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A metal detector is a portable electronic device that penetrates the ground magnetically in order to find traces of metal. This metal could either be discarded pieces of aluminum or valuable coins, jewelry, or other buried treasures. Part of the appeal of using the device is this unknown factor, keeping amateurs and professionals on a constant scan for new sources of metal and more promising locations. These tools can usually penetrate sand, soil, wood, and other non-metallic substances, making most areas fair game for treasure hunters.
A basic metal detector consists of an electronic box and battery case on one end, with a brace or handle for the operator's arm. An insulated wire wraps around a telescoping shaft and into a round plastic disk called the coil. This disk comes off the shaft at an angle, which allows it to be held parallel to the ground. The operator straps on or grips the electronic box and turns on the power. The idea is to slowly sweep the coil end over the ground until an electronic signal is heard. This lets the user know that some metallic element is buried directly beneath the area swept by the coil.
Some devices can discriminate between various metals, allowing users to decide if a particular discovery would be worth digging up. A significant percentage of hits are indeed pieces of metallic trash or discarded building materials. Part of the appeal of this hobby, however, is discovering a lost class ring or a piece connected with local history. Some professional treasure hunters use very discriminating detectors set only for valuable metals, but hobbyists tend to explore even the less lucrative hits.
Metal detectors work on the principle of electromagnetics and their effects on conductive metals. There are actually two separate elements in the coil of a typical unit. The first is a high-powered coil of metal called the transmitter that uses the battery power to generate a penetrating magnetic field. As the elecromagnetic field enters the ground, anything metallic will become magnetized, similar to how a paper clip reacts after it comes into contact with a standard bar magnet.
The coil also contains a very sensitive wire array called the receiver. This receiver reacts to any charged magnetic field it passes over, especially the newly-magnetized metal elements. When the receiver 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. The phase shifts of all other metallic elements are electronically squelched by the microprocessing unit.
These devices must also be adjusted to eliminate false positives generated by natural deposits of metal in the soil or sand itself. Most units allow users to change the sensitivity of the coil in order to cancel out the background clutter. Metal detecting technology is also used for security inspections at airports, government buildings, and other public places. Construction crews and woodworkers also use hand-held metal detectors to find dangerous nails or other metallic debris in reclaimed building materials and trees.