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Metal detectors use electromagnetic induction to detect metal. Uses include demining (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 metal, which is electrically conductive, 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.

The first detectors

Metal detectors have been around for much longer than most people realize. 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 (IB) system, which was incorporated into metal detectors a hundred years later. Early machines were crude and used a lot of battery power, and only worked to a very limited degree. The Scottish physicist, Alexander Graham Bell, used such a device to attempt to locate a bullet lodged in the back of American President James Garfield in 1881.

Modern developments

The modern development of the metal detector began in the 1930's. Gerhard Fischer had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Dr 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 the Second World War. 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 for profit. The hobby of metal detecting had been born.

Further refinements

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, and are still at the leading edge of detector innovation today. Another leader in detector technology was Charles Garrett, who pioneered the BFO (Beat Frequency Oscillator) machine, and whose company is still one of the world leaders in design. 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 artefacts were coming out of the ground. Suddenly, there was a huge requirement 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 jewellery, coins, bullets, and other various artefacts buried shallowly underground.

Discriminators

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 certainty that, sooner or later, the user would scan over, and ignore, a valuable find - perhaps a diamond engagement ring on a beach.

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; 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 70's, 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 70's 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 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 jewellery could now be located even under highly-mineralised '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 mineralised 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.

Future detectors

Metal detectors have come a long way, from the simple one-coil BFO, to today's sophisticated machines. Modern top models are fully computerised, 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. We can expect to see more improvements as designers continue to apply the latest Electronics and Computer technologies to the task of making ever better metal detectors.

While these advances push technology to its full potential, 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.

Metal detectors in archaeology

The use of metal detectors to search for archaeological finds is practised both by archaeologists and hobbyists. In some European countries including France and Sweden the use of a metal detector is forbidden by law, unless one has special permission. This is intended to protect archaeological sites but rarely means that illicit metal detecting ('nighthawking') does not take place and has the effect that new sites found by metal detector are never publicised or investigated fully. Instead, they are slowly plundered for their metal items, often disturbing the stratigraphy and forcing the artefacts on to the Black Market.

In the United Kingdom 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. In England and Wales 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.

Countries with heavy restrictions are in danger of sites being regularly raided (night hawking) and their contents sold on without the information they provide ever being known. Strict legislation often results in exacerbation.

Archaeology is beginning to recognise the contribution responsible metal detecting provides in adding to the knowledge of our past. One example is utilising 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.

As a hobby

Many people use consumer metal detectors to look for coins on the beach. Most metal detectors are only good to detect metal 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 four major types of hobbyist activities involving metal detectors:

  • Coin shooting - looking for coins after an event involving many people, like a baseball game, or simply looking for any old coins
  • Prospecting - looking for valuable metal like gold and silver
  • Relic hunting - looking for items that are historically significant, like weapons used during a war
  • Treasure hunting - looking for items that are rumoured to be hidden
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