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How Metal Detectors Work

Metal detectors use electromagnetic induction to detect metal. Uses include demining (the detection of landmines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archeology 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 it’s 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. 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 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. 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 Fischer 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. 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, jeweled daggers, arrowheads- all types of metal artifacts 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 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 certainty 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 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. 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 jewelry 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. Metal detectors have come a long way, from the simple one-coil BFO, to today’s sophisticated machines. 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. 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.

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 the 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 rumored to be hidden

Before any user of a metal detector can confidently operate their machine, they need to know several vital steps that apply to most metal detector uses. The first step is “tuning” in your detector to the ground. This means that the detector is adjusted so that it recognizes the ground as a medium for targets to be in. Most new digital units have a “self-tuning” feature installed. For the ones that don’t, you must manually tune them in. After this is achieved, you’re ready to begin searching for your target, whether it’s a coin, gas line, or what have you. Placement of the loop above the ground is critical for desired performance. Hold the detector at arm’s length with the loop barely touching the ground. For most purposes, a “motion” mode is used. This means the loop must be moving at all times for it to function correctly. With the detector now ready to go begin to sweep it back and forth along the ground. Each sweep should take close to a second from one side to the other. Additionally, each sweep should overlap the last forming a tight sweep pattern. This way you will be less likely to pass over the target. While you are searching with your detector, a slow even pace should be established.

How to Pan for Gold

1): Fill the gold pan about half full of gravel, then submerge it deep enough in water (stream or tub) so it is just under the surface of the water. Next give the pan several vigorous shakes back and forth and from side to side, but not too hard as to wash material out of the pan, you want to shake the gravel so the heavy gold sinks to the bottom of the pan and the less dense rocks and minerals float to the top of the pile in your pan.

2): Change from the shaking motion to a gentle circular movement, so the remaining material starts turning in a circle. This process will cause most of the dirt and clay to dissolve and wash out of the pan. If roots and moss surface, work them over your pan with your fingers to dissolve any lumps. Pick out the larger rocks, pebbles, and debris after making sure that they are washed clean of any mud or sand. You want to try and recover as much gold as possible but you will lose some in the process so don’t be to gentle in the operation. Do this until you have the larger rocks and pebbles washed out of the pan.

3): Now wash away the lighter sand and gravel. Hold the pan just under the water and tilt it slightly away from you. Begin to swirl the water from side to side, with a slight forward tossing motion. Take care, but with sufficient force to move the surface and the lighter gravel out over the edge of the pan. Leveling the pan out from time to time and shaking it back and forth will cause the light material to come to the surface and the gold to settle to the bottom. Keep working the pan until you are down to black sand and color(gold).

4): Now you will want to start carefully working the black sand. Now is the time to slow down and be careful. Leaving just a little water in the gold pan, lift the pan out of the water and use your tweezers to pick out any nuggets and large flakes and place these in your poke. You can use a magnet to separate the magnetite (black sand) from the gold. Continue panning the con down until it is about a third gold. As you pan you will notice that the gold will form in a tail behind the black sand. At this time you can clean up the gold with your tweezers and eye dropper. Keep panning until you are out of gold or out of black sand in the bottom of the pan.

Learning How to Pan for Gold

The easiest way to learn how to pan for gold, is to watch someone else do it first. You can’t appreciate how vigorously you have to shake the pan at the beginning until you see a master gold panner in operation. Many of the commercial gold panning tours in Alaska will teach you how to pan for gold, and will get you into the color in about an hour. If you want to learn prior to coming up to Alaska, you should consider joining a local gold prospecting club. You will get the chance to practice and also see what other people are using. If you don’t ave the time or desire to join a club the next best thing is to buy a video or book. I normally buy this type of specialized material from Amazon gold panning.

Gold Panning Kits

Many prospecting stores sell a gold panning kit. These kits normally consist of a gold pan, book and/or video, and a bag of sand with gold ore. The store will place a number of gold flakes and small nuggets in the bag of sand/gravel so that you will know the amount of gold you should be recovering. With these gold panning kits it is recommended that you pan the pay-dirt over a small tub or child’s play pool so you can recover the sand and gold you missed while you learn. It isn’t a bad way to go but not nearly as exciting as learning on a stream in the great outdoors.