Metal Detecting History
200 B.C. - Ancient Chinese documents indicate that a metal detector was in use more then 200 years before the birth of Christ. A Chinese emperor had a doorway metal detector constructed to protect himself against assassination. His craftsman built the doorway of a magnetic mineral called magnetite with the frame possibly built something like a horseshoe magnet. Through a combination of heating and striking the magnetite with hammers, an iron metal “attractor” was created. The heating and jarring caused the molecules to align themselves in the direction of the Earth’s magnetic field. If a person attempted to carry iron objects such as armor, swords or other weapons through the doorway, these objects would be drawn against the doorway shown on the following page and held fast.
1830 - English geologist and mining engineer R. W. Fox first discovered that electricity will flow through metallic ores as well as solid metal objects. Thus he devised a simple metal locator which consisted of nothing more than a battery, several metal rods and a suitable length of wire. His first method of detection was as follows: one metal rod would be driven into the earth where the suspected vein of ore was located; it was connected to one terminal of the battery. The other battery terminal was connected to a floating wire. Other metal rods were driven into the ground at several different points and successively touched with the floating wire. Where a spark occurred, it was an indication that metal was present.
1879 - Professor D. E. Hughes demonstrated to the Royal Society in London his Induction Balance machine. Hughes and his instrument maker, William Groves, soon recognized the potential of the I.B. as a metal locator, and several were supplied to various London Hospitals for locating metal objects in human bodies.
Early 1880s, Alexander Graham Bell experimented with a metal detecting device. To make it, Bell had to study the theory of electromagnetism which had been previously demonstrated by Joseph Henry. The Induction Balance was also influenced by Bell's previous experience in his home - where he could hear the ticking of his clock while wearing a phone earpiece when a piece of metal was brought near the device - reason for which he deduced that the electric balance had been disturbed and this could be suitable to detect a hidden metal. He therefore studied these concepts and published ``Upon New Methods of Exploring the Field of Induction of Flat Spirals``
1890 - A test was made to locate sulfides through the medium of conductivity, using a telegraphic receiver connected in series with a battery and a wire brush. Electrical contacts were made in the earth, and a brush was then moved over the surface. Whenever it touched sulfides, the brush would complete the circuit, indicated by a click in the receiver. Since it could be used only on exposed mineralized surfaces, the method was of limited value. Further attempts at metal detection were made, using the Wheatstone bridge circuit for measuring resistance. Here again, conductivity was the determining factor, but the conductivity between two points on the earth’s surface had to be calculated indirectly by first measuring resistance. This method also proved impractical.
Still another earth conductivity method was given considerable attention. Since electrical currents flowing through the ground cause electrical potential lines to be created, equal potential points across the ground could be measured by galvanometers and plotted. The presence of an ore body caused these lines to warp or distort. Although the method was somewhat successful, many variables were involved. In addition, water layers, areas of uneven moisture and other substances in the soil gave indications which could be misconstrued as indicating the presence of an ore body. Too, failure to indicate ore would not necessarily mean barren ground. The oxidized condition existing around sulfide ore bodies forms an almost perfect insulator that prevents accurate measurement.
1902 - The London Electric Ore Finding Company filed an application at the British Patent Office for an entirely new type of metal detector. This was a very advanced instrument for its time, having a range of one hundred yards. It operated as follows: a bank of batteries supplied a high-voltage, heavy-duty current to a spark generator; its output was chopped by a motorized contact breaker to achieve a signal at audio frequency, which in turn was fed to two transmitter probes driven into the earth. At a suitable distance away, two similar probes were connected to receiving apparatus, and equipotential lines of conducting material (ores or solid metal objects) could be plotted.
1928 - American businessman Shirl Herr received a patent for a similar design that was effective to depth of 8 feet. His invention quickly went international and was used in Antarctic exploration and the recovery of ancient Italian artifacts.
1928 - With the very rapid development of wireless techniques during World War I, it was only natural that this technique would be adapted to metal locators and prospecting equipment. One of the first pioneers to exploit this technology for locating buried treasure was Englishman George Williams, who was the wireless operator aboard the salvage ship RACER during the recovery of gold from the wreck of the LAUREN TIC. Being fully conversant with wireless techniques, and seeing the somewhat primitive treasure locators available then, he decided he could improve the existing technology by designing a Radio-Locator (as metal detectors were known then).
1928 – The London Times newspaper reported that Englishman George Williams found “a solid gold altar two feet high,” while C. B. Driscoll in DOUBLOONS expands the finds list even further.
He also describes the Williams’ detector as a Transmit-Receive instrument operating at radio frequency. Williams, with his locator, arrived in Panama in July 1925, hence he certainly was one of the pioneers of T-R technique.
1933 - Gerhard Fischer (spelling it Fisher) founded Fisher Research Laboratory, which remains a leading manufacturer of metal detectors to this day.
Gerhard Fisher immigrated to the United States from Germany after studying electronics at the University of Dresden. While working as a Research Engineer in Los Angeles, California his work with aircraft radio detection finders led him to the idea of a portable metal detectors. Fisher shared the idea with Albert Einstein who correctly predicted the proliferation of hand held metal detector use.
Fisher Labs became the first company to offer metal detectors to the public on a large scale, in 1931. His Metallascope, selling for $200 and weighing “only” 22 lbs, was a huge success with public utilities companies, as it made finding buried cables and pipes a simple matter.
By the mid-1930’s, there was an abundance of gold and treasure locators, the most notables being: the Alpha by George Maher, Terrasearch from Engineering Research Corporation, Radioscope by Goldak, Inc.; and, of course, Metalloscope from Gerhard Fisher. The Metalloscope was undoubtedly the most popular treasure finder of all time having survived in continuous production right up to the solid-state era, hence the M-scopes of the current Fisher line of detectors.
