Metal detectors are instruments designed to locate metallic objects buried underground, hidden within walls, or obscured in other environments. These devices operate by sensing the presence of metals through their interaction with electromagnetic fields. Their fundamental purpose is to assist in finding items ranging from historical artifacts and lost valuables to utility lines and security threats.
How Metal Detectors Work
A metal detector functions by generating an electromagnetic field from a coil in its search head. When this magnetic field encounters a metallic object, it induces small electrical currents, known as eddy currents, within the metal. These induced eddy currents then create their own secondary magnetic field. The detector’s receiver coil detects this secondary magnetic field, a subtle disturbance in the original transmitted field. The device processes this signal and alerts the user through an audible tone or a visual display, indicating the presence of metal.
Metals Readily Detected
Metal detectors identify metals with high electrical conductivity. This includes non-ferrous metals like gold, silver, copper, and aluminum, which allow eddy currents to flow freely when exposed to an electromagnetic field. These metals produce clear and strong signals due to their excellent conductivity, making them easy to detect. For instance, a gold ring or a copper coin generates a distinct response from a metal detector because of the strong secondary magnetic field created by induced currents.
Ferrous metals, such as iron and steel, are also readily detected, though for different reasons. While conductive, their magnetic properties, known as ferromagnetism, play a significant role. The detector’s primary magnetic field can directly interact with and be distorted by these materials’ magnetic properties, in addition to inducing eddy currents. This dual interaction results in a strong and easily identifiable signal, making items like old iron tools or steel relics common finds for metal detector enthusiasts.
Metals That Are Harder or Not Detected
Certain metals and materials pose challenges for standard metal detectors, being difficult to find or completely undetectable. Some forms of stainless steel have lower magnetic permeability and electrical conductivity compared to common iron or other steels, making them harder to detect, especially in small or thin pieces. Lead, while conductive, is also challenging to detect if in small fragments or deeply buried, as its conductivity is not as high as copper or gold.
Materials not electrically conductive or lacking magnetic properties cannot be detected by conventional metal detectors. This category includes common non-metallic objects such as wood, plastic, glass, and ceramics. Natural rocks and minerals also fall into this category unless they contain concentrations of conductive or magnetic metallic ores. A metal detector will not alert a user to the presence of a buried plastic pipe or a glass bottle.
Factors Affecting Detection
Beyond the inherent properties of the metal, several external factors influence a metal detector’s ability to locate an object. The size and shape of a metallic item significantly impact its detectability; larger objects produce stronger signals than smaller ones, and a flat, disc-shaped object is easier to detect than an irregularly shaped one of the same mass. The depth at which an object is buried is a limiting factor, as the electromagnetic field strength diminishes rapidly with distance. A coin just below the surface is much easier to find than the same coin buried a foot deep.
The object’s orientation relative to the detector’s coil also affects signal strength. An object lying flat presents a larger surface area for eddy current induction than one standing on its edge. The type of soil or medium plays a role; highly mineralized ground, containing naturally occurring conductive or magnetic minerals, can interfere with detector signals, masking targets. Adjusting sensitivity settings helps mitigate some challenges, but extreme conditions still make detection difficult.