Magnetometer Basics: Types and Applications

This article covers the basics of magnetometers, explores different types, and outlines their applications.

Introduction to Magnetometers

A magnetometer is a device used to measure magnetic fields present in the surrounding environment. These magnetic fields are generated by current-carrying conductors and magnets. Even the Earth possesses its own magnetic field, the strength of which is inversely proportional to the cube of the distance (1/r³).

Figure-1: Earth Magnetometer, Alphalab Inc.

The measurement of magnetic fields is crucial for research, navigation, and mineral exploration. A magnetometer contains a sensor that measures magnetic flux density (B) or magnetic field strength (H). B is measured in Teslas (T), equivalent to N*m/A, while H is measured in A/m.

Magnetometers can detect fluctuations in the Earth’s magnetic field caused by materials like magnetite, which possesses its own inherent magnetic field.

Magnetometer Applications

Magnetometers have a wide range of applications:

• Navigation: Used as a navigational tool in various devices and systems.
• Space Exploration: Used for magnetic field measurements in space missions.
• Mineral Exploration: Used in the exploration of minerals such as iron, copper, silver, and gold.
• Defense Systems: Integrated into defense systems like drones (UAVs) and submarines.
• Anti-lock Braking Systems: Used in anti-lock braking systems (ABS) for vehicles.
• Smartphones: Used as a compass in smartphones for direction finding.

Magnetometer Types

Magnetometers are classified into different types based on their working principles and functions:

Vector Magnetometer

Vector magnetometers measure magnetic flux density in a specific direction within three-dimensional space. A typical example is the fluxgate magnetometer.

Scalar Magnetometer

Scalar magnetometers measure the total strength of a magnetic field, regardless of direction. They measure only the magnitude of the vector passing through the sensor. A quantum magnetometer is an example of a scalar magnetometer.

Based on their construction and specific applications, magnetometers can be further categorized as follows:

Coil Magnetometer

This type utilizes the principle that the current flowing through a wire creates a magnetic field with flux density directly proportional to the current. Conversely, changes in the magnetic field generate current in the wire. Coil magnetometers are used to detect buried objects and mines.

Hall Effect Sensor Magnetometer

These magnetometers operate on the Hall effect principle, which states that when a current-carrying conductor is placed in a magnetic field, a voltage is generated perpendicular to the direction of the field and current flow. Hall effect sensors are polarity-dependent; if either the current or the field direction changes, the Hall voltage polarity flips. Because the Hall voltage is typically very low, it requires amplification.

Magnetoresistive Sensor Magnetometer

Magnetoresistance is a phenomenon where the resistance of a material changes when exposed to a magnetic field. This principle is used in magnetoresistive sensor magnetometers, which offer higher accuracy compared to Hall effect sensors.

Magnetostrictive Magnetometer

These magnetometers rely on the Joule effect (contraction and expansion of a material under the influence of a magnetic field) and the Villari effect (changes in the susceptibility of a material when subjected to mechanical stress). The magnetostrictive effect is very small and requires indirect methods for measurement.

Fluxgate Magnetometer

Fluxgate magnetometers sense the intensity and orientation of magnetic fields. They are widely used to detect gravitational fields and their orientation in planets and the moon.

Proton Precession Magnetometer

These magnetometers measure the resonance frequency of protons or hydrogen nuclei in a magnetic field, offering an accuracy of about 1 ppm. This high accuracy is due to the fact that the precession frequency depends only on atomic constants and the ambient magnetic field strength.

Overhauser Magnetometer

Overhauser magnetometers use a special liquid combined with hydrogen atoms that is exposed to secondary polarization by an RF magnetic field. RF frequencies are transparent to the Earth’s DC magnetic field, making them suitable for use in magnetic devices. Overhauser magnetometers offer higher sensitivity than proton precession magnetometers.

Optically Pumped Magnetometer

These magnetometers include one nuclear magnetometer (helium 3) and four electron resonance magnetometers (helium 4, rubidium, cesium, potassium).

SQUID Magnetometer

SQUID stands for Superconducting Quantum Interference Device. SQUID magnetometers are the most sensitive type, capable of sensing down to 10^-15 Tesla. They operate at very low temperatures (around 4.2 K) but can also be designed to operate at higher temperatures.