A device for measuring the strength and direction of magnetic fields. Karl Friederich Gauss built an early magnetometer in 1832, comprising a freely rotating magnet suspended from a torsion wire in which the period of oscillation of the magnet measured the field strength. Modern magnetometers include the fluxgate magnetometer, in which paired electromagnets driven by an external alternating voltage generate an electrical signal in a coil surrounding the pair in response to the field under test, and cryogenic or SQUID magnetometers. Magnetometers are used in many systems where magnetic fields are present, and also in geophysics.
A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.
Earth's magnetism varies from place to place and differences in the Earth's magnetic field (the magnetosphere) can be caused by a couple of things:
The differing nature of rocks The interaction between charged particles from the sun and the magnetosphereUses
Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic pull of iron.
A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.
Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can even see the light from the aurora.
Types
Magnetometers can be divided into two basic types:
scalar magnetometers measure the total strength of the magnetic field to which they are subjected, and vector magnetometers have the capability to measure the component of the magnetic field in a particular direction.The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined.
A magnetograph is a special magnetometer that continuously records data.
Proton precession magnetometer
One type of magnetometer is the proton precession magnetometer, which operates on the principle that protons are spinning on an axis aligned with the magnetic field.
An inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with the newly created field. The field is then interrupted, and as protons are realigned with Earth's magnetic field, spinning protons precess at a specific frequency.
Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good.
If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive.
Overhauser magnetometer
The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom.
RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e.
The unbound electrons in the special liquid transfer their excited state (i.e.
The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons.
Overhauser magnetometers achieve some 0.01 nT/√Hz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.
Cesium vapor magnetometer
A basic example of the workings of a magnetometer may be given by discussing the common "optically pumped cesium vapour magnetometer" which is a highly sensitive (0.004 nT/√Hz) and accurate device used in a wide range of applications.
The device broadly consists of a photon emitter containing a cesium light emitter or lamp, an absorption chamber containing cesium vapour and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.
Calibration
The basic principle that allows the device to operate is the fact that a cesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus.
Detection
Given that this theoretically perfect magnetometer is now calibrated it can be exposed to the environment.
Applications
When removed from an isolated environment, the cesium vapour can never be 'perfectly' calibrated and the system is subject to environmental interference as are all scalar magnetometers.
SQUID magnetometer
SQUIDs, or Superconducting Quantum Interference Devices, are used to measure extremely small magnetic fields;
These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint.
Early magnetometers
In 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper entitled "On the intensity of the Earth's magnetic field expressed in absolute measure".
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