engineer and innovator who is credited with inventing the first practical telephone....

 used such a device to attempt to locate a bullet lodged in the chest of American President James Garfield
James Garfield
James Abram Garfield served as the 20th President of the United States, after completing nine consecutive terms in the U.S. House of Representatives. Garfield's accomplishments as President included a controversial resurgence of Presidential authority above Senatorial courtesy in executive...

 in 1881; the attempt was unsuccessful because the metal coil spring bed Garfield was lying on confused the detector.

Modern developments

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. However, it was one Lieutenant
Lieutenant
A lieutenant is a junior commissioned officer in many nations' armed forces. Typically, the rank of lieutenant in naval usage, while still a junior officer rank, is senior to the army rank...

 Jozef Stanislaw Kosacki
Józef Kosacki
Józef Stanisław Kosacki was a Polish professor engineer, inventor, and an officer in the Polish Army during World War II. He is best known as the inventor of the Polish mine detector, the first man-portable mine detector, whose basic design has been in use with various armies for over 50...

, a Polish officer attached to a unit stationed in St Andrews
St Andrews
St Andrews is a university town and former royal burgh on the east coast of Fife in Scotland. The town is named after Saint Andrew the Apostle.St Andrews has a population of 16,680, making this the fifth largest settlement in Fife....

, Fife
Fife
Fife is a council area and former county of Scotland. It is situated between the Firth of Tay and the Firth of Forth, with inland boundaries to Perth and Kinross and Clackmannanshire...

, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

, during the early years of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, who refined the design into a practical Polish mine detector
Polish mine detector
The Mine detector Mark I was a metal detector for landmines developed during World War II in the winter of 1941–1942 by Polish lieutenant Józef Kosacki.- History :...

. 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
Second Battle of El Alamein
The Second Battle of El Alamein marked a major turning point in the Western Desert Campaign of the Second World War. The battle took place over 20 days from 23 October – 11 November 1942. The First Battle of El Alamein had stalled the Axis advance. Thereafter, Lieutenant-General Bernard Montgomery...

 when 500 units were shipped to Field Marshal Montgomery
Bernard Montgomery, 1st Viscount Montgomery of Alamein
Field Marshal Bernard Law Montgomery, 1st Viscount Montgomery of Alamein, KG, GCB, DSO, PC , nicknamed "Monty" and the "Spartan General" was a British Army officer. He saw action in the First World War, when he was seriously wounded, and during the Second World War he commanded the 8th Army from...

 to clear the minefields of the retreating Germans, and later used during the Allied invasion of Sicily
Allied invasion of Sicily
The Allied invasion of Sicily, codenamed Operation Husky, was a major World War II campaign, in which the Allies took Sicily from the Axis . It was a large scale amphibious and airborne operation, followed by six weeks of land combat. It launched the Italian Campaign.Husky began on the night of...

, the Allied invasion of Italy
Allied invasion of Italy
The Allied invasion of Italy was the Allied landing on mainland Italy on September 3, 1943, by General Harold Alexander's 15th Army Group during the Second World War. The operation followed the successful invasion of Sicily during the Italian Campaign...

 and the Invasion of Normandy. As it was a wartime research operation to create and refine the design of the detector, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.

Further refinements

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.

Larger portable metal detectors are used by archaeologists and treasure hunters to locate metallic items, such as jewelry, coins
COinS
ContextObjects in Spans, commonly abbreviated COinS, is a method to embed bibliographic metadata in the HTML code of web pages. This allows bibliographic software to publish machine-readable bibliographic items and client reference management software to retrieve bibliographic metadata. The...

, bullets, and other various artifacts
Artifact (archaeology)
An artifact or artefact is "something made or given shape by man, such as a tool or a work of art, esp an object of archaeological interest"...

 buried shallowly underground.

Discriminators

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
Phase response
In signal processing and electrical engineering, phase response is the relationship between the phase of a sinusoidal input and the output signal passing through any device that accepts input and produces an output signal, such as an amplifier or a filter....

 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.

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 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 jewelry could now be located even under highly-mineralized black sand
Black sand
Black sand is sand that is black in color. One type of black sand is a heavy, glossy, partly magnetic mixture of usually fine sands, found as part of a placer deposit. Another type of black sand, found on beaches near a volcano, consists of tiny fragments of lava.While some beaches are...

.

