Magnetometer

A magnetometer is an instrument designed to measure magnetic fields or magnetic dipole moments. These devices form an essential part of geophysics, navigation, materials science, space exploration and defence applications. Their designs range from simple mechanical compasses to highly sensitive quantum-based devices capable of detecting minute field variations.

Fundamental concepts of magnetic fields

Magnetic fields are vector quantities possessing both magnitude and direction. In the International System of Units, magnetic flux density is measured in tesla (T), whereas the centimetre–gram–second system employs the gauss (G), with 10,000 gauss equal to one tesla. Measurements of the Earth’s magnetic field are typically expressed in nanotesla (nT), also known as gamma, and range from approximately 20,000 to 80,000 nT depending on geographical location. Natural fluctuations are usually around 100 nT, and anomalies within geological formations can fall within the picotesla range.
Magnetometers designed to measure fields above 1 millitesla may be called gaussmeters or teslameters, while instruments designed for weaker fields are commonly referred to simply as magnetometers.

Types of magnetometer

Magnetometers may be categorised by what they measure or how they measure it.
Vector magnetometers determine the directional components of a magnetic field, providing three-dimensional information. In geophysics they may express measurements in terms of magnetic declination and inclination relative to the Earth’s surface.
Scalar or total-field magnetometers measure the magnitude of a magnetic field irrespective of direction. These instruments are used extensively in survey work because of their robustness and simplicity of interpretation.
Magnetometers may also be classified by mode of operation:

• Absolute magnetometers use internal calibration or fundamental physical constants to obtain precise measurements of field magnitude or direction.
• Relative magnetometers, or variometers, measure changes in magnetic field strength relative to an uncalibrated baseline. These are used to detect temporal variations or anomalies.

Finally, magnetometers can be organised according to their operational context:

• Stationary magnetometers, permanently installed for long-term monitoring.
• Portable magnetometers, used in mobile surveys or fieldwork.
• Laboratory magnetometers, which measure the magnetic properties of materials under controlled conditions.
• Survey magnetometers, used in geological mapping, archaeology, detection of unexploded ordnance and mineral exploration.

Principles of operation and instrumentation capabilities

The performance of a magnetometer is defined by several technical specifications:

• Sample rate: the number of readings per second. High sample rates improve spatial resolution in mobile surveys.
• Bandwidth: the ability to accurately follow rapid changes in magnetic field, limited by the Nyquist criterion or any onboard filtering.
• Resolution: the smallest field change detectable by the sensor, influenced by quantisation and system design.
• Absolute error and drift: the deviation from the true value and how that deviation changes over time.
• Thermal stability: sensitivity to temperature variations.
• Noise level: random fluctuations arising from sensing elements or electronics, often described in nT/√Hz.
• Heading error: measurement deviations caused by instrument orientation.
• Dead zones: orientations that yield unreliable readings, common in optically pumped and Overhauser devices.
• Gradient tolerance: the ability to conduct accurate measurements in environments with strong magnetic gradients, such as surveys for buried metallic objects.

These capabilities determine whether a magnetometer is suitable for a particular application, from high-precision scientific measurements to ruggedised field surveys.

Early development of magnetometers

The earliest magnetometers originated from attempts to quantify the Earth’s magnetic field. The magnetic compass, based on a magnetised needle aligned with the Earth’s field lines, represents the simplest example. The frequency of oscillation of such a needle provides a crude measure of field strength.
By the early nineteenth century, more sophisticated instruments emerged. William Scoresby contributed important advances in maritime magnetic science, and in 1823 he described a magnetimeter designed to measure magnetic attraction and dip. Carl Friedrich Gauss significantly advanced the field in 1833 with his absolute magnetometer, based on oscillation measurements of a suspended permanent magnet. His contributions were later honoured through the naming of the gauss unit.
Francis Ronalds and Charles Brooke introduced magnetographs in 1846, enabling continuous photographic recording of magnetic variations. These instruments became mainstays in global magnetic monitoring and supported large-scale geomagnetic surveys through the late nineteenth and early twentieth centuries.

Applications in science, navigation and industry

Magnetometers have wide-ranging uses across scientific and technological fields.
In geophysics, they detect magnetic anomalies arising from variations in rock composition, aiding mineral exploration and archaeological investigations. In the 1950s, airborne magnetometry led to major discoveries such as ore bodies in Manitoba.
In navigation, vector magnetometers assist in determining aircraft heading within attitude and heading reference systems. Miniaturised magnetometers embedded in integrated circuits now serve as electronic compasses in consumer devices.
In materials science, laboratory magnetometers measure magnetisation, magnetic moment and the behaviour of electrons within solids. They enable classification of materials as diamagnetic, paramagnetic, ferromagnetic or antiferromagnetic, supporting research into superconductivity, spintronics and condensed-matter physics.
In military contexts, sensitive magnetometers can detect the magnetic signature of submarines or function as triggering devices for magnetic mines. Because of their strategic importance, the export of high-sensitivity magnetometers may be restricted by some governments.
Magnetometers are also used as metal detectors, capable of locating ferrous materials at far greater distances than conductivity-based detectors. Large iron objects, such as vehicles, may be detected at significant ranges depending on the sensitivity of the instrument.

Miniaturised and modern magnetometers

Recent technological progress has led to the development of compact, low-cost magnetometers incorporated into microelectromechanical systems. These devices act as three-axis compasses and are ubiquitous in smartphones, drones and wearable technologies. Their small size, low power consumption and integration with digital electronics allow real-time orientation sensing for navigation, gaming and motion tracking.
More advanced magnetometers, including fluxgate instruments used aboard spacecraft, perform precise measurements of interplanetary and planetary magnetic fields. Space missions such as Pioneer 10 and 11 employed such devices to characterise the magnetospheres of outer planets.