The Metal Detector
Toward the end of the 19th century, many scientists and engineers implemented their growing knowledge of electrical theory in an attempt to devise a machine which would pinpoint metal. The implementation of such a device to find ore-bearing rocks would give a huge advantage to any miner who employed it. The German physicist Heinrich Wilhelm Dove created the induction balance system, which was incorporated into metal detectors a hundred years later. Early machines were crude, used a lot of battery power, and worked only to a very strict degree. Alexander Graham Bell used such a device to attempt to locate a bullet lodged in the chest of American President James Garfield in 1881; the attempt was unsuccessful because the metal coil spring bed Garfield was lying on mislead the detector.
The modern evolution of the metal detector started in the 1920s. Gerhard Fisher had developed a system of radio direction-finding, which was to be implemented for accurate navigation. The system worked extremely well, but Fisher saw that there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would locate metal using a search coil resonating at a radio frequency. In 1925 he applied for, and was given, the first patent for a metal detector. Although Gerhard Fisher was the earliest person given a patent for a metal detector, the earliest to apply was Shirl Herr, a businessman from Crawfordsville, Indiana. His file for a hand-held Hidden-Metal Detector was filed in February of 1924, but not patented until July 1928. Herr assisted Italian leader Benito Mussolini in getting items remaining from the Emperor Caligula's galleys at the bottom of Lake Nemi, Italy, in August of 1929. Herr's invention was implemented by Admiral Richard Byrd's Second Antarctic Expedition in 1933, when it was used to locate objects left behind by earlier explorers. It was productive up to a depth of eight feet. However, it was one Lieutenant Jozef Stanislaw Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland, during the early years of World War II, who refined the creation into a practical Polish mine detector. They were heavy, operated on vacuum tubes, and needed separate battery packs.
The design created by Kosacki was used extensively during the clearance of the German mine fields during the Second Battle of El Alamein when 500 units were shipped to Field Marshal Montgomery to clear the minefields of the retreating Germans, and later used during the Allied invasion of Sicily, the Allied invasion of Italy and the Invasion of Normandy. As it was a wartime research operation to invent and refine the design of the detector, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.
Many makers of these new devices brought their own ideas to the market. White's Electronics of Oregon began in the 1950s by building a machine named the Oremaster Geiger Counter. Another leader in detector technology was Charles Garrett, who pioneered the BFO (Beat Frequency Oscillator) device. With the invention and advancement of the transistor in the 1950s and 1960s, metal detector manufacturers and designers made smaller lighter machines with improved circuitry, running on small battery packs. Companies sprang up all over the USA and Britain to supply the growing popularity.
Modern top units are fully computerized, using integrated circuit 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. Observed to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.
Larger portable metal detectors are implemented by archaeologists and treasure hunters to locate metallic items, such as jewelry, coins, bullets, and other various artifacts buried shallowly underground.
The biggest technical change in detectors was the advancement of the induction-balance system. This technique involved two coils that were electrically balanced. When metal was brought to their vicinity, they would become unbalanced. What let detectors to discriminate between metals was the fact that every metal has a different phase response when exposed to alternating current. Scientists had long known of this fact by the time detectors were created that could selectively detect desirable metals, while ignoring undesirable ones.
Even with discriminators, it was still a test to avoid undesirable metals, because some of them have similar phase responses e.g. tinfoil and gold, particularly in alloy form. Thus, improperly tuning out certain metals raised the risk of passing over a valuable find. Another problem of discriminators was that they reduced the sensitivity of the machines.
New coil designs
Coil designers also tried out breakthrough designs. The original induction balance coil system consisted of two similar coils placed on top of one another. Compass Electronics produced a new system: two coils in a D shape, mounted back-to-back to form a circle. This design was widely used in the 1970s, and both concentric and D type (or widescan as they became known) had their fans. Another step in evolution was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-biased mode. It worked best at lower frequencies than those implemented before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the individual to switch between the discriminate (biased) mode and the non-discriminate (non-biased) mode. Later advancements switched electronically between both modes. The advancement of the induction balance detector would ultimately result in the motion detector, which constantly checked and balanced the background mineralization.
At the same time, inventors were looking at using a different technique in metal detection called Pulse Induction. Unlike the Beat Frequency Oscillator or the Induction Balance machines which both implemented a uniform alternating current at a low frequency, the pulse induction machine simply fired a high-voltage pulse of signal into the ground. In the absence of metal, the 'spike' decayed at a uniform rate, and the time it took to fall to zero volts could be precisely measured. However, if metal was present when the machine fired, a small current would flow in the metal, and the time for the voltage to drop to zero would be raised. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and tell the presence of metal at a reasonable distance. These new devices had one major advantage: they were completely impervious to the effects of mineralization, and rings and other jewelry could now be located even under highly-mineralized black sand.