Archaeology

England and Wales
In England and Wales
England and Wales
England and Wales is a jurisdiction within the United Kingdom. It consists of England and Wales, two of the four countries of the United Kingdom...

 metal detecting is legal provided that permission is granted by the landowner, and that the area is not a Scheduled Ancient Monument
Scheduled Ancient Monument
In the United Kingdom, a scheduled monument is a 'nationally important' archaeological site or historic building, given protection against unauthorized change. The various pieces of legislation used for legally protecting heritage assets from damage and destruction are grouped under the term...

, a site of special scientific interest
Site of Special Scientific Interest
A Site of Special Scientific Interest is a conservation designation denoting a protected area in the United Kingdom. SSSIs are the basic building block of site-based nature conservation legislation and most other legal nature/geological conservation designations in Great Britain are based upon...

 (SSSI), or covered by elements of the Countryside Stewardship Scheme
Countryside Stewardship Scheme
The Countryside Stewardship Scheme is an agri-environment scheme run by the United Kingdom Government set up in 1991.Originally introduced as a five-year pilot project by the Countryside Commission, the scheme aimed to improve the environmental value of farmland throughout England...

.

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
Portable Antiquities Scheme
The Portable Antiquities Scheme is a voluntary programme run by the United Kingdom government to record the increasing numbers of small finds of archaeological interest found by members of the public...

 or the UK Detector Finds Database
UK Detector Finds Database
The UK Detector Finds Database is an initiative by some members of the metal-detecting community in the United Kingdom to promote good practice within the hobby...

 is encouraged.
Scotland
The situation in Scotland is very different. Under the Scots law principle of bona vacantia
Bona vacantia
Bona vacantia is a legal concept associated with property that has no owner. It exists in various jurisdictions, with consequently varying application, but with origins mostly in English law.-Canada:...

, 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.

As a hobby

There are six major types of hobbyist activities involving metal detectors:
  • Coin shooting is looking for coins after an event involving many people, like a baseball game, or simply looking for any old coins. Serious coin shooters will spend hours, days and months doing historical research to locate long lost sites that have the potential to give up historical and collectible coins.
  • Prospecting
    Prospecting
    Prospecting is the physical search for minerals, fossils, precious metals or mineral specimens, and is also known as fossicking.Prospecting is a small-scale form of mineral exploration which is an organised, large scale effort undertaken by mineral resource companies to find commercially viable ore...

     is looking for valuable metals like gold and silver in their natural forms, such as nuggets or flakes.
  • General metal detecting is very similar to coin shooting except that the metal detectorist is after any type of historical artifact. Metal detectorists may be dedicated to preserving historical artifacts, and often have considerable expertise. Coins, bullets, buttons, axe heads, and buckles are just a few of the items that are commonly found by relic hunters; in general the potential is far greater in Europe and Asia than many other parts of the world. More valuable finds in Britain alone include the Staffordshire Hoard
    Staffordshire Hoard
    The Staffordshire Hoard is the largest hoard of Anglo-Saxon gold and silver metalwork . Discovered in a field near the village of Hammerwich, near Lichfield, in Staffordshire, England on 5 July 2009, it consists of some 3,500 items that are nearly all martial in character...

     of Anglo-Saxon
    Anglo-Saxon
    Anglo-Saxon may refer to:* Anglo-Saxons, a group that invaded Britain** Old English, their language** Anglo-Saxon England, their history, one of various ships* White Anglo-Saxon Protestant, an ethnicity* Anglo-Saxon economy, modern macroeconomic term...

     gold, sold for £3,285,000, the gold Celtic Newark Torc
    Newark Torc
    The Newark Torc is a complete Iron Age gold alloy torc found by a metal detectorist on the outskirts of Newark-on-Trent, Nottinghamshire, England in February 2005....

    , the Ringlemere Cup
    Ringlemere Cup
    The Ringlemere Gold Cup is a Bronze Age vessel found in the Ringlemere barrow near Sandwich in the English county of Kent in 2001 by metal detectorist Cliff Bradshaw. Although badly crushed by recent plough damage it can be seen to have been 14 cm high with corrugated sides...

    , West Bagborough Hoard
    West Bagborough Hoard
    The West Bagborough Hoard is a hoard of 670 Roman coins and 72 pieces of hacksilver found in October 2001 by metal detectorist James Hawkesworth near West Bagborough in Somerset, England.-Discovery, excavation and valuation:...

