Editing Metal detector (section)

==History and development==
[[File:Metal detector from World War 1.jpg|thumb|upright=1.3|An early metal detector, in 1919, used to find un-exploded bombs in France after [[World War I]]]]

In 1841, Professor [[Heinrich Wilhelm Dove]] published an invention he called the "differential inductor".<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015065411269&view=1up&seq=321 "Ueber die durch Magnetisiren des Eisens vermittelst Reibungselektricität inducirten Ströme"], H. W. Dove, ''Annalen der Physik und Chemie'', 1841 series 2 vol. 54, p. 305.</ref>  It was a 4-coil induction balance, with 2 glass tubes each having 2 well-insulated copper wire solenoids wound around them. Charged Leyden jars (high-voltage capacitors) were discharged through the 2 primary coils; this current surge induced a voltage in the secondary coils.<ref>[https://archive.org/details/atreatiseonelec00walkgoog/page/424/mode/2up "Influence that is exercised upon Induction by metal Masses"], Auguste de la Rive, ''A Treatise on Electricity in Theory and Practice'', 1853 vol. I, p. 424.</ref>  When the secondary coils were wired in opposition, the induced voltages cancelled as confirmed by the Professor holding the ends of the secondary coils. When a piece of metal was placed inside one glass tube the Professor received a shock. This then was the first magnetic induction metal detector, and the first pulse induction metal detector.

In late 1878 and early 1879, Professor (of music) [[David Edward Hughes]] published his experiments with the 4-coil induction balance.<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015033442891&view=1up&seq=76 "On an Induction-Currents Balance, and Experimental Researches made therewith"], Professor D. E. Hughes, ''Proceedings of the Royal Society of London'', 1879 May 15 vol. 29, p. 56.</ref>  He used his own recent invention the microphone and a ticking clock to generate regular pulses and a telephone receiver as detector. To measure the strength of the signals he invented a coaxial 3-coil induction balance which he called the "electric sonometer".<ref>{{Cite web|url=http://davidedwardhughes.com/wp-content/gallery/induction/10.Laboritory-Instrument-Made-by-Groves.jpg|title=Professor Hughes Induction Balance and Sonometer}}</ref>  Hughes did much to popularize the induction balance, quickly leading to practical devices that could identify counterfeit coins. In 1880 Mr. J. Munro, C.E. suggested the use of the 4-coil induction balance for metal prospecting.<ref>[https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837364&view=1up&seq=113 "Prospecting metal veins by the Induction Balance"], Mr. J. Munro, ''The Electrician'', 1880 Jan 17, p. 103</ref>  Hughes's coaxial 3-coil induction balance would also see use in metal detecting.

In July 1881, [[Alexander Graham Bell]] initially used a 4-coil induction balance to attempt to locate a bullet lodged in the chest of American President [[James Garfield]].<ref>{{Cite web|url=http://www.scitechantiques.com/belldiscovery/|title=Alexander Graham Bell's 1881 efforts to help save President Garfield's Life.}}</ref>  After much experimenting the best bullet detection range he achieved was only 2 inches (5 centimeters). He then used his own earlier discovery, the partially overlapping 2-coil induction balance, and the detection range increased to 5 inches (12 centimeters). But the attempt was still unsuccessful because the metal coil spring bed Garfield was lying on confused the detector. Bell's 2-coil induction balance would go on to evolve into the popular double D coil.

On December 16, 1881, Captain Charles Ambrose McEvoy applied for British Patent No. 5518, Apparatus for Searching for Submerged Torpedoes, &c., which was granted Jun 16 1882. His US269439 patent application of Jul 12 1882 was granted Dec 19 1882.<ref>[https://patents.google.com/patent/US269439A/en "Apparatus for Finding Torpedoes, &c."], C. A. McEvoy, 1882 Dec 19</ref>  It was a 4-coil induction balance for detecting submerged metallic torpedoes and iron ships and the like.<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015084573206&view=1up&seq=190 "McEvoy's Electric Submarine Detector"], ''Engineering'', 1882 Aug 18, p. 154.</ref>  Given the development time involved this may have been the earliest known device specifically constructed as a metal detector using magnetic induction.

In 1892, George M. Hopkins described an orthogonal 2-coil induction balance for metal detecting.<ref>"Unscientific and Scientific Divining Rods", George M. Hopkins, ''Scientific American'', 1892 Aug 20, p. 114</ref>

In 1915, Professor [[Camille Gutton]] developed a 4-coil induction balance to detect unexploded shells in farmland of former battlefields in France.<ref>[https://gallica.bnf.fr/ark:/12148/bpt6k3114b/f71.image "Sur une balance d'induction destinée à la recherche des obus enterrés dans les terrains à mettre en culture"], M. C. Gutton, ''Comptes Rendus'', 1915 July 26, pp. 71–73.</ref> Unusually both coil pairs were used for detection.<ref>[https://babel.hathitrust.org/cgi/pt?id=pst.000063000122&view=1up&seq=431 "Detecting Buried Shells with Induction Balance"], ''Scientific American'', 1915 Nov 13, front cover & p. 425.</ref> The 1919 photo at the right is a later version of Gutton's detector.

