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Metal detectors use Lenz’s law to generate an oscillating field that detects conductive objects. Pulse induction technology can detect objects under mineralized sand. Applications include locating mines and searching for buried treasure.
According to Lenz’s law of electromagnetism, when a conductor falls within a certain range of an oscillating (alternating) magnetic field, it generates its own oscillating field, which opposes the primary field. A magnetometer can detect the resulting changes in the overall field, signaling the nearby presence of a conductive object, typically a piece of metal. The range of metal detectors varies from a few feet for the smallest coils, to 10 feet (3 m) for coils from 12 to 15 inches (30.5 to 38.1 cm).
The key to a functioning metal detector is the presence of stray currents generated by conductive objects in the environment. Just as pushing a paddle through a lake of water can cause small eddies to appear on the surface, producing an oscillating field in the environment causes electromagnetic vortices as electrons in the metal generate their own oscillating field. Frequencies from 3 to 20 kHz have been known to produce the best results, and some more modern metal detectors even allow the operator to change the frequency of the alternating field.
A different, newer type of metal detector uses a technology called pulse induction. This metal detector blasts the ground with a large electromagnetic pulse and watches the time it takes for the voltage to drop to ambient levels. If there is a conductive object under the ground, it will take longer for the voltage to drop. It’s a small effect, but modern sensors can pick it up well. This technique has some advantages over conventional metal detectors, such as the ability to detect objects under highly mineralized “black sand”.
The applications of metal detectors are numerous and generally well known. Perhaps the most important application for any metal detector is to locate mines or improvised explosive devices buried just below the surface. In some countries where landmines from old wars still remain, such as Vietnam, people are advised to use metal detectors when walking in unfamiliar areas known to be at risk from landmines. This can save many lives.
Another common use of the metal detector is to search for “buried treasure”: coins and relics from years or even millennia in the past. Searching for a beach that has many visitors can turn up lost and found items that have only been a few days old. This isn’t a viable way to make a living, but some people enjoy it as a hobby.
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