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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 two things:
  • The differing nature of rocks
  • The interaction between charged particles from the sun and the magnetosphere


    • Uses Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.

    A magnetometer can also be used by satellites like GOES to measure both the Magnitude (mathematics) and Direction (geometry, geography) of the earth's magnetic field.

    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 the 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 Auroral light. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field.

    Types Magnetometers can be divided into two basic types:

    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 and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.

    A magnetograph is a special magnetometer that continuously records data.

    Proton precession magnetometer One type of magnetometer is the proton precession magnetometer, which measures the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to Nuclear Magnetic Resonance (NMR).

    A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precession at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).

    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. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.

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

    The strength of the Earth's magnetic field varies with time and location, so that the frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5kHz near the equator to 2.5kHz near the poles.

    Electron-spin resonance In contrast to nuclear magnetic resonance, electron- spin resonance (ESR) is observed only in a restricted class of substances. These substances include transition elements--that is, elements with unfilled inner electronic shells--free radicals (molecular fragments), metals, and various paramagnetic defects and impurity centers. Another difference from NMR is a far greater sensitivity to environment; whereas the resonance frequencies in NMR in general are shifted from those of bare nuclei by very small amounts because of the influence of conduction electrons, chemical shifts, spin-spin couplings, and so on, the ESR frequencies in bulk matter may differ greatly from those of free spins or free atoms because the unfilled subshells of the atom are easily distorted by the interactions occurring in bulk matter. A model that has been highly successful for the description of magnetism in bulk matter is based on the effect of the crystal lattice on the magnetic center under study. The effect of the crystal field, particularly if it has little symmetry, is to reduce the magnetism caused by orbital motion. To some extent the orbital magnetism is preserved against ligand fields of low symmetry by the coupling of the spin and orbital momenta. The total energy of the magnetic center consists of two parts: (1) the energy of coupling between magnetic moments due to the electrons and the external magnetic field, and (2) the electrostatic energy between the electronic shells and the ligand field, which is independent of the applied magnetic field. The energy levels give rise to a spectrum with many different resonance frequencies, the fine structure Another important feature of electron-spin resonance results from the interaction of the electronic magnetization with the nuclear moment, causing each component of the fine-structure resonance spectrum to be split further into many so-called hyperfine components. If the electronic magnetization is spread over more than one atom, it can interact with more than one nucleus; and, in the expression for hyperfine levels, the hyperfine coupling of the electrons with a single nucleus must be replaced by the sum of the coupling with all the nuclei. Each hyperfine line is then split further by the additional couplings into what is known as superhyperfine structure. The key problem in electron-spin resonance is, on one hand, to construct a mathematical description of the total energy of the interaction in the ligand field plus the applied magnetic field and, on the other hand, to deduce the parameters of the theoretical expression from an analysis of the observed spectra. The comparison of the two sets of values permits a detailed quantitative test of the microscopic description of the structure of matter in the compounds studied by ESR. The transition elements include the iron group, the lanthanide (or rare-earth) group, the palladium group, the platinum group, and the actinide group. The resonance behaviour of compounds of these elements is conditioned by the relative strength of the ligand field and the spin-orbit coupling. In the lanthanides, for instance, the ligand field is weak and unable to uncouple the spin and orbital momentum, leaving the latter largely unreduced. On the other hand, in the iron group, the components of the ligand field are, as a rule, stronger than the spin-orbit coupling, and the orbital momentum is strongly reduced. The advent of ESR has marked a new understanding of these substances. Thus, it was formerly thought that in the iron group and the lanthanide group ions of the crystal were bound together solely by their electrostatic attraction, the magnetic electrons being completely localized on the transition ion. The discovery of superhyperfine structure demonstrated conclusively that some covalent bonding to neighbouring ions exists. With few exceptions, the magnetic moments of imperfections such as vacancies at lattice sites and impurity centers in crystals that give rise to an observable ESR have the characteristics of a free electronic spin. In the study of these centers, hyperfine and superhyperfine structure provide a mapping of the electronic magnetization and make it possible to test the correctness of the model chosen to describe the defect. The most widely studied by resonance are those of phosphorus, arsenic, and antimony, substituted in the semiconductors silicon and germanium. Studies of hyperfine and superhyperfine structure give detailed information on the status of these impurities.

    Free radicals are ideally suited for study by electron- spin resonance. They can be studied in a concentrated form or in very dilute solutions. The sensitivity of ESR is particularly important for the study of very short-lived species. The ESR of free radicals in solutions gives an extreme wealth of hyperfine lines because the magnetic electron is not localized on one nucleus but interacts with several nuclei of the radical.

