Magnetometer

A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the Sun and the magnetosphere of a planet. Magnetometers are a frequent component instrument on spacecraft that explore planets.
Further information: Earth's magnetic field

* Uses

Magnetometers are used in ground-based electromagnetic geophysical surveys (such as magnetotellurics) to assist with detecting mineralization and corresponding geological structures. Airborne geophysical surveys use magnetometers that can detect magnetic field variations caused by mineralization, using airplanes like the Shrike Commander.[1] Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects, and in metal detectors to detect metal objects, such as guns in security screening. Magnetic anomaly detectors detect submarines for military purposes.

They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so that both the inclination and azimuth of the drill bit can be found.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is published on the K-index.[2]

A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions.[3] A dual technique magnetometer is part of the Cassini-Huygens mission to explore Saturn.[4] This system is composed of a vector helium and fluxgate magnetometers.[5] Magnetometers are also a component instrument on the Mercury MESSENGER mission. A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of a planet's or moon's magnetic field.
Further information: Spacecraft magnetometer
* Mobile phones

Magnetometers are appearing in mobile phones. The HTC Dream, Apple iPhone 3GS, Motorola Droid[6], Nokia N97, Nokia E72, Nexus One and the HTC Hero all have a magnetometer and come with compass apps for showing direction.[7][8]
* 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, relative to the spatial orientation of the device.

The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF magnetometer. Some scalar magnetometers are discussed below.

A magnetograph is a special magnetometer that continuously records data.
* Rotating coil magnetometer

The magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete.
* Hall effect magnetometer
NMR monitoring device containing three Hall-effect sensors.

The most common magnetic sensing devices are solid-state Hall effect sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.
* Proton precession magnetometer

Proton precession magnetometers, also known as proton magnetometers, measure the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to nuclear magnetic resonance (NMR). Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer is very good. They are widely used.

A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing some of the protons to align themselves with that field. The current is then interrupted, and as protons realign themselves with ambient magnetic field, they precess at a frequency that is directly proportional to the magnetic field. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor, amplified electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength or output as digital data.

The relationship between the frequency of the induced current and the strength of the magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hz/nT.

These magnetometers can be moderately sensitive if several tens of watts are available to power the aligning process. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained. Variations of about 0.1 nT can be detected.

The two main sources of measurement errors are magnetic impurities in the sensor and errors in the measurement of the frequency.

The Earth's magnetic field varies with time, geographical location, and local magnetic anomalies. The frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5 kHz near the equator to 2.5 kHz near the geomagnetic poles. Typical short-term magnetic field variations at a particular location during Earth's daily rotation is about 25 nT (i.e., about 1 part in 2,000), with variations over a few seconds of typically around 1nT (i.e., about 1 part in 50,000).[9]

Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful to detect variations of magnetic field in space or in time (often referred to as magnetic anomalies), caused by submarines, skiers buried under snow, archaeological remains, and mineral deposits.
* Gradiometer

Magnetic gradiometers are pairs of magnetometers with their sensors separated by a fixed distance (usually horizontally): the readings are subtracted in order to measure the differences between the sensed magnetic fields (i.e. field gradients caused by magnetic anomalies). This is one way of compensating both for the variability in time of the Earth's magnetic field and for other sources of electromagnetic interference, allowing more sensitive detection of anomalies. Because nearly equal values are being subtracted, the noise performance of the magnetometers used the performance requirements for the magnetometers is more extreme. For this reason, high performance magnetometers are the rule in this type of system.
* Fluxgate magnetometer