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A metal detector is a tool a person can use to help find metal nearby. It is useful for finding metal within other objects, or for finding metal underground. Most metal detectors will make a sound or have a needle move on a sensor in order to show where objects are. Most of the tiem a metal detector will tell you how close you are to an object - for example, by making a higher-pitched sound when you are closer to an object.
History of the metal detector
Alexander Graham Bell invented a kind of metal detector in 1881 in a to try and find the bullet in President James Garfield's body after he had been shot. The metal detector worked but it was confused between the bullet and a coiled bed-spring which he had been lying on. Gerhard Fischer made a better version in the 1920s but the detector got confused when someone was approaching ore-bearing rocks. He was given a patent in 1925.
A businessman named Shirl Herr had also tried to get a patent for a metal detector but was not given one until 1928.