A magnetometer is a scientific instrument which measures magnetic fields. In addition to determining the strength of a magnetic field, a magnetometer can also determine its orientation and direction. There are a wide range of uses for magnetometers, along with several basic styles which can be used, depending on the circumstances. Some of these devices are so sensitive that they are capable of detecting the signs of increased sun activity before these signs are visible to observatories.
One of the most obvious uses for a magnetometer is in scientific observations of the Earth's magnetic field, which can fluctuate in various circumstances. Magnetometers are also used by geologists to learn more about specific regions of the Earth's crust, or to look for magnetic minerals like iron. These devices can also measure the impact of various human activities on the Earth's magnetic field. Several long-running scientific surveys keep magnetometers continuously running for the purpose of gathering comparison data.
Archaeologists also use magnetometers, because these tools can be used to identify archaeological sites and ship wrecks by detecting various magnetic substances which may be concentrated in these regions. Many ships also keep a magnetometer on board to identify potential hazards such as unmarked shipwrecks or wrecked ships which have drifted since they were last charted. Casual archaeologists also use magnetometers, in the form of metal detectors.
The study of the Earth's magnetic field can yield interesting information about the development of the Earth and the Universe in general. Scientists have documented clear changes in the Earth's magnetic field, some of which are actually recorded in the fossil record, in the form of iron particles which have reoriented themselves in response to polarity switches. Magnetometers can also be used to look at the impact of solar activity on the Earth, with scientists using data from the past to chart and predict solar flares.
The complexity and size of these machines varies widely. Many are designed for fieldwork, so they are at least reasonably portable. Chances are that you probably own a magnetometer, since many people own compasses. Compasses use the Earth's magnetic field to move directional needles which can be used to orient the person utilizing the compass. More delicate and precise magnetometers are maintained by numerous scientific institutions and universities.