A metal detector is a device that finds metal that may not be visible. The basic metal detector is composed of an oscillator producing an alternating current that passes through the coil producing an alternating magnetic field. If electrically conductive metal is close to the coil then the magnetic field is disrupted and thus the metal detector can sense the hidden metal.
The first metal detectors were developed in the 1960s and were used for mining and other industries. They were used for de-mining, for detecting weapons, geophysical prospecting, archaeology, and treasure hunting. Scientists and engineers used growing knowledge to create a device that would pinpoint metal and give a huge advantage to any miner who used it. Heinrich Wilhelm Dove invented the induction balance system which was used in metal detectors later on.
Gerhard Fisher developed a system of radio direction-finding used for accurate navigation. He noticed anomalies in his system when over terrain in ore-bearing rocks and thought that he might be able to design a machine that could detect metal at a certain radio frequency. He received the first patent for a metal detector in 1937. Jozef Stanislaw Kosacki refined Fisher’s design into a practical mine detector; however, they were heavy, ran on vacuum tubes, and needed separate battery packs. The first practical metal detector was kept secret for over 50 years due to its invention during war time. After the war mine detectors were purchased by relic hunters which developed the hobby industry.
Whites Electronics of Oregon began by building a machine called the Oremaster Geiger Counter. The transistor helped propel new designs to lighter machines with improved circuitry that ran on small battery packs.
Modern models are fully computerized and use integrated circuit technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, and they hold these parameters in memory for future use.
The induction-balance system was the biggest technical change in detectors. Using two coils that are electrically balanced metal is detected by noting when the coils become unbalanced. Each metal has a different phase response when exposed to alternating current, thus it is possible to find only desirable metals and ignore undesirable ones.
Discriminators did prove to have some disadvantages because they reduced the sensitivity of the machines. It was still also possible to miss desirable metals because some of them have similar phase responses as undesirable metals and thus would be skipped over.
Detectors in the 70’s had a switch that enabled the user to switch between the discriminate mode and the non-discriminate mode. Later the machines switched between modes electronically. Developers were attempting to use a different technique in metal detecting called Pulse Induction. This device simply fired a high voltage pulse directly into the ground. The spike would decay at a uniform rate in the absence of metal, however, when metal was present it would take longer for the spike to decay. Although these differences were minute it was still possible to measure them accurately and identify metal. In Scotland the crown has claim over any object that is found using a metal detector. Any item found must be reported and then the panel determines what will happen to the found item. Failure to report is a criminal offense.
Metal detectors are also used in airports in order to detect weapons and to prevent hijackings. They are also used in food production to detect and prevent any metal shards from entering food.