*reprinted by permission of First Texas and Dave Johnson
File: El Paso Chapter GPAA talk 12 Feb 08 Dave Johnson minor update 25 march 09
The History of Metal Detectors, with Emphasis on Gold Prospecting
Good evening! My name is Dave Johnson, and I’m the Chief Design Engineer for the world’s largest manufacturer of hobby type metal detectors located right here in El Paso. The brand names we manufacture include Fisher Research Lab, Bounty Hunter, Teknetics, and a number of private labels. I’ve been designing metal detectors for 27 years for several different companies, so there’s half a chance that if you swing a metal detector, I had something to do with it.
My presentation this evening is on the history of metal detectors, with an emphasis on gold machines. I’m not here to sell my company’s products, but it’d be downright discourteous of me to show up and not bring some literature and a Gold Bug 2 to demo. So after the meeting if anyone is interested I’ll be happy to talk with you.
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The earliest electromagnetic device used for gold prospecting was the magnetic dip needle, a sort of compass turned sideways. It’s been around for about 500 years. It didn’t locate gold, but could identify concentrations of iron minerals associated with gold. Modern electronic magnetometers have replaced the dip needle for mapping subsurface geology, but the basic idea is still the same.
The earliest electromagnetic metal detectors date from the late 1800’s. When President Garfield was shot, surgeons were unable to locate the bullet. Alexander Graham Bell, inventor of the telephone, built a surgical probe metal detector for the purpose of helping the surgeons out. The President later died without the bullet having been successfully located; I don’t remember if Bell’s metal detector ever got used on the President. I believe its design was the induction balance, a basic principle that remains the basis of the vast majority of metal detectors to this day because of its sensitivity.
The earliest discriminating “metal detector” was a benchtop unit patented in the late 1800’s, designed for distinguishing counterfeit coins from the real thing. It was a VLF induction balance which used a motorized commutator as a phase-sensitive synchronous demodulator. It’s the same basic physics principle behind virtually all modern discriminating metal detectors. The amazing thing is that this was done even before the invention of the vacuum tube. What the old timers lacked in off-the-shelf technology they made up for in cleverness and careful attention to detail.
During the early 1900’s, audio frequency induction balances requiring no electronics were occasionally used for locating large masses of metal. The advent of the vacuum tube made it possible to increase sensitivity to pipes and cables by raising the operating frequency above the audio range, and then converting the detected signal back into the audio range, just as with an AM radio. Fisher Research Lab was founded in 1934 to manufacture pipe and cable locators of that type. During this period hand held metal detectors for detecting coins etc. didn’t exist as far as I have been able to determine.
World War II brought the need for hand-held mine detectors. These were audio frequency induction balances with vacuum tube amplification. They didn’t use synchronous demodulation. After WWII the designs were improved but they weren’t much good for civilian use. During the 50’s and 60’s several companies began to produce metal detectors better suited for civilian use. At first they were BFO’s but later radio frequency induction balances with both the transmitter and receiver coils build into a single search head became more popular. These were called “TR” metal detectors, for “Transmit-Receive”.
The late 60’s and early 70’s saw a revolution in metal detecting technology that is still going on today—the VLF synchronously demodulated induction balance—l800’s technology made practical by transistors and integrated circuits. What the so-called “VLF” did that the other technologies couldn’t do, is cancel the signal from iron minerals in the ground, which is often tens or hundreds of times as strong as the signal from a buried metal object.
The earliest VLF’s didn’t offer discrimination, but gold prospectors didn’t need that, they needed to see through ground minerals. The older BFO and TR units wouldn’t detect small nuggets even lying on the surface, and wouldn’t detect larger ones buried, so they weren’t much use for anything other searching quartz mine tailings. The VLF’s would detect small nuggets on the surface and larger ones in the ground. Hillslope areas that were previously unmineable by any method were now accessible to electronic nugget hunting.