    , Milton Keynes Hoard
    Milton Keynes Hoard
    The Milton Keynes Hoard is a hoard of Bronze Age gold found in September 2000 in a field near Monkston, Milton Keynes, England. The hoard consisted of two torcs, three bracelets, and a fragment of bronze rod contained in a pottery vessel...

    , Roman Crosby Garrett Helmet
    Crosby Garrett Helmet
    The Crosby Garrett Helmet is a brass Roman cavalry helmet dating to the late 1st to mid 3rd century AD. It was found by an unnamed metal detectorist near Crosby Garrett in Cumbria, England, in May 2010, close to a Roman road, but a distance from any recorded Roman settlements. The helmet is thought...

    , Stirling Hoard
    Stirling hoard
    The Stirling torcs make up a hoard of four gold Iron Age torcs, a type of necklace, all of which date to between 300 B.C. and 100 B.C. and which were buried deliberately at some point in antiquity. They were found by a metal detectorist in a field near Blair Drummond, Stirlingshire, Scotland on 29...

    , Collette Hoard
    Collette Hoard
    The Collette Hoard was found in fields near Berwick-upon-Tweed, Northumberland, England by metal detectorist John Minns in April 2005. The hoard is named after Collette, the eight year old daughter of Minns, rather than the location it was found at, in order to keep the find location secret.The...

     and thousands of smaller finds.
  • Beach combing is hunting for lost coins or jewelry on a beach. Beach hunting can be as simple or as complicated as one wishes to make it. Many dedicated beach hunters also familiarize themselves with tide movements and beach erosion. There are two main techniques for beach hunting. The first one is called "gridding", which is when you search in a pattern. For example, you start from the beach line, and work your way down to the shoreline, move to the side a little, and repeat the process. The next technique is called "Random searching". Random searching is when you walk around the beach in no particular pattern, hoping to cover more ground.
  • Metal detecting clubs across the United States, United Kingdom and Canada exist for hobbyists to learn from others, show off finds from their hunts and to learn more about the hobby.

Security screening

A series of aircraft hijacking
Aircraft hijacking
Aircraft hijacking is the unlawful seizure of an aircraft by an individual or a group. In most cases, the pilot is forced to fly according to the orders of the hijackers. Occasionally, however, the hijackers have flown the aircraft themselves, such as the September 11 attacks of 2001...

s led the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 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
Outokumpu
Outokumpu is a group of companies headquartered in Espoo, Finland, aimed at stainless steel. The company has approx. 8 000 employees in about 30 different countries worldwide...

 adapted mining metal detectors, 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
Rapiscan Systems
Rapiscan Systems is a global privately held company that specializes in walk-through metal detectors, and x-ray machines for airport luggage and cargo screening...

 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

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
Mars Bar
Mars is a chocolate bar manufactured by Mars, Incorporated. It was first manufactured in Slough, Berkshire in the United Kingdom in 1932 as a sweeter version of the US Milky Way bar which Mars, Inc. produced...

.

Basic Operation

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
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

. 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 where by 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.

Civil engineering

In civil engineering, special metal detectors (cover meter
Cover meter
A cover meter is an instrument to locate rebars and measure the exact concrete cover. Rebar detectors are less sophisticated devices that can only locate metallic objects below the surface...

s) are used to locate rebar. Rebar detectors are less sophisticated, and can only locate metallic objects below the surface.

See also

  • DEMIRA Deutsche Minenraeumer e.V.
  • Portable Antiquities Scheme
    Portable Antiquities Scheme
    The Portable Antiquities Scheme is a voluntary programme run by the United Kingdom government to record the increasing numbers of small finds of archaeological interest found by members of the public...

  • Inductive sensor
    Inductive sensor
    An inductive sensor is an electronic proximity sensor, which detects metallic objects without touching them.The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls asymptotically toward zero from its initial level when...

  • Access control vestibule
    Access control vestibule
    The Access Control Vestibule is a security screening system for the detection of individuals carrying weapons into any facility without authorization, therefore minimizing the possibility of an armed takeover. The ACV is composed of an aluminum frame, separate entrance and exit doors, a metal...

  • Cover meter
    Cover meter
    A cover meter is an instrument to locate rebars and measure the exact concrete cover. Rebar detectors are less sophisticated devices that can only locate metallic objects below the surface...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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