The metal detector is a ubiquitous part of life today, from scanners in airports and public buildings, to finding buried utility pipes and cables. Tens of thousands of people worldwide enjoy the hobby of metal detecting. There are even shows on TV following metal detectorists in their adventures.
The news often regales us with tales of lost wedding rings discovered, ancient coins found, or gold nuggets unearthed, all with the help of the modern metal detector. But the groundwork for these treasure hunting marvels was laid well over a century ago, in the 1870s.
Early metal detector
searching for unexploded artillery shell
Scientific luminaries such as Faraday and Alexander Graham Bell studied the effects of magnetic metals on electrical current. In fact, Bell was researching ways to prevent telegraph cables from interfering with early telephone lines when President Garfield was shot by an assassin in 1881. Gathering together other scientists, he quickly developed a machine that could detect a bullet in a human body, in an effort to find the assassin's bullet in the President's body. His efforts were thwarted by the steel springs in the bed Garfield was resting on, and Garfield eventually died from infection. Much of the experimentation with induction balance in the earliest years was focused on finding bullets in a human body, so surgeons knew where to operate.
Technological strides were made through the turn of the century, with reports of a bench-top machine that was used to sort counterfeit coins from real ones being used in the late 1800s. Large, two-man metal detectors were developed in France to find buried unexploded ordnance and large shell fragments after WWI.
Shirl Herr with his
magnetic balancemetal detector
By the 1920s, the technology was being used in walk-through security measures to prevent factory workers from stealing tools, and to detect buried pipes, cables, and electrical lines. Gerhard Fisher was the first person to be awarded a patent for a man-portable metal detector. He had begun work on the device after discovering deposits of iron-bearing ores were affecting his airborne radio direction-finding (RDF) devices. 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.
At about the same time, Shirl Herr developed his own portable metal detector, gaining a patent on the technology in 1928. Herr's detectors, which could detect large objects at depths up to 8 feet and silver dollars at “several inches,” were used by Admiral Byrd in the Antarctic to find supply caches buried in previous expeditions, and by Benito Mussolini's archaeologists to discover lost treasures of the Roman emperor Caligula.
Most metal detectors during this era were made from scratch by experimenters or electrical engineers. The first story of successful “treasure hunting” with a metal detector was recorded in the New York Times in 1927, in a report of three men who used a “beat frequency oscillator” metal detector to find buried Spanish gold treasure in Panama. Several valuable objects were recovered, including gold chains and plate. Unlike the induction balance metal detectors, a beat frequency oscillator produced a steady beat of sounds that would change when metal disturbed the electrical pulses.
The year 1927 also saw the first book on metal detecting published. Modern Divining Rods: Construction and Operation of Electrical Treasure Finders by R. J. Santschi became a bestseller.
The 1930s not only saw an explosion of people building their own radios, but also of plans and articles to use many of the same parts to build homemade metal detectors. One version used two 28-inch wooden bicycle tire rims as frames for the coils. “Do it Yourself” kits were also sold by several companies, including the ubiquitous HeathKit company. Metal detectors of this era generally could not spot targets smaller than a silver dollar, and mostly were used to prospect for gold ore veins or large objects.
British 8th Army soldiers
hunting mines in North Africa 1942
World War II and the 50s
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.
One of the many things that these wartime minesweepers introduced was a manual ground balance function using frequency modulation. The operator could tune the detector to the local mineralization conditions, so he could better hear the signals indicating a land mine.
As transistors replaced vaccum tubes, metal detectors not only became lighter, they became more powerful. By the late 1950s, the resolution on the new detectors allowed them to find small targets. This marked the beginnings of “coin shooting” as a new category of metal detecting.
One of the early post-war metal-detecting pioneers was Ken White. White got his start in 1950, selling uranium detectors that looked much like contemporary metal detectors. When the U.S. government stopped buying uranium from the public in 1958, White changed gears and joined Fisher in the commercial metal detector market. They were followed by Garrett in 1964, Bounty Hunter in 1970, Tesoro in 1980, and Minelab in 1985. All these companies still sell metal detectors today.
The Transistor Revolution
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.
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.
The Computer Age in Metal Detecting
Photo By: Minelab
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.
Today, advancements in technology have brought such features as full color displays, custom search profiles for different types of hunts, wireless headphones, even an on-board GPS that interfaces with Google Maps so you can mark those new, remote hotspots and return later! From $99 to $5000, there is a metal detector out there for any budget and any type of hunting you want.