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 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.
Inside a metal detector (like gold bug metal detector), there are two sets of copper wire windings. An electrical current is passed through one of the windings and this creates the electromagnetic field. This is called the transmit winding.
As metal conducts electricity, any metal object brought into the field will alter it and this alteration is picked up by the second winding, known as the receive winding. This is then sent to the control box which the operator can use to control the types of metals he or she wants to detect. This information is then displayed through a meter or LCD display and, often, with an audio signal. The more sophisticated the detector the more information is displayed.
Some detectors are used for detecting metal in elevator, airport, and other occasions, such as security metal detector. This will depend on the quality of the detector. It also depends on the size of the object. There are various refinements to metal detectors designed to filter out unwanted metals and give a signal on the wanted metals only. This is called discrimination and is achieved by the detector being set for a specific level of conductivity. Different metals conduct electricity in different degrees. Silver is an excellent conductor of electricity for example whereas nails are a very low conductor of electricity. Each metal's conductivity is known of course and so a metal detector can be set to detect gold or silver and reject any others. The size of the metal found is not particularly important in this case. It is just the ability or the 'willingness' of metal to conduct electricity that is detected.
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