Don Crawley had looked for treasure with his metal detector for years, finding nothing. But, recently, while on a visit to a farm in Suffolk, England, he found the prize of a lifetime! A hoard of buried Anglo Saxon silver pennies minted during the reign of Aethelred II, King of England who ruled from AD 978-1016.
As Don explains: “It was my first visit to this farmer’s land and after walking up an incline in the field, my detector gave off such a strong signal and within a short space of time I had recovered 93 coins.” The collection was taken to the British Museum which examined the booty, saying they were probably worth $60,000. Not a bad day’s work.
A device that can sense the presence of a nearby piece of metal has been around a long time. When American President James Garfield was shot in 1881, Alexander Graham Bell brought to the doctors attending him an electrical apparatus to attempt to locate the bullet lodged in his chest. Unfortunately, because the physicians did not let Bell run it himself and kept him out of the room, the detector did not work because later on they acknowledged using it while Garfield was lying on a metal coil spring bed.
Towards the end of the 19th century, many scientists and engineers used their growing knowledge of electrical theory trying to devise a machine which would pinpoint metal in ore-bearing rocks, particularly nuggets of gold or silver. Early machines were crude, used a lot of battery power, and worked only to a very limited degree.
The modern development of the metal detector began in the 1920s when Gerhard Fischer, a radio engineer, noticed there were anomalies in the transmission of radio signals 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 metallic objects using a search coil resonating at a radio frequency. In 1925 he was granted the first patent for a metal detector.
The advent of WWII led a Polish Officer, Lieutenant Józef Stanisław Kosacki, a skilled electronics technician, to refine the Fischer design into a practical mine detector. Although these units ran on vacuum tubes and needed separate battery packs and were therefore quite heavy, over 500 units were used during the Second Battle of El Alamein and shipped to Britain’s Field Marshal Montgomery to clear the minefields left by the retreating Germans.
There are basically three different types of metal detector topology in use today. The oldest and simplest design uses beat frequency oscillators. One oscillator is crystal controlled to a frequency of say, 1 MHz, and the other oscillator, using a big loop of wire as an inductor, is tuned just a little bit higher. Because there is a difference in frequency, any mixing of the signals yields a tone that can be heard in earphones. When the search loop is brought near metal, it detunes the second oscillator, changing the audio pitch in the receiver. This type is one of the most common metal detectors, simple to use and capable of looking for coin-sized items situated approximately 6 inches underground. Due to the high magnetic permeability of iron alloys, objects made of that metal can be detected over quite a distance. A buried horseshoe can probably be found at a depth of two feet.
The next sensitive detector of the group is called a VLF type, which stands for Very Low Frequency. This is the kind that a serious treasure hunter would use because it can detect a coin at a foot and also selectively filter out signals from unwanted objects such as aluminum cans or steel nails. Inside the search head is a transmitter coil of wire. Electronic current is driven through this to create an AC electro-magnetic field. A second coil of wire placed nearby, the receiver, is arranged by orientation and electronics to disregard any signal produced by the transmitter field. If however, there is a nearby metal object, a signal will be sensed in the receive coil because the metal generates its own magnetic field that is shifted in phase from the transmitter signal. The largest phase shift will occur for metal objects which are primarily large and thick. Objects made from excellent conductors like gold, silver, or copper make the largest phase shifts from the transmitter signal. Smaller shifts are typical for objects which are primarily resistive such as thinner objects, or those composed of less conductive materials and are probably unwanted such as iron bottle caps, bent nails, or aluminum foil. The tiny current in the receive coil is amplified and processed by the detector’s electronics and displayed on a screen which shows the target’s likely identity.
The third type of detector is one that uses high power pulse induction. These are specifically employed to look deep into the ground for larger relics, gold, and treasure. A single coil is employed for this purpose and it transmits a magnetic field far into the ground. They are intended for maximum depth under extreme search conditions such as salt-water beaches and highly mineralized ground where VLF types cannot be used.
Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at firstname.lastname@example.org or email@example.com.
A small block of wood is floating in a beaker of water. Joe places a stack of 30 pennies on the wood. If he places 31 the block will sink. He makes a mark on the side of the beaker of the water level. Suddenly the floating block tips over and all pennies fall in. Is the water level now higher or lower than the mark?
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