How Do Metal Detectors Work?
In 1881, Alexander Graham Bell constructed one of the world's first metal detectors in an attempt to find an assassin's bullet in President James Garfield. Fischer patented a portable version in 1931. From treasure hunting to security screening, metal detectors are used in many a field.

Metal detectors are electronic devices that are used to find traces of metal usually from the ground, a person, or cargo. This metal could be anything from discarded pieces of aluminum to buried treasures.

These devices can penetrate sand, soil, wood and other non-metallic substances.

A basic consists of an electronic box and a battery case on one end, with a handle for the operator's arm. There is a coil, which consists of an insulated wire around a telescoping shaft and into a round plastic disk. This disk comes off the shaft at an angle, which allows it to be held parallel to the ground. The operator grips the electronic box and turns on the power to slowly sweep the coil end over the ground until an electronic signal is heard.

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the presence of some metallic element buried beneath the area swept by the coil.

Metal detectors work on the principal of electromagnetism and their effects on conductive metals. The high-powered coil of metal, called the transmitter, uses the battery power to generate a penetrating magnetic field. As the electromagnetic field enters the ground, anything metallic will become charged with magnetism. When the receiver in the coil detects the electromagnetic signature, it sends a signal to the electronic box. A speaker amplifies this signal and the operator hears a beep.

Microprocessors located in the electronic box can actually measure the time between the charging and the receiving (called a phase shift) and determine which metals may be present. This is how high-end metal detectors can be adjusted to only search for certain metals.

Metal detectors use one of three technologies:

A beat frequency oscillator detector uses a coil as an inductor in an oscillator. When its frequency changes, so does its inductance. Another oscillator produces a close frequency, and audible beats between them signal metal.

An induction balance detector uses two coils that overlap each other.

A sine wave is transmitted with one coil and received with the other. The coils are adjusted so that there is no signal in the receiving coil when there is no metal nearby.

A pulse induction detector generates pulses by cutting off an indicator. This pulse is sent through a coil and the detector listens for echoes..