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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING History of metal detectors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur The theory of electromagnetism was first demonstrated by the American Joseph Henry and independently by Michael Faraday in 1831. Henry soon made successful experiments with induction and self-induction, which became the basis for the telegraph, telephone, and radio. He extended his experiments with induction using flat spirals of insulated wire - the first coils. A lot of experiments, set by various researchers, studied the effect of metal objects on inductance, as well as the principle of balancing inductive effects on one part of the circuit with equal and opposite effects on the other part. An early form of induction balance for this purpose was apparently invented in Germany by Professor Dove around 1841. At about the same time similar apparatus was independently invented in America by Professor Henry Rowland. In 1976, Professor Alexander Graham of Bell turned his attention to inductance balancing due to the problem of telephone noise caused by telegraph equipment on lines running close to telephone wires. The interference was eliminated by using two conductors instead of one, since the currents induced in one conductor were exactly equal and opposite in direction to the currents induced in the other conductor; thus an inductive balance was formed, and the circuit had a zero signal at the output. This method was patented in England in 1877 by Bell, and during the winter of 1877-78 in London Bell was experimenting with this method. He found that when the circuit is balanced, a piece of metal placed in an inductance field causes sound in the telephone (receiver). When a half-crown silver coin or florin moved in front of the coils placed in parallel, the silence on the telephone was interrupted three times. Bell's English acquaintance, music professor Daniel Hughes, experimented with inductive balance in 1878 and demonstrated in July 1879 a more promising device for inductive balance using four coils, in which, using the latest patented electric microphone and the ticking of a clock, an electrical disturbance was created in a circuit containing two main coils and two secondary coils connected to telephone headphones. When a piece of metal was placed in the vicinity of one pair of coils, the balance was disturbed, and the ticking of the clock became audible in the headphones.
When Bell returned to America, he published an article "On New Methods for Investigating the Induction Field of Flat Coils" in August 1879, at the request of Gardner Hubbard, who saw here a possible way to discover deposits of valuable metals in the earth. On July 1881, XNUMX, President Gardfield was shot in the back by an assassin. For the next hours and days, the whole world waited in hope and fear, but no one could dare to predict the end, since the position of the bullet in the body remained unknown. Bell, who was in the city of Washington at the time, offered his help. He quickly made some preliminary experiments. On July 11, 1881, George Hopkins of Scientific American published his results using improved Hughes inductive balance methods in the New York Tribune. Bell, with the help of Summer Tainter, contacted Hopkins, and, with Hughes, Rowland, and John Throwbridge of Harvard, organized a community to help develop a bullet detection device. They experimented with balanced devices of different sizes, different lengths and diameters of coils, different batteries, and eventually added a capacitor to the circuit so that now a similar lead bullet would be found at a distance of two inches in a clenched fist. On July 26, Bell brought his equipment to the White House. After tuning, he heard hissing sounds and found that the detection range seemed to be insufficient. The instrument was unable to detect the bullet. It was later found that the capacitor was connected to only one of the two coils. Bell returned in August and heard a faint sound from the instrument over a large area of Garfield's body. The next day, he discovered that the president's mattress was supported by steel springs. Later, on September 19, the president died. The autopsy showed that the bullet was too deep to be detected by Bell's equipment. On October 24, 1881, Bell was in Paris, where he successfully demonstrated the method of induction balance and published the article "Successful application of induction balance for the painless detection of metal objects in the human body." His equipment could detect a bullet at a distance of 2,5 inches, 5 inches when the bullet was on the axis of the reel, and 1 inch at the edge. In conclusion, he explained that the depth at which an object lies below the surface of the earth cannot be determined if the shape of the object and the angle of its projection are unknown. Bell's attention was drawn to other work until December 1882, when he made an experiment with a coil to detect metal veins in the ground, also the purpose of the experiment was to detect underground telegraph wires.
In February 1887, Dr. John Ginder of New York, who had listened to Bell's speech 5 years earlier, published the results of his experiments to detect metal objects in the human body. His apparatus consisted of a two-chromium battery of six cells, an ordinary interrupter with an interruption frequency of about 600 Hz. The searchcoils were mounted in a wooden case, which he called the "Explorer", the other coils were called "tuning". This device could detect a bullet at a depth of 6 inches in the human body, in the ground the range was less. At the end of the century, Captain McEvoy, who had been experimenting with Hughes's apparatus, reduced the metal detector to a size that would allow it to be used underwater. The portable, sealed case contained tuning coils, an interrupter, a two-cell battery that could be replaced by a small magnetoelectric generator producing alternating current, and headphones. An insulated cable connected pairs of coils. Rubber washers, ivory screws and hard rubber handles were used to reduce interaction with metal parts. When the coil was immersed in water, if it was moved near the bottom and a piece of metal appeared in its field - a torpedo body, a chain, an underwater cable, then the balance was disturbed and the sound in the telephone, which was very weak before, became very loud and clear. The only drawback was that a metal object lying exactly under the coil did not affect it.
