What is a metal detector? 
A metal detector is a device which responds to metal that may not be readily apparent. 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 in the metal, and this produces an alternating 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. continue..

Mention the words metal detector and you'll get completely different reactions from different people. For instance, some people think of combing a beach in search of coins or buried treasure. Other people think of airport security, or the handheld scanners at a concert or sporting event.continue..

In the beginning metal detectors, couldn’t pick up any thing deeper than one inch down. As time passes and inventors learn more, the capabilities of the metal detector have greatly improved. On July 2, 1881, American President James Garfield was shot in the chest, and the bullet was left behind. Alexander Graham Bell, tried using one of the first original metal detectors, in order to help locate the bullet. Alexander Graham Bell wasn't able to locate the fatal bullet that was left behind. The following day, it was discovered that the metal detector was confused by the metal coils located in the bed he was laying on. In 1925, Gerard Fisher was the first person to apply for and receive a patent for the first portable tool known as a metal detector. This was a revamped version of the one that Alexander Graham Bell used to try to help James Garfield with. Metal detectors have many uses. They are used medically to find metal in the human body..continue..

This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio.This is nothing but a Colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.This is the set up.When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected..continue..

BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency. One of these oscillators uses a coil of wire that we call the search loop. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. This gives a beat note of 250hz to 0 to 250hz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5hz ( motor boating ) any slight change being noticeable..continue..

The first crude metal detector was devised in 1830 by British geologist and mining engineer R.W. Fox. Numerous refinements increased the efficiency of metal detection technology over the decades, starting an amateur treasure hunting craze in the 1920s and 1930s with metal detector models such as the Metalloscope. In addition to amateur treasure hunting, metal detectors are also used for civilian and military security purposes, as well as construction work..continue..

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