A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature. Some countries such as the USA, Canada and Australia classify the more sensitive magnetometers as military technology, and control their distribution.
The International System of Units unit of measure for the strength of a magnetic field is the tesla. As this is a very large unit, workers in the earth sciences commonly use the nanotesla (nT) as their working unit of measure. Engineers often measure magnetic fields in Gauss. 1 Gauss = 100,000 nT or 1 Gauss = 100,000 gamma.
The Earth's magnetic field (the magnetosphere) is a potential field. It varies both temporally and spatially for various reasons, including inhomogeneity of rocks and interaction between charged particles from the Sun and the magnetosphere.
The earth's magnetic field is relatively weak. A simple magnet that may be purchased in a hardware store produces a field many hundreds of times stronger than the earth's field. The earth's magnetic field varies from around 20,000 nT near the equator to 80,000 nT near the poles. It also varies with time. There is a daily variation of around 30 nT at mid latitudes and hundreds of nT at the poles. Geomagnetic storms can cause much larger variations.
Magnetometers, which measure magnetic fields, are distinct from metal detectors, which detect hidden metals by their conductivity. When used for detecting metals, a magnetometer can detect only magnetic (ferrous) metals, but can detect such metals buried much deeper than a metal detector. Magnetometers are capable of detecting large objects like cars at tens of meters, while a metal detector's range is unlikely to exceed 2 meters.