During this time, Georges Hopkins, who continued to study metal detection, invented a device for finding metal ores that did not use an induction balance, the coils of which were installed perpendicularly. A typical 6 or 8 inch coil could detect minerals lying at the surface at a depth of several inches.
During the First World War, some attention was drawn to bomb detectors, but no documentation of the practical use of these detectors has been found. In 1915 M.S. Gutton from France experimented with a similar device, but he did not manage to completely balance it. His apparatus consisted of two transformers in the form of five coils connected to a Maxwell bridge. After experimenting with the Gutton apparatus and the Anderson bridge, in 1922 the US Bureau of Standards published an article "Inductive Balance for the Detection of Metallic Bodies". In early 1924, Daniel Chilson of Los Angeles invented and patented an electromagnetic detector known as the "radio" detector. His apparatus used a new beat circuit that became known as the "Chilson Bridge". The first successful search for buried treasure using a "violet beam" or "radio" device that indicated the presence of the treasure was reported by James Young in the New York Times in 1927. The search was arranged by one American and two English adventurers with a four-year government license in the Isthmus of Panama. The finds included gold chains, jewelry, and plates hidden by pirates. Mr. Young went on to state that it had only been a year or two since it had been possible to board the sunken ship for the treasure. He participated in organizing the search for lost treasures on a large scale. Radio apparatus, he said, had brought success where man had searched in vain for more than two centuries, and he predicted that future success with the use of new radio treasure-hunting devices would no doubt come from the West Indies, the Florida Keys, and the coast of Mexico. Obviously, the first book on metal detection was R.J. Santsky, Modern Dowsing: The Construction and Use of Electronic Metal Detectors, published in 1927. It became so popular that it was reprinted in 1928, 1931, and 1939. In 1929, Gerhard Gischer, of Hollywood, California, a research engineer advising the Radio Corporation (known for its geophysical surveys for the mining industry), patented the "Metalscope". It weighed 22 pounds (10 kg) and was equipped with dry batteries, vacuum tubes and headphones. To work with him did not require any qualifications or special training. The operator was between a vertical transmitter and a horizontal receiver, which were connected to each other by wooden handles. The tube voltmeter recorded the perturbations caused by the metal. The depth of the objects could not be measured, but if you notice the angle of the transmitter at which the arrow deviates as much as possible, then making measurements from different points, and then plotting on paper using trigonometry, you can get the position of the objects with quite acceptable accuracy. Selling for $200, the device became widely used by utility companies to quickly and accurately find old pipelines, cables, conduits, steel rails, and other buried objects, and it was also used by prospectors to find ore veins near the surface. In addition, Fisher prepared drawings and instructions and made them available to hobbyists using standard radio components. Soon this device, called the "M-Scope", was used as a "treasure finder" by those who believed they knew the approximate location of buried treasures. The simplest set, sold for $ 95 - MT-Scope, which had an average sensitivity and adjustable detection depth, used a tube voltmeter as an indicator. A third Fisher circuit was later developed, but never made it to the commercial market. She used only three lamps and one double coil instead of separate coils for the transmitter and receiver. Fisher also noted that the longer the buried object, the easier it is to detect. Shortly thereafter, the Fisher M-Scope was a market success, with blueprints published to assemble a homemade "radio finder" that could find a silver dollar several inches underground, indicated by a buzzing sound in headphones. The spools used were 28″ wooden bike rims. In 1930, physicist Theodor Theodorsen, working for the National Advisory Committee for Aeronautics, reported that Langley's laboratory had developed an "Instrument for Detecting Metallic Bodies in the Earth" designed to directly detect unexploded bombs dropped from aircraft. The bombing site was near a new seaplane testing canal at Langley Field, Virginia, which was being renovated at the time. The new "detector" successfully located many bombs buried in or nearby, including 17 pound bombs at a depth of 2 feet. This detector, known as the NACA bomb detector, was of simple design and did not require a skilled operator. The design was based on the work of M.S. Gutton from France. Three coils were wound on a hollow wooden frame 3 feet in diameter and 1-1,5 feet high. The coils were hung on a ladder-shaped frame, two people were required to operate the device. The device was powered by 110 volt batteries placed in a large box. In 1935, a metal detector was designed to search for underground wells outside the walls of a leading American university. The search radio device soon proved to be a sensitive treasure-hunting tool, and drawings of it were made available to hobbyists in popular magazines. Like most detectors of the time, it had to be at an acceptable distance from the target to work, and it could not distinguish between ferrous and non-ferrous metals. And although some detectors were able to compensate for the influence of the operator's body and the ground, others reacted to strips of wet soil and wet plant roots. But even the best detectors were useless on the ocean beaches, which contained a lot of magnetic black sand. During this time, the "Invisible Weapon Detector" was used in prisons to detect magnetic metals. The presence of metal could be judged by the