The first mention of the detectors can be found in ancient Chinese documents of the II century BC, which describes the entrance into the apartments of the Emperor, made of natural magnetite in the form of an arch, having the shape of a horseshoe magnet. Like this the magnet has a high magnetic force and attracts any iron objects, including weapons made in the room of the Emperor.
In the late nineteenth century in different countries conducted research fashionable phenomena the magnetic induction. In USA it is actively engaged Alexander Graham bell, inventor phone. In 1881 he was mortally wounded President George W. Bush. Hartfield. Physicians, having heard about the experiments of bell, was asked to help find the bullet in the body of the President. However, the device Bella was still very primitive and the bullet could not be found.
In the beginning of XX century metal detectors were used mainly in search of useful minerals, in particular electrically conductive sulphide ores in the United States and England. These the devices were relatively bulky and therefore were mounted on cars. Powerful DC generator with a huge radiating coil created electromagnetic field, which penetrated into the ground for a few meters and, Bouncing off the metal and other conductive bodies (some ores metals), and captured take-up spool. To change the characteristics of secondary field (amplitude and phase shift) it was possible to judge the presence of a the object in the ground. Subsequently, these powerful machines have been banned in countries signatories to the Convention on the limiting levels of electromagnetic waves.
In 1925, in Germany appeared first arched metal detectors, allowing to detect metal objects that are working secretly carried with the territory of the factory. At about the same time a German researcher W. Herr developed the principle of magnetic induction balance and invented the metal detector, working on this principle.
This idea was picked up by various firms, in particular Radio Metal Locating Company in the U.S., which first began the production of hand-held metal detectors at the end the 20-ies. Reception and transmission coils were separated by a distance of 2 meters wooden frame, to eliminate mutual electronic interference and to ensure the condition of the induction balance. Any metal object, trapped in the electromagnetic field of the transmitter, violated the induction balance, and in the receiving coil arose voltage, which is amplified in an audio signal warn the operator about the discovery. The unit worked for 6 tubes, was quite bulky, but successfully found the pipe at a depth of 3 m. However, in the early 30-ies the company went bankrupt.
After the great depression in the United States came a building boom, and reappeared the need for devices to search for cables and pipes. This prompted Gerhard Fischer, a German engineer, who emigrated in 1923 in the USA, to develop of such devices. In 1937, he received his first patent for a metal detector, named metalloscope, and opened the firm for its production. Metalloscope had a more complex scheme, which used 9 bulbs. The operating frequency was downgraded to kHz, which provides deep penetration of the electromagnetic fields in the ground. In 1938 diagram of the device was described in detail in a number of popular magazines, so many hams have made such devices for its hands.
At the same time, another American firm, Goldak Company, has released similar metal detector with spaced coils called radiokop. For the first time solved the problem ground balancing. The same firm was first patented the design of detectors with a round search coil, which have almost all modern metal detectors. It should be noted that the search devices 30's, he worked at radio tubes, were made mostly of wood and weighed from 15 to 25 kg.
At the end of the 30-ies were mainly developed devices with round search coil, rod and the electronic unit is mounted on the rod. This the design was more comfortable and allowed how to find small objects and hidden treasures. Widespread search for pipe received the instruments on the beat that would later be used to find the min.
The use of mine detectors to search for the treasure began in the late 40-ies in the USA, when military stockpiles began to sell off obsolete equipment. Used them at the time, mainly to search for treasure and gold nuggets. To work with so heavy and uncomfortable devices could only enthusiasts. And only when the end of the 50s, appeared small and light devices on transistors, new the craze has begun to spread widely across America, stimulating the emergence of numerous firms producing the metal detectors. These devices worked or the beating or by the principle of induction balance. The sensitivity was low (10-15 cm for coins), ground balancing and discrimination was absent, but nevertheless they were much more comfortable military mine detectors and allowed to find many coins, rings and other jewelry jewelry Main producers of metal detectors in the USA in the 60s were G. Fischer, C., Garrett, E., Flight, W. Megan.
Significant interest in searching for gold and coins originated in the 70s in the United States in connection with a sharp increase in the cost of gold. The fact that in the United States from 1933 to 1974 U.S. citizens were forbidden to have a native gold in private hands. The government supported in this point, the artificially low price of gold at 35 us dollars per ounce). After the abolition of this law, the price of gold began to rise sharply, reaching at the end 70s up to $ 800 per ounce. This served as a new impetus in the search for gold and stimulated the production of metal detectors that have become widely used for these objectives. There were dozens of companies producing such devices. However the acute competition only a few firms in the 70s managed to survive it Garrett Electronics, Inc., Fisher Research Laboratory (in 2006 this company bought firm First Texas International), White's Electronics, Inc. (USA) and C-Scoop (England). In the 80s originated and successfully developed, finding its niche, firms like Tesoro Electronics, Inc.,Ltd. and Teknetics in USA, Minelab in Australia. Appeared small, stable and very sensitive devices.
As you progress in the design of the detectors were successfully solved such the problem, as the detuning from metal debris and conductive minerals soil (iron oxides and salts), whereas until the end of the 70s. these two processes it was impossible to fulfill simultaneously. Operating frequency of the devices was reduced from 100 kHz to 1 to 5 kHz. Invented the scheme of continuous tuning of the device during search, appeared efficient switching devices with discrimination, etc. Metal detectors every year it becomes more difficult and, unfortunately, harder. Listening to the complaints of the search engines, G. Fischer in the early 80-ies has developed brand new unit (1260s), which includes automatic detuning from the ground, and automatic discrimination using new electronic components and a new layout of the device. It was lightweight, easy to handle and quite the effective detector dynamic action, and soon other companies began to use this principle of operation and the same layout.
Signal processing from the facility every year became more and more precise and significantly improved in the mid-90s with the advent of computer devices. The first patent on computer the metal detector has received a firm Garrett Electronics, Inc., however on the market the first computer device is manufactured by White's Electronics, Inc., last modification of which is a Spectrum XLT and DFX are currently the best in the world. All other leading firms now also produce computer devices.
In Russia before the end of the 80s metal detectors in household virtually no, though interest in treasure hunting showed not only individual citizens, but also some office. This is typically used mine detectors. Enthusiastic Amateurs collected devices by the schemes published in magazines, upgraded mine detectors, making them more sensitive and stable, but it was individual instances. At the beginning of the 80s the best Amateur design was, perhaps, pulse metal detector In Gorchakov. First foreign devices appeared on sale in Russia in the late 80s.
Modern metal detectors can do almost everything except digging findings: to detect metal to determine to which group it belongs, what are its size, the depth of lies, to establish its exact location not to react to the minerals of the soil and metal debris. But while in these devices it has limitations. They can't find a coin at a depth of 50 cm, show the screen shape of the subject, to determine the chemical composition of the metal, can't find only the gold. The detectors of the future, undoubtedly, will have such opportunities.
Author: L. V. Bulgak