    Maxwell's equations Four equations that, together, form a complete description of the production and interrelation of electric and magnetic fields. The physicist James Clerk Maxwell in the 19th century based his description of electromagnetic fields on these four equations, which express experimental laws. The statements of these four equations are, respectively: (1) electric field diverges from electric charge, an expression of the Coulomb force, (2) there are no isolated magnetic poles, but the Coulomb force acts between the poles of a magnet, (3) electric fields are produced by changing magnetic fields, an expression of Faraday's law of induction, and (4) circulating magnetic fields are produced by changing electric fields and by electric currents, Maxwell's extension of AmpSre's law (q.v.) to include the interaction of changing fields. The most compact way of writing these equations in the metre-kilogram- second (mks) system is in terms of the vector operators div (divergence) and curl. In these expressions the Greek letter rho, , is charge density, J is current density, E is the electric field, and B is the magnetic field; here, D and H are field quantities that are proportional to E and B, respectively. The four Maxwell equations, corresponding to the four statements above, are: (1) div D = rho} (2) div B = 0, (3) curl E = -dB/dt, and (4) curl H = dD/dt + J Nuclear magnetic resonance In the absence of atomic motion in rigid lattices (crystals), NMR makes it possible to determine molecular structures not observable by other means. In many solids, even at low temperatures, there occur atomic diffusion and rotation of groups of atoms. These movements affect the shape of the NMR absorption peak. A study of these effects as a function of temperature can supplement other physical measurements. In metals, the nuclei are influenced by an interaction between the spins of the conduction electrons (electrons not bound to atoms that move freely through the metal) and the applied field. This condition results in a shift of the resonance frequency from the value observed for the same nucleus when it is present in an insulator. These so-called metallic shifts provide important information on the magnetic susceptibility, the quantum mechanical wave functions that describe energy states, and the density of states of conduction electrons in the metal. In superconductors, the shape of the NMR spectral peaks provide detailed information on the penetration and internal distribution of the magnetic field. In ferromagnets or antiferromagnets (crystals in which not all electrons are paired), the NMR is influenced by the internal magnetic fields produced by the array of ordered electronic spins. In ferromagnets the shift is a measure of the lattice magnetization; in an antiferromagnet there are at least two shifts that give the magnetization of each antiferromagnetic sub lattice separately, a result unattainable by conventional magnetic measurements. For certain nuclei, the NMR spectrum reveals the existence of nuclear electric quadrupole moments (an electric quadrupole consists of a charge distribution equivalent to a special arrangement of two electric dipoles) that interact with the electric fields that exist at the nuclear sites. These interactions provide information on the microscopic distribution of electric charge around the nucleus. The most important consequence of the extraordinary sharpness of nuclear magnetic resonance (NMR) lines in liquids is the possibility of measuring the chemical shifts--that is, the separations between NMR lines from nuclear spins of the same species but in different molecular environments. The physical origin of chemical shifts is the following: an external magnetic field polarizes the closed electron shells of the atoms and produces a small magnetic field, proportional to the external field, which shifts the NMR line with respect to its position for the bare nucleus--e.g., one that is devoid of electrons. The bare nucleus itself is never observed, but the atomic diamagnetic shifts that correspond to atoms located in different molecular sites are slightly different, and it is their differences that produce the chemical shifts. As an example, the proton NMR spectrum of ethyl alcohol exhibits three peaks, with relative weights or intensities of 3:2:1. In more complicated molecules such spectra contain much chemical information and can help in the determination of unknown molecular structures. The multiplicity of lines is further increased by the interaction between nuclear spins. As already mentioned in connection with motion narrowing in liquids, the usual magnetic dipolar interactions are averaged out by molecular motion and do not split the NMR spectra. There exists, however, an indirect interaction between nuclear spins, caused by the electrons, that splits the resonance line of a specific nuclear spin into many components. High resolution nuclear magnetic resonance has become one of the most prized tools in the fields of organic chemistry and biochemistry. On the experimental side, the requirements to be met by the equipment are severe. In order to match natural line widths of a fraction of a cycle, the applied magnetic fields must have a relative stability and homogeneity throughout the sample better than one part in 10. Special magnets that give uniform fields and are stabilized, devices that twirl samples in order to smooth out the magnetic inhomogeneity, and sophisticated radio- frequency detection equipment are commercially available. The trend toward higher fields (over 100 kilogauss), resulting from super conducting solenoids, improves the resolution by increasing the chemical shift splittings and the signal-to-noise ratio.

    The measurement of the precession frequency of proton spins in a magnetic field can give the value of the field with high accuracy and is widely used for that purpose. In low fields, such as the Earth's magnetic field, the NMR signal is expected to be weak because the nuclear magnetization is small, but special devices can enhance the signal 100 or 1000 times. Incorporated in existing portable magnetometers, these devices make them capable of measuring fields to an absolute accuracy of about one part in 1,000,000 and detecting field variations of about 10 gauss. Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful whenever a phenomenon is linked with variations of magnetic field in space or in time, such as anomalies arising from submarines, skiers buried under snow, archaeological remains, and mineral deposits

    Fluxgate magnetometer A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily magnetised in alignment with that field and less easily magnetised in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field.

    Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospection. In Britain the commonest such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).

    Overhauser magnetometer The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This NMR effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source).

    RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's direct current magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

    The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

    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. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant.

    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. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

    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. When a cesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this stage the device can be said to be perfectly calibrated.

    Detection Given that this theoretically perfect magnetometer is now calibrated it can be exposed to the environment. It is easy to imagine that the environment is constantly emitting quanta of energy and that some of these will pass through the chamber. When they do, they may hit one of our cesium atoms and cause it to jump into a new energy state, which may in turn be one in which it can absorb a photon from our cesium emitter. If this is the case it will cause a decrease in the number of photons reaching our detector and this can be easily recorded. Scaling from this simple example to account for the vast number of energy transactions occurring within the cesium vapour, it is easy to see how the system works.

    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. However, by the application of feedback systems and an averaging of the detection rates seen in a benign environment, the instrument can be calibrated sufficiently well in a real-world environment to make it accurate and useful for detection.

    Spin-exchange-relaxation-free (SERF) atomic magnetometers At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free atomic magnetometers containing potassium vapor operate similarly to the cesium magnetometers described above yet can reach sensitivities lower than 1 fT/√Hz . Large volume detectors have achieved 200 aT/√Hz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.

    SQUID magnetometer SQUIDs, or Superconducting Quantum Interference Devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.

    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. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).

    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". It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer) . It consisted of a permanent bar magnet suspended horizontally from a gold fibre . A magnetometer is also called a gaussmeter.

    See also

    External links

    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 two things:
  • The differing nature of rocks
  • The interaction between charged particles from the sun and the magnetosphere


    • Uses Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.

    A magnetometer can also be used by satellites like GOES to measure both the Magnitude (mathematics) and Direction (geometry, geography) of the earth's magnetic field.

    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 the 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 Auroral light. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field.

    Types Magnetometers can be divided into two basic types:

    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 and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.

    A magnetograph is a special magnetometer that continuously records data.

    Proton precession magnetometer One type of magnetometer is the proton precession magnetometer, which measures the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to Nuclear Magnetic Resonance (NMR).

    A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precession at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).

    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. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.

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

    The strength of the Earth's magnetic field varies with time and location, so that the frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5kHz near the equator to 2.5kHz near the poles.

    Electron-spin resonance In contrast to nuclear magnetic resonance, electron- spin resonance (ESR) is observed only in a restricted class of substances. These substances include transition elements--that is, elements with unfilled inner electronic shells--free radicals (molecular fragments), metals, and various paramagnetic defects and impurity centers. Another difference from NMR is a far greater sensitivity to environment; whereas the resonance frequencies in NMR in general are shifted from those of bare nuclei by very small amounts because of the influence of conduction electrons, chemical shifts, spin-spin couplings, and so on, the ESR frequencies in bulk matter may differ greatly from those of free spins or free atoms because the unfilled subshells of the atom are easily distorted by the interactions occurring in bulk matter. A model that has been highly successful for the description of magnetism in bulk matter is based on the effect of the crystal lattice on the magnetic center under study. The effect of the crystal field, particularly if it has little symmetry, is to reduce the magnetism caused by orbital motion. To some extent the orbital magnetism is preserved against ligand fields of low symmetry by the coupling of the spin and orbital momenta. The total energy of the magnetic center consists of two parts: (1) the energy of coupling between magnetic moments due to the electrons and the external magnetic field, and (2) the electrostatic energy between the electronic shells and the ligand field, which is independent of the applied magnetic field. The energy levels give rise to a spectrum with many different resonance frequencies, the fine structure Another important feature of electron-spin resonance results from the interaction of the electronic magnetization with the nuclear moment, causing each component of the fine-structure resonance spectrum to be split further into many so-called hyperfine components. If the electronic magnetization is spread over more than one atom, it can interact with more than one nucleus; and, in the expression for hyperfine levels, the hyperfine coupling of the electrons with a single nucleus must be replaced by the sum of the coupling with all the nuclei. Each hyperfine line is then split further by the additional couplings into what is known as superhyperfine structure. The key problem in electron-spin resonance is, on one hand, to construct a mathematical description of the total energy of the interaction in the ligand field plus the applied magnetic field and, on the other hand, to deduce the parameters of the theoretical expression from an analysis of the observed spectra. The comparison of the two sets of values permits a detailed quantitative test of the microscopic description of the structure of matter in the compounds studied by ESR. The transition elements include the iron group, the lanthanide (or rare-earth) group, the palladium group, the platinum group, and the actinide group. The resonance behaviour of compounds of these elements is conditioned by the relative strength of the ligand field and the spin-orbit coupling. In the lanthanides, for instance, the ligand field is weak and unable to uncouple the spin and orbital momentum, leaving the latter largely unreduced. On the other hand, in the iron group, the components of the ligand field are, as a rule, stronger than the spin-orbit coupling, and the orbital momentum is strongly reduced. The advent of ESR has marked a new understanding of these substances. Thus, it was formerly thought that in the iron group and the lanthanide group ions of the crystal were bound together solely by their electrostatic attraction, the magnetic electrons being completely localized on the transition ion. The discovery of superhyperfine structure demonstrated conclusively that some covalent bonding to neighbouring ions exists. With few exceptions, the magnetic moments of imperfections such as vacancies at lattice sites and impurity centers in crystals that give rise to an observable ESR have the characteristics of a free electronic spin. In the study of these centers, hyperfine and superhyperfine structure provide a mapping of the electronic magnetization and make it possible to test the correctness of the model chosen to describe the defect. The most widely studied by resonance are those of phosphorus, arsenic, and antimony, substituted in the semiconductors silicon and germanium. Studies of hyperfine and superhyperfine structure give detailed information on the status of these impurities.

