____________ are devices that detect changes in a magnetic field.

Magnetometers are specifically designed to measure the strength and direction of magnetic fields. They detect changes in a magnetic field by responding to variations in the magnetic lines of force in their vicinity. This capability enables magnetometers to be used in various applications, including navigation, geological surveys, and military operations, where understanding the Earth's magnetic field or the presence of ferromagnetic materials is crucial.

While accelerometers measure acceleration and tilt, thermometers measure temperature, and seismometers detect seismic waves related to earthquakes, none of these instruments are primarily designed to detect magnetic field changes. Thus, the distinct function of magnetometers in detecting magnetic fields sets them apart as the correct answer.