How Does a Magnetometer Measure Magnetic Fields?
A magnetometer or a compass is a navigation device that measures the strength of the magnetic field or magnetic dipole moment. A magnetometer is an instrument with a sensor that measures the magnetic flux density.
Magnetometers determine the direction, strength, or relative variation in the magnetic field at a certain location.
We also use magnetometers to calibrate electromagnets and permanent magnets and to determine the magnetization of the material.
The first magnetometer was invented by Carl Friedrich Gauss in1833 that was capable of measuring absolute magnetic intensity.
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Types of Magnetometers
There are two types of magnetometers; these are:
Vector magnetometers
Scalar magnetometers
Vector Magnetometers
Vector magnetometers measure the flux density value in a specific direction in three-dimensional space. One such type is a fluxgate magnetometer. It measures the strength of the component of the earth’s field by positioning the sensor in the direction of the desired component.
Types of Vetor Magnetometers
A fluxgate magnetometer is also called the saturation magnetic circuit. It has two parallel bars wrapped with a coil placed close to each other. The magnetic field in the fluxgate magnetometer is caused by applying an alternating current through it.
A fluxgate magnetometer consists of a magnetically susceptible core wound by two coils of wire, where one coil is supplied an AC and the changing field produces an induced current in the other coil. Thus the alternating magnetic field and the induced output current is produced.
Squid Magnetometer
Squid magnetometers contain two superconductors set apart by two thin insulating layers to form two junctions.
These magnetometers are highly sensitive to the low range intensity fields and most are commonly used in measuring the magnetic field generated by the brain and heart.
Search-Coil Magnetometers
Search-coil magnetometers function on the phenomenon of Faraday’s law of induction. It contains copper coils that are wrapped around the magnetic core. When the core gets magnetized by the magnetic field lines produced inside the coil, the fluctuations in the field lines result in the flow of electric current.
The changes in this current due to changing voltage is measured and recorded by the magnetic field.
Scalar Magnetometers
Scalar magnetometers measure the scalar value of the magnetic field with accuracy. It is divided into the following types:
Types of Scalar Magnetometers
Proton Precession Magnetometers
This magnetometer uses nuclear magnetic resonance to determine the resonance frequency of the protons in a magnetic field.
When a polarizing DC is passed via a solenoid, a magnetic flux generates around the hydrogen-rich fuel, viz: kerosene, because of which a few protons align with the flux.
When the polarizing flux is released, the frequency of the precession of protons to the usual rearrangement is used to measure the magnetic field.
These magnetometers measure the magnitude of the vector passing through the sensor irrespective of the direction. A quantum magnetometer is an example of such a type.
Overhauser Effect Magnetometer
The working of the overhauled effect magnetometer is similar to the proton precession magnetometer; however, the difference lies in the type of radiofrequency employed. An overhauled magnetometer uses low power radio frequency signals to align the protons.
When an electron-rich liquid combines with hydrogen, it is subjected to an RF signal, under the overhauled effect, protons couple to the nuclei of the liquid. As soon as the precession frequency becomes linear with the magnetic flux density, we can measure the magnetic flux density.
The best part of such types of magnetometers is, these devices consume less power and have low sampling rates.
Ionized Gas Magnetometers
These magnetometers are more precise than the proton precession magnetometers. They comprise a photon emitter light and a vapor chamber filled with vapors of cesium, helium, and rubidium.
When the atom of the cesium (Cs) hits the photon of the lamp, the energy levels of the electrons vary corresponding to the external magnetic field. This frequency variation helps measure the magnetic field strength.
Uses of Magnetometers
We find the applications of magnetometers in the following fields:
In doing geographical and archaeological surveys.
As a metal detector.
To detect mineralization and geographical structures.
In the oil and gas industry for the directional drilling process.
Applications of Magnetometers
In Archaeology
Magnetometers serve as an aid in archaeological study. It helps to detect the archaeological sites viz: buried and submerged objects.
In Coal Exploration
For locating the sills and other obstructions that can result in an explosion.
In Military Applications, Defense, and Aerospace
Magnetometers are used in defense and navy to perform submarine activities.
In Oil and Gas Exploration
For drilling the discovered wells.
Health Care Monitoring
Used to perform cardiac operations viz: a diagnostic system for a non-invasive measure of the heart rate.
Pipeline Monitoring
Underground systems have higher chances of getting corroded; therefore, to detect this issue, we use magnetometers.
We find the application of magnetometers in the following field of geological Physics:
Compasses.
Applications in space.
Image processing of the magnetic data.
