A metal detector is an instrument that detects the nearby presence of metal. Metal detectors are useful for finding metal objects on the surface, underground, and under water. A metal detector typically consists of a control box, an adjustable shaft, and a variable-shaped pickup coil. When the coil nears metal, the control box signals its presence with a tone, numerical reading, light, or needle movement. Signal intensity typically increases with proximity or metal size and composition. A common type are stationary "walk through" metal detectors used at access points in prisons, courthouses, airports and psychiatric hospitals to detect concealed metal weapons on a person's body. The simplest form of 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 electrically conductive metal is close to the coil, eddy currents will be induced (inductive sensor) in the metal, and this produces a 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 industrial metal detectors came out in the 1960s, and were used for finding minerals, among other things. Metal detectors help find land mines. They also detect weapons like knives and guns, which is important for airport security. People most commonly use them to search for buried objects, like in 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.
History and development
First idea The first metal detector was likely the simple electric conduction metal detector ca. 1830. Electric conduction was also used to locate metal ore bodies by measuring the conductivity between metal rods driven into the ground. In 1841, Professor Heinrich Wilhelm Dove published an invention he called the "differential inductor". It was a 4-coil induction balance, with 2 glass tubes each having 2 well-insulated copper wire solenoids wound around them. Charged Leyden jars (high-voltage capacitors) were discharged through the 2 primary coils; this current surge induced a voltage in the secondary coils. When the secondary coils were wired in opposition, the induced voltages cancelled as confirmed by the Professor holding the ends of the secondary coils. When a piece of metal was placed inside one glass tube the Professor received a shock. This then was the first magnetic induction metal detector, and the first pulse induction metal detector. In 1862, Italian General Giuseppe Garibaldi was shot and wounded in the foot. It was difficult to distinguish between bullet, bone, and cartilage. So Professor Favre of Marseilles quickly built a simple probe that was inserted into the track of the bullet. It had two sharp points connected to a battery and a bell. Contact with metal completed the circuit and rang the bell. In 1867, Mr. Sylvan de Wilde had a similar detector and an extractor also wired to a bell. In 1870, Gustave Trouvé, a French electrical engineer also had a similar device however his buzzer made a different sound for lead and iron. The electric bullet locators were in use until the advent of X-rays. In late 1878 and early 1879, Professor (of music) David Edward Hughes published his experiments with the 4-coil induction balance. He used his own recent invention the microphone and a ticking clock to generate regular pulses and a telephone receiver as detector. To measure the strength of the signals he invented a coaxial 3-coil induction balance which he called the "electric sonometer". Hughes did much to popularize the induction balance, quickly leading to practical devices that could identify counterfeit coins. In 1880 Mr. J. Munro, C.E. suggested the use of the 4-coil induction balance for metal prospecting. Hughes's coaxial 3-coil induction balance would also see use in metal detecting. In July 1881, Alexander Graham Bell initially used a 4-coil induction balance to attempt to locate a bullet lodged in the chest of American President James Garfield. After much experimenting the best bullet detection range he achieved was only 2 inches (5 centimeters). He then used his own earlier discovery, the partially overlapping 2-coil induction balance, and the detection range increased to 5 inches (12 centimeters). But the attempt was still unsuccessful because the metal coil spring bed Garfield was lying on confused the detector. Bell's 2-coil induction balance would go on to evolve into the popular double D coil. On December 16, 1881, Captain Charles Ambrose McEvoy applied for British Patent No. 5518, Apparatus for Searching for Submerged Torpedoes, &c., which was granted Jun 16 1882. His US269439 patent application of Jul 12 1882 was granted Dec 19 1882. It was a 4-coil induction balance for detecting submerged metallic torpedoes and iron ships and the like. Given the development time involved this may have been the earliest known device specifically constructed as a metal detector using magnetic induction. In 1892, George M. Hopkins described an orthogonal 2-coil induction balance for metal detecting. In 1915, Professor Camille Gutton developed a 4-coil induction balance to detect unexploded shells in farmland of former battlefields in France. Unusually both coil pairs were used for detection. The 1919 photo at the right is a later version of Gutton's detector.
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