According to Lenz's law of electromagnetism, when a conductor falls within a certain range of an oscillating (alternating) magnetic field, it generates an oscillating field of its own, which opposes the primary field. A magnetometer can pick up 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 12 to 15-inch (30.5 to 38.1 cm) coils.
The key to a functioning metal detector is the presence of eddy currents generated by conductive objects in the environment. Just like pushing a paddle through a lake of water can cause little vortices to appear on the surface, producing an oscillating field in the environment causes electromagnetic vortices when the electrons in metal generate their own oscillating field. Frequencies of 3 to 20 kHz are 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 observes the length of time it takes for the voltage to decrease to ambient levels. If there is a conductive object under the ground, it will take a longer amount of time for the voltage to decrease. It is a small effect, but modern sensors can pick it up well. This technique has certain 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 under the surface. In some countries where mines still remain from old wars, such as Vietnam, people are advised to use metal detectors when walking through unfamiliar areas known to be at risk for the presence of land mines. This can save many lives.
Another common use for the metal detector is searching for "buried treasure" - coins and relics from years or even millennia in the past. Searching a beach that has many visitors can bring up lost items from only a few days past. This is not a viable way to make a living, but some people enjoy it as a hobby.