FAQs on Magnetometer: Principle, Types & Uses
1. What is a magnetometer and what fundamental principle does it operate on?
A magnetometer is a scientific instrument used to measure the strength and, in some cases, the direction of a magnetic field or a magnetic moment. The fundamental principle behind most magnetometers involves detecting a change caused by a magnetic field. This could be through Faraday's law of induction, where a changing magnetic field induces an electric current in a coil, or by measuring the effect of a magnetic field on the physical properties of a material, such as the Hall effect or nuclear magnetic resonance.
2. What are the main types of magnetometers available?
Magnetometers are broadly classified into two main categories based on what they measure:
Scalar Magnetometers: These measure the total strength (magnitude) of the magnetic field. A common example is the Proton precession magnetometer.
Vector Magnetometers: These measure the magnetic field's strength in a specific direction, allowing for a complete three-dimensional measurement of the field vector. Examples include Fluxgate, SQUID (Superconducting Quantum Interference Device), and Hall effect magnetometers.
3. What are some of the most important real-world applications of a magnetometer?
Magnetometers have a wide range of important applications across various fields, including:
Geophysical Surveys: Used in mineral prospecting to find ore bodies and in oil exploration to detect geological structures that might contain hydrocarbons.
Archaeology: To detect buried metallic artifacts, kilns, and other structures without excavation by mapping magnetic anomalies in the soil.
Navigation: Modern electronic compasses in smartphones and aircraft use magnetometers to determine orientation relative to the Earth's magnetic field.
Space Exploration: Spacecraft carry magnetometers to map the magnetic fields of planets, moons, and the sun.
Security: Walk-through metal detectors in airports are a form of magnetometer used to detect ferromagnetic metals.
4. What is the core difference between a scalar and a vector magnetometer?
The core difference lies in the information they provide. A scalar magnetometer measures only the total magnitude (strength) of the magnetic field, providing a single value for that point in space. In contrast, a vector magnetometer measures both the magnitude and the direction of the magnetic field, typically by measuring the field's components along three orthogonal axes (X, Y, and Z). This provides a complete vector representation of the field.
5. How does the magnetometer in a smartphone work, and what is it used for?
The magnetometer in a smartphone is a tiny, solid-state chip, often a Hall effect sensor or a magnetoresistive sensor. It works by measuring the force (Lorentz force) exerted on moving charge carriers by the Earth's magnetic field. This force creates a measurable voltage difference across the sensor, which is proportional to the magnetic field's strength. Its primary uses in a phone are for compass applications, providing directional information for map orientation, and in some games or augmented reality apps that require positional awareness.
6. Can you explain the working principle of a fluxgate magnetometer?
A fluxgate magnetometer works by using a highly permeable ferromagnetic core wrapped with two coils: a drive coil and a sense coil. The drive coil passes an alternating current that continuously drives the core into and out of magnetic saturation. In the absence of an external magnetic field, the signal induced in the sense coil is symmetrical. However, when an external magnetic field is present, it biases the core's magnetisation, causing an asymmetry in the output signal. The magnitude and phase of this asymmetry are directly proportional to the strength and polarity of the external magnetic flux, allowing for precise measurement.
7. Why are SQUID magnetometers so sensitive, and what is their main limitation?
SQUID (Superconducting Quantum Interference Device) magnetometers are incredibly sensitive due to their use of quantum mechanical effects. They operate with a superconducting loop containing one or two Josephson junctions. An external magnetic field passing through the loop induces a change in the quantum state, which can be measured as a change in voltage with extreme precision. This allows them to detect magnetic fields thousands of times weaker than those detected by other magnetometers. Their main limitation is the need for cryogenic temperatures; the superconducting components must be cooled with liquid helium to near absolute zero, making them expensive and complex to operate.
8. How does a deflection magnetometer work to compare the magnetic moments of two bar magnets?
A deflection magnetometer works based on the tangent law of magnetism. It consists of a small magnetic compass needle free to rotate in a horizontal plane. To compare two bar magnets, the first magnet is placed at a known distance from the compass, perpendicular to the Earth's magnetic meridian. The magnet's field deflects the compass needle by an angle. The experiment is repeated with the second magnet at the same distance. According to the tangent law, the magnetic moment of a magnet is proportional to the tangent of the angle of deflection it produces. By comparing the tangents of the angles, one can determine the ratio of the magnetic moments of the two magnets.
9. Can a magnetometer distinguish between a piece of unmagnetised iron and a permanent magnet?
Yes, a sensitive magnetometer can distinguish between them. An unmagnetised piece of iron has no external magnetic field of its own. However, when brought near the magnetometer, it will interact with the Earth's magnetic field, creating a local distortion due to induced magnetism. The magnetometer would detect this weak, localised anomaly. A permanent magnet, on the other hand, possesses its own strong, persistent magnetic field. It would produce a much stronger and more distinct signal on the magnetometer. Furthermore, a vector magnetometer could also identify the distinct north and south poles of the permanent magnet, which is not possible with the unmagnetised iron.