During the 1970’s and 80’s there were rapid advancements in VLF technology as engineers became more adept in designing stable low noise circuitry and in using bandpass filters and electronic differentiators to boost signals from metal targets while suppressing interference from ground minerals. The early 1980’s saw the introduction of slow-motion double-derivative discriminators and of target identification meters. The ancestry of many metal detectors manufactured today is traceable to platforms developed in the early 1980’s.
These advances added a lot of value to the coinshooting and relic hunting customer, so that’s where most of the product development was concentrated. Early on there were several gold machines which were adaptations of metal detector designs originally intended for coinshooting. The best of these was probably the Garrett A2B.
In 1986, Fisher introduced the Gold Bug. It was first metal detector designed from the ground up for one purpose only—to find gold nuggets. It became the instant industry standard for gold machines. It was such a solid design that it remained in production for 16 years.
In the late 80’s and early 90’s, White’s introduced its Goldmaster series, which were hotter than the Gold Bug, and Minelab introduced gold machines which included the feature of electronic tracking ground balancing, which eliminated the need for manual ground balancing. In ’95, Fisher introduced the Gold Bug 2, which was basically a higher frequency major revision of the original Gold Bug. In ’97 Tesoro came out with the Lobo Supertraq, which included electronic ground balancing. In 2000, White’s introduced the GMT which had the performance of the GMT series combined with computerized ground balancing and other features. It was a decade of rapid innovation driven by several different manufacturers.
We can’t discuss that decade without mentioning the Minelab SD series including the recent GP. Introduced about 1997, the SD’s owed nothing to the earlier commercial products, not even Minelab’s own tracking VLF units. The Minelab SD was based on the pulse induction principle, which is quite different from VLF induction balance. Pulse induction has some very interesting advantages and also some very difficult disadvantages. Minelab saw that despite the disadvantages of pulse induction, its advantages could be developed into a metal detector which would go deeper on larger nuggets than a VLF in the highly mineralized laterite ground typical of Australian goldfields. It created a modern gold rush in Australia, and became popular in the USA as well.
The 1990’s also saw the introduction of multiple frequency metal detectors. This technology proved itself well for saltwater beach work, and for coinshooting and relic hunting requiring target identification and discrimination. However multiple frequency technology is not by its nature particularly sensitive to small metal objects, and that kept it from having any impact on gold prospecting.
Since the turn of the century, there hasn’t been a lot of basic innovation in gold prospecting machines, it’s been mostly improvement of existing designs. The Tesoro Lobo Supertraq and the White’s MXT created a new market niche—the general purpose machine based on a gold prospecting platform, so that it can do a decent job of gold prospecting. Here in El Paso, Teknetics and Fisher introduced the T2 and the F75 which are general purpose machines with serious gold prospecting capability, but those models proved so good as relic hunting machines that so far their gold prospecting capabilities have been pretty much ignored. We’ve announced the F70 and F5 which are also general purpose machines with gold prospecting capability, and these should be shipping in several weeks. (Update 25 March 09: these units have been in production for about a year now.)
The Fisher Gold Bug II was such a solid design that it’s still selling well. Those who prefer it like it mostly because of its manual ground balance and its extraordinary sensitivity to the smallest nuggets.
So, y’all might be wondering, do we have any new gold machines planned for the future? Well, that’s confidential information. What I can tell you is that the rejuvenated Fisher Research Lab now located in El Paso has made new product development a key part of its business, and that our engineering capability is first-rate.
Chief Design Engineer, Fisher Research Labs and First Texas Products (Teknetics and Bounty Hunter)
Designer of the following gold machines:
Fisher Gold Bug (no longer in production)
Fisher Gold Bug 2 (in production)
Tesoro Diablo Micromax (my personal all-time favorite gold machine, no longer in production)
Tesoro Lobo Supertraq (general purpose, in production)
White’s GMT (in production)
White’s MXT (general purpose, in production)
And also of the following general-purpose machines with serious gold prospecting capability:
Troy X-5 (no longer in production)
Teknetics T2 (in production)
Fisher F75 (in production)
Fisher F70 (in production)
Fisher F5 (in production)