sharp deflection of the cathode ray tube beam. The device provided good sensitivity, but was difficult to set up. In 1938, a tunable inductive bridge circuit was developed to detect metal particles in cigars. This circuit had good sensitivity and stability and could operate under any temperature, humidity, dust and vibration. It was also a feature of the circuit that was easy to adjust and compact, and this circuit was more stable than beat devices. In 1939, Harry Faure published a circuit for an inexpensive detector using a Chilson beat-on bridge, unresponsive to external interference and tuned to zero beats. He used a single coil and the detection signal was "clucking sounds" made by headphones with a resistance of 4 kOhm. When properly adjusted, the instrument could detect a 3-inch square of metal at a depth of 12 inches, and a 10 cent coin at a depth of several inches. In December 1939, Dr. Lincoln La Paz of Ohio State University presented a paper on meteorite detectors to the Astronomical Society. The three instruments were designed and built using research done by Theodorsen in developing the bomb detector. The first instrument was a large three-coil detector driven by a generator powered by a gasoline engine. The device could fit in the trunk of a car. The second design also had a three-coil system driven by a tube oscillator and was small enough to be carried in a backpack. Searchcoils of any size could be connected to the device as easily as screwing a light bulb into a socket. The third design proved to be the most successful. It consisted of a search and emitting coils, and compared to commercial devices had half the power consumption when powered by batteries. Weighing less than 15 pounds, this device could be used anywhere a person could get. The development of the Second World War required the immediate development of mine detectors. The work was carried out by the research department of the Ministry of Supply. They soon developed nine experimental detectors. The problem was to develop a device capable of withstanding harsh operating conditions and that its weight would be acceptable for a soldier. In addition, it needed to be uncomplicated, require a minimum number of people to operate, and be made from simple, interchangeable parts for quick replacement. In the end, a single-tube generator designed by William Osborne in 1928 was used. At the beginning of October 1941, the research team was near the final stage when they received details of a new model developed independently by two lieutenants of the Polish Army. It did not contain new principles, but its layout promised advantages in production and operation. It immediately became clear that the Polish design was very good, so test models were created based on this design. Production from began in 1941. The detector consisted of a flat disk - a search coil, and had dimensions of 8x15 inches. The movable rod was attached to the center of the coil, there were two control knobs on the rod handle. Everything else was in the operator's shoulder bag. The first order for the production of detectors was placed among various British firms producing radio equipment. These "modernized" detectors have become standard designs and are still in use today. Significant experimental work in 1942 led to the introduction of the frequency modulation detector. Known as the FM Locator, it proved to be very stable and featured ground balance adjustment.
In 1943, William Blackmer improved the beat circuit. In the same year, a Winston bridge was developed for measuring resistance in a mine detector. This device, pushed forward along the ground like a scrubber, was assembled from 250 components contained in 29 blocks. In the immediate aftermath of the war, as stores selling military equipment scraps spread throughout North America and Europe, thousands of metal detectors were offered to the public at prices ranging from $5 to $50 dollars. Needless to say, this spawned a new wave of experimenters and treasure hunters. In 1946, Harry Faure published drawings for building an electrically coupled beat detector based on British Army research. Its design was aimed at advanced experimenters, and has not yet held the excellent position of the original Chilson detector as strong as commercial instruments. In addition, many improvements have been added to the design. The instrument could detect a one-square-foot plate of metal at a distance of 12 inches. Indication was carried out by increasing or decreasing "clucking" sounds. The mine detector research carried out during the war was a boon to those interested in discovering hidden treasures. As new instruments with greater sensitivity and modernized appearance grew in popularity, many small companies began to manufacture and sell detectors and treasure hunting equipment. The three main types of detectors were the bridge circuit, the beat circuit, and the radio balance circuit. Another technological breakthrough, the transistor, changed the design and performance of detectors for over a decade. Today, nearly half a century later, the metal detecting hobby and industry is still growing and thriving. And while the underlying principles have remained unchanged for a long time, some amazing innovations have been made in the current generation of detectors: Discrimination, Very Low Frequency Motion Discrimination, Notch Discrimination, Visual Target Identification and Depth Indication, One-Button Adjustment and Auto-Setup, precise manual balance and automatic ground balance, multi-frequency capability, advanced pulse design, high performance computerized and miniaturized detectors, ergonomic housing designs, and more. One can only dream of what tomorrow will bring! Roy T. Roberts is currently researching the history of metal detectors and treasure hunting and would like to enlist the support of WE&N readers. His address is 20609 Dundas Street, London, Ontario, Canada NSW 2Z1. Author: Roy T. Roberts
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