What Is A Magnetometer?

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“The earth is a giant magnet, with an axial tilt of about 23.5°,” reminds you of the stuff you read in geography books. Adding science makes it much more fun when you can actually measure the values of magnetism. A magnetometer lets you measure the values of magnetization along with the strength and direction of the magnetic field at a point in space.

Ever since the development of the first magnetometer back in 1833,  it has gone through many changes. It is an instrument with a sensor that measures magnetic flux density (B) in Tesla or As/m2. Magnetometers refer to sensors used for sensing magnetic fields or to systems which measure magnetic field using one or more sensors. Let’s take a look into these magnetic measuring devices in order to understand them more.

Types of magnetometer

Magnetometers used in laboratory are meant to be used on sample materials. The testing method is also different, with a requirement to place the sample inside the magnetometer. This type of measurement allows understanding the magnetic properties of an unknown material. An easily understood type of magnetometer is the inductive pickup coils type. You can make this one by yourself if you like. It uses the basic principle of magnetic induction. A magnet induces current in a moving coil.

Optical magnetometry on the other hand, makes use of various optical techniques to measure magnetization. Kerr magnetometry technique for example, makes use of the magneto-optic kerr effect, or MOKE. Incident light directed at the sample’s surface magnetizes the surface non-linearly. A detector measures the elliptical polarization of the reflected light.

There are two basic types of magnetometer measurements for surveys. Vector magnetometers measure the flux density value in a specific direction in 3D. A fluxgate magnetometer for example can measure the strength of any component of the Earth’s field by orienting the sensor in the direction of the desired component. Scalar magnetometers measure only the magnitude of the vector passing through the sensor regardless of the direction. Quantum magnetometers are an example.

Interestingly, there are magnetometers in some phones as well. The magnetometer in your phones is enclosed in an electronic chip that often incorporate other sensors that help correct the raw magnetic measurements using tilt information from the auxiliary sensor.

Dejan Nedelkovski explains the working of a hall-effect sensor in the video below. The most common applications of the hall effect sensor type magnetometer is in phones, vehicles and position sensors.

The art of measurement

The art of measurement is something in itself. Below table gives an overview of the common types of magnetic field measurement techniques, or magnetometry.

Type of Magnetometry

Explanation

SQUID (Superconducting quantum interference device)

SQUID magnetometry is an extremely sensitive absolute magnetometry technique resulting in the equipment being noise sensitive. It makes them impractical in high DC magnetic fields and in pulsed magnets.

Inductive pickup coils

It measures magnetization by detecting the current induced in a coil due to the changing magnetic moment of the sample material.

VSM (Vibrating Sample Magnetometer)

Here the detection of magnetization is done by mechanically vibrating the sample inside an inductive pickup or a SQUID coil.

Pulsed Field Extraction Magnetometry

Here the sample is secured and the external magnetic field is changed rapidly,  for example in capacitor-driven magnets.

Magnetic torque magnetometry

Here we measure the torque acting on a sample’s magnetic moment as a result of a uniform magnetic field.

Faraday force magnetometry

The magnetic force on the sample can be measured by a scale, or by detecting the displacement against a spring.

Optical magnetometry

In this technique, incident light is directed at the sample’s surface. Light interacts with a magnetized surface non-linearly and causes elliptical polarization of the reflected light.

The wonder that is a magnetometer

Magnetometers are not just magnetic field measuring devices. These are used as metal detectors as well. Not all metals though, just the magnetic or ferrous ones. A metal detector for example, can detect a car at about two meters, whereas a magnetometer can do the same for tens of meters. Sadly, it does not yet measure Magneto’s powers. But if the name is any indication, who knows, maybe one day….

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