Metal detectors use electromagnetic induction to detect metal. Uses include de-mining (the detection of land mines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archaeology and treasure hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors.
In its simplest form, 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.
Technological changes were taking place at a rapid rate too, and very few of the smaller companies managed to stay in competition with the big outfits. GOLDAK, METROTECH, IGWT, TEC, and, quite recently, ARADO ceased production of hobby machines. Some devotees of metal detecting still treasure their Arado machines, which had a reputation for being difficult to set up, but were reputed to be the deepest-seeking hobby detectors ever made. The biggest technical change in detectors was the development of the induction-balance system, where two coils are set up in an electrical equilibrium to produce a 'null' or zero balance. Introducing metal to the vicinity of the coils caused them to unbalance, producing a change of tone in the machine's speaker. Scientists had long known that every metal has a specific response to stimulation by alternating current. Each metal produces a time lag or 'phase angle' in its induced current, in relation to the drive current. This meant that detectors could now be set up to ignore unwanted phase angles, and respond positively only to desired metals. But there was also a downside to the development of the 'discriminator' detectors. Introducing discrimination always had the effect of reducing the sensitivity of the machine, so it was less able to find deep objects. In addition, there was the fact that some desirable metals were quite near the area of unwanted metals, such as iron. Gold, particularly in alloy form, was quite close to tinfoil in the overall spectrum, so the discrimination control had to be used carefully. The price to be paid for setting up a detector to ignore iron and tinfoil was the possibility that, sooner or later, the user would scan over, and ignore, a valuable find - perhaps a diamond engagement ring on a beach.
Modern top models are fully computerized, using microchip technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, etc., and hold these parameters in memory for future use. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.
New genres of metal detector have made their appearance. BB (Beat Balance) and CCO (Coil Coupled Operation) were unveiled by the electronics press in 2004. Both were invented by electronics writer and designer Thomas Scarborough and combine unprecedented simplicity with good sensitivity.
Uses of metal detectors