1939 - During the early years of WWII, Polish officer Józef Stanisław Kosacki refined the design into the practical Polish mine detector. The design invented 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.
1941 -World War II brought the urgent need for rugged, simple devices for detecting landmines. A Polish Lieutenant named Kosacki, who had made it to England after his country fell to the Nazis, came up with a design that was soon adopted. The war provided the impetus and financing to make great strides in metal detecting technology, but the thousands of minesweepers that flooded the market after the war weren't designed for hunting individual coins or small nuggets. The capabilities of these machines, and the ones that followed immediately after the war, was limited to finding relatively large pockets of ore, buried cables and pipes, and (of course,) land mines and buried munitions.
In the 1950s, there were very few machines capable of finding a coin sized item. A few did exist prior to 1960, that were sensitive enough to find individual coins, bullets, etc... But most of them from this '50s decade were strictly for hub-cap sized items (like mine detectors, or 2-box units).
Early 1960s - Word was getting out, and makers were starting to step up to demand. Transistors came about, etc... There were a few machines by the early '60s that might do 4`` and were more user friendly. But still, many others that were still worthless junk, drifted horribly, etc.... Still no disc, still can't handle minerals, etc. But some amazing stories come from this early '60s era of guys who were the first to detect in front of concession stands, or on virgin beaches, etc.... virgin schools, etc... Even though they only got 3 or 4`` deep, yet all the coins were still silver then, and the pulltab hadn't been invented yet.
Late 1960s - Pacific Northwest Instruments (PNI) in Klamath Falls, Oregon originated the Bounty Hunter brand of metal detectors sometime in the late 1960's or early 1970's
These were likely the first models offered by PNI and all three probably shared pretty much the same parts and circuitry. Just the addition of a few controls or a larger meter and case created the differences between them. Inside was a basic Beat Frequency Oscillator circuit that was no doubt very similar to what any of the competition was offering at the time. Anyone who could handle a soldering iron and read a simple schematic could design and build one.
The 1970s saw the greatest technological leaps metal detecting had ever seen. In 1974, VLF (Very Low Frequency) metal detectors, which operated from 1Khz to 30Khz, were introduced. Until then, metal detectors operated near 100 Khz. While the VLF detectors allowed operators to “ground balance” - tune out the soil mineralization – it couldn't discriminate between metal types.
Manufacturers soon combined VLF and high frequency operation into one machine. VLF was called “all metal” mode, and the high frequency was called “discriminating” mode, which allowed the operator to “dial out” iron and foil.
1978 gave us the first VLF-only discrimination mode, based on comparing the signal when the coil was rapidly moved over the target. At their heart, though, these machines still used the “induction balance” design from the earliest detectors. Induction balance machines still make up most of the detectors sold even today.
Early 1970s - The very first discriminators were seen, but for the most part, it was all metal till the early 1970s. Even then, there were still some md'rs who ``looked with suspicion`` on these new-fangled discriminators (with cautionary claims that you'd ``miss rings``, etc).
So some people stubbornly held on to their all-metal TR’s. Naturally that attitude changed when old-timers saw the disc. guys effortlessly passing tabs and foil in junky turf! When knocking out foil you would loose some depth. When cranking it higher to pass tabs, you would loose even more depth, as disc. and depth were directly correlated on TR disc. of that era. And they were a bear to keep tuned. Depth on the best of the TR discriminators might have been 6″ on a dime, if you tried hard. Depended on your ear, the minerals, etc. The TRs were next to useless in some nastier soils (the continental divide mountaneous regions, for instance).
By the 1980s, detectors had become sensitive enough to (approximately) gauge the depth of a target, with the assumption that it was coin-sized.
Manufacturers also started including indications of the type of metal a target was, either by different audio tones, or marked on a meter. The 80s also saw an actual divergence in metal detecting technologies between the machines intended for gold prospecting and the “coinshooter” crowd, who were looking for old buried coins. The “coinshooter” machines, operating at a higher frequency, were also better at finding low-purity 14k gold jewelry.
1980 - Jack Gifford, a former Electronics Technician at Bounty Hunter, announced the founding of Tesoro Electronics Inc.
Jack wrote a letter to US dealers declaring:
“It has become apparent to me over the past few years that there is a place for, even a need for, a metal detector manufacturer who is dedicated to the independent dealer market. The products should utilize the best of today’s technology to provide maximum sensitivity. They should be rugged, but light enough that anyone can use them. They should do their job without “bells and whistles” that only increase the cost. And finally, the company should remain loyal to their dealers and be responsive to their needs and desires. It is with these principles in mind, that TESORO was founded in May of this year.
Tesoro is the Spanish word for treasure. The Spanish certainly seemed to understand and appreciate tesoro. Never in the history of the New World has there been a treasure hunt like the Spaniards held. It’s hard to think of a better name for a company associated with treasure hunting, or a better symbol of our goals than the conquistador. As the Spaniards were yesterday’s undisputed masters of treasure hunting, we at TESORO will always endeavor to meet our goal of making TESORO Metal Detectors today’s undisputed masters of treasure hunting.”
The 1990s saw integrated circuits replace most of the transistors in metal detectors, just as transistors had replace vacuum tubes
These circuit boards allowed for visual displays, notch discrimination, and more accurate metal identification. The 90s saw the first underwater metal detectors outside of the military hit the market, as well as the introduction of Pulse Induction (PI) metal detectors. PI detectors cannot discriminate out iron, but they can function in highly mineralized soils, and have a deeper penetration – two features that made gold prospectors flock to the new technology. PI machines also opened up beach detecting in highly mineralized “black sand,” and their deeper depth means targets beyond the reach of other treasure hunters can be found.