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Magnetometer basics | Magnetometer types, Magnetometer applications

This article covers Magnetometer basics and Magnetometer types including its applications. It describes different types of Magnetometer viz. Vector magnetometer, Scalar magnetometer, coil magnetometer, hall sensor magnetometer, magnetoresistive sensor, magnetostrictive sensor, fluxgate magnetometer, proton magnetometer overhauser magnetometer, optically pumped magnetometer, Squid magnetometer etc.

What is Magnetometer?

Introduction
Magnetometer is a device used to measure magnetic fields present around us. Magnetic fields are generated by current carrying conductor. The magnets creates magnetic field. Even our earth has its own magnetic field. Earth's magnetic field strength is proportional to 1/r3.

Earth Magnetometer
Figure-1: Earth Magnetometer, Alphalab Inc.

Measurement of magnetic field is essential for research, navigation and mineral exploration. Magnetometer is a device having sensor which measures magnetic flux density (B) or magnetic field strength (H). B is measured in units of N*m/A or Tesla. H is measured in units of A/m. Magnetometer measures fluctuations in the earth's magnetic field due to materials such as magnetite which has its own magnetic fields.

Magnetometer applications

Following are the applications of magnetometer:
• It is used as navigational tool.
• It is used in space for magnetic field measurements.
• It is used in mineral exploration such as iron, copper, silver, gold etc.
• It is used in defence systems such as drones (i.e. UAVs), submarines etc.
• It is used in anti-lock bracking.
• It is used in smartphones as compass for direction finding.

Magnetometer Types

Based on their working principles and functions, magnetometer types are as follows.

Vector Magnetometer: It is used to measure magnetic flux density in specific direction (in three dimensional space) . Example of vector magnetometer is fluxgate magnetometer.

Scalar Magnetometer: It is used to measure total strength of a magnetic field. It measures only magnitude of vector passing through sensor irrespective of direction. Example of scalar magnetometer is quantum magnetometer.

Based on their construction and applications magnetometer types are as follows.

Coil Magnetometer: The current flowing through a wire creates a magnetic field with flux density which is directly proportional to current (in amperes). Conversely, change in magnetic field will generate current in the wire. This principle is being used in coil magnetometer. It is used to detect buried objects and as mine detection when it moves above them.

Hall Effect Sensor Magnetometer: It works on 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 flow of current". Hall effect sensor is polarity dependent. If the current changes direction or field changes direction, hall voltage polarity flips. Hall voltage is very low in value and hence needs amplification.

Magneto-resistive sensor Magnetometer: Magnetoresistance is a phenomenon in which resistance of material changes fractionally when it is exposed to magnetic field. This principle is used in such magnetometer design. This magnetometer type offers higher accuracy compare to hall effect sensor.

Magneto-strictive Magnetometer: It involves two effects viz. contraction and expansion of material under influence of magnetic field (known as Joule effect) and inverse effect of changes in susceptibility of material when subjected to mechanical stress (known as villari effect). The magnetostrictive effect is very small and hence needs indirect methods for its measurement.

Fluxgate Magnetometer: It senses the intensity and orientation of magnetic fields. It is used widely to detect gravitational fields and their orientation of the planets and the moon.

Proton Precession Magnetometer: It measures resonance frequency of protons or hydrogen nuclei in the magnetic field. It offers accuracy of about 1 ppm. This is due to the fact that precession frequency depends on atomic constants and ambient magnetic field strength only.

Overhauser Magnetometer: In this magnetometer type, a special liquid is combined with hydrogen atoms which is exposed to secondary polarization by RF magnetic field. RF frequencies are transparent to DC magnetic field of the Earth. Hence they are used in magnetic devices. Overhauser magnetometer offers higher sensitivity compare to proton precession magnetometer.

Optically pumped Magnetometer: These magnetometer type include one nuclear magnetometer (viz. helium 3) and four electron resonance magnetometers (viz. helium 4, rubidium, cesium, potassium).

Squid Magnetometer: SQUID stands for Superconducting Quantum Interference Device. It is most sensitive compare to all and it can sense down to 10-15. They can operate at very low temperatures ( 4.2 0K) as well as higher temperatures.



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