=== Modern developments ===
The modern development of the metal detector began in the 1920s. [[Gerhard Fischer (inventor)|Gerhard Fischer]] had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fischer noticed 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 detect metal using a search coil resonating at a radio frequency. In 1925 he applied for, and was granted, the first patent for an electronic metal detector. Although Gerhard Fischer was the first person granted a patent for an electronic metal detector, the first to apply was Shirl Herr, a businessman from Crawfordsville, Indiana. His application for a hand-held Hidden-Metal Detector was filed in February 1924, but not patented until July 1928. Herr assisted Italian leader [[Benito Mussolini]] in recovering items remaining from the [[Caligula|Emperor Caligula's]] galleys at the bottom of [[Lake Nemi]], Italy, in August 1929. Herr's invention was used by Admiral Richard Byrd's Second Antarctic Expedition in 1933, when it was used to locate objects left behind by earlier explorers. It was effective up to a depth of eight feet.<ref>
{{Cite book
 | first= Thomas C.
 | last= Poulter
 | title= Outline of the Scientific Accomplishments of the Byrd Antarctic Expedition II, 1933–1935
}}
</ref>
However, it was one [[Lieutenant]] [[Józef Kosacki|Józef Stanisław 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 design into a practical [[Polish mine detector]].<ref>
{{Cite book
 | first= Tadeusz
 | last= Modelski
 | title= The Polish Contribution to The Ultimate Allied Victory in The Second World War
 | location= Worthing, England
 | year= 1986
 | page= 221
}}
</ref>
These units were still quite heavy, as they ran on vacuum tubes, and needed separate battery packs.

The design invented by Kosacki was used extensively during the [[Second Battle of El Alamein]] when 500 units were shipped to [[Bernard Montgomery, 1st Viscount Montgomery of Alamein|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]].<ref>
{{Cite book
 | first1= Mike
 | last1= Croll
 | first2= Leo
 | last2= Cooper
 | title= The History of Landmines
 | year= 1998
 | publisher= Pen & Sword Books
 | isbn= 978-0-85052-628-8
}}
</ref>

As the creation and refinement of the device was a wartime military research operation, the knowledge that Kosacki created the first practical metal detector was kept secret for over 50 years.

===Beat frequency induction===
Many manufacturers of these new devices brought their own ideas to the market. White's Electronics of Oregon began in the 1950s by building a machine called the Oremaster Geiger Counter. Another leader in detector technology was Charles Garrett, who pioneered the BFO ([[beat frequency oscillator]]) machine. With the invention and development 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 United States and Britain to supply the growing demand. Beat Frequency Induction requires movement of the detector coil; akin to how swinging a conductor near a magnet induces an electric current.

===Refinements===
Modern top models 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. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.

State-of-the-art metal detectors have further incorporated extensive wireless technologies for the earphones, connect to [[Wi-Fi]] networks and [[Bluetooth]] devices.  Some also utilize built in [[GPS]] locator technology to keep track of searching location and the location of items found.  Some connect to [[smartphone]] applications to further extend functionality.

===Discriminators===
The biggest technical change in detectors was the development of a tunable induction system. This system involved two coils that are electro-magnetically tuned. One coil acts as an [[RF]] transmitter, the other as a receiver; in some cases these can be tuned to between 3 and 100&nbsp;kHz. When metal is in their vicinity, a signal is detected owing to eddy currents induced in the metal.  What allowed detectors to discriminate between metals was the fact that every metal has a different [[phase response]] when exposed to alternating current: longer waves (low frequency) penetrate deeper into the ground, and select for high-conductivity targets like silver and copper; shorter waves (higher frequency) are less ground-penetrating, and select for low-conductivity targets like iron. Unfortunately, high frequency is also sensitive to ground [[Mineralization (geology)|mineralization]] interference. This selectivity or discrimination allowed detectors to be developed that could selectively detect desirable metals, while ignoring undesirable ones.

Even with discriminators, it was still a challenge 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 increased the risk of passing over a valuable find. Another disadvantage of discriminators was that they reduced the sensitivity of the machines.

===New coil designs===
Coil designers also tried out innovative designs. The original induction balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design: two coils in a D shape, mounted back-to-back to form a circle. The system was widely used in the 1970s, and both concentric and double D type (or widescan as they became known) had their fans. Another development was the invention of detectors which could cancel out the effect of [[Mineralization (geology)|mineralization]] in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20&nbsp;kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode. Later developments switched electronically between both modes. The development of the induction balance detector would ultimately result in the motion detector, which constantly checked and balanced the background mineralization.

===Pulse induction===
At the same time, developers were looking at using a different technique in metal detection called pulse induction.<ref>{{cite web|url=http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector4.htm|title=How Metal Detectors Work|date=23 May 2001}}</ref> Unlike the beat frequency oscillator or the induction balance machines, which both used a uniform alternating current at a low frequency, the pulse induction (PI) machine simply magnetized the ground with a relatively powerful, momentary current through a search coil. In the absence of metal, the field decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured. However, if metal was present when the machine fired, a small eddy current would be induced in the metal, and the time for sensed current decay would be increased. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and identify the presence of metal at a reasonable distance. These new machines had one major advantage: they were mostly impervious to the effects of [[Mineralization (geology)|mineralization]], and rings and other jewelry could now be located even under highly [[Mineralization (geology)|mineralized]] [[black sand]]. The addition of computer control and digital signal processing have further improved pulse induction sensors.

One particular advantage of using a pulse induction detector includes the ability to ignore the minerals contained within heavily [[Mineralization (geology)|mineralized]] soil; in some cases the heavy mineral content may even help the PI detector function better.{{citation needed|date=March 2021}} Where a VLF detector is affected negatively by soil [[Mineralization (geology)|mineralization]], a PI unit is not.