    Free radicals are ideally suited for study by electron- spin resonance. They can be studied in a concentrated form or in very dilute solutions. The sensitivity of ESR is particularly important for the study of very short-lived species. The ESR of free radicals in solutions gives an extreme wealth of hyperfine lines because the magnetic electron is not localized on one nucleus but interacts with several nuclei of the radical.

    Maxwell's equations Four equations that, together, form a complete description of the production and interrelation of electric and magnetic fields. The physicist James Clerk Maxwell in the 19th century based his description of electromagnetic fields on these four equations, which express experimental laws. The statements of these four equations are, respectively: (1) electric field diverges from electric charge, an expression of the Coulomb force, (2) there are no isolated magnetic poles, but the Coulomb force acts between the poles of a magnet, (3) electric fields are produced by changing magnetic fields, an expression of Faraday's law of induction, and (4) circulating magnetic fields are produced by changing electric fields and by electric currents, Maxwell's extension of AmpSre's law (q.v.) to include the interaction of changing fields. The most compact way of writing these equations in the metre-kilogram- second (mks) system is in terms of the vector operators div (divergence) and curl. In these expressions the Greek letter rho, , is charge density, J is current density, E is the electric field, and B is the magnetic field; here, D and H are field quantities that are proportional to E and B, respectively. The four Maxwell equations, corresponding to the four statements above, are: (1) div D = rho} (2) div B = 0, (3) curl E = -dB/dt, and (4) curl H = dD/dt + J Nuclear magnetic resonance In the absence of atomic motion in rigid lattices (crystals), NMR makes it possible to determine molecular structures not observable by other means. In many solids, even at low temperatures, there occur atomic diffusion and rotation of groups of atoms. These movements affect the shape of the NMR absorption peak. A study of these effects as a function of temperature can supplement other physical measurements. In metals, the nuclei are influenced by an interaction between the spins of the conduction electrons (electrons not bound to atoms that move freely through the metal) and the applied field. This condition results in a shift of the resonance frequency from the value observed for the same nucleus when it is present in an insulator. These so-called metallic shifts provide important information on the magnetic susceptibility, the quantum mechanical wave functions that describe energy states, and the density of states of conduction electrons in the metal. In superconductors, the shape of the NMR spectral peaks provide detailed information on the penetration and internal distribution of the magnetic field. In ferromagnets or antiferromagnets (crystals in which not all electrons are paired), the NMR is influenced by the internal magnetic fields produced by the array of ordered electronic spins. In ferromagnets the shift is a measure of the lattice magnetization; in an antiferromagnet there are at least two shifts that give the magnetization of each antiferromagnetic sub lattice separately, a result unattainable by conventional magnetic measurements. For certain nuclei, the NMR spectrum reveals the existence of nuclear electric quadrupole moments (an electric quadrupole consists of a charge distribution equivalent to a special arrangement of two electric dipoles) that interact with the electric fields that exist at the nuclear sites. These interactions provide information on the microscopic distribution of electric charge around the nucleus. The most important consequence of the extraordinary sharpness of nuclear magnetic resonance (NMR) lines in liquids is the possibility of measuring the chemical shifts--that is, the separations between NMR lines from nuclear spins of the same species but in different molecular environments. The physical origin of chemical shifts is the following: an external magnetic field polarizes the closed electron shells of the atoms and produces a small magnetic field, proportional to the external field, which shifts the NMR line with respect to its position for the bare nucleus--e.g., one that is devoid of electrons. The bare nucleus itself is never observed, but the atomic diamagnetic shifts that correspond to atoms located in different molecular sites are slightly different, and it is their differences that produce the chemical shifts. As an example, the proton NMR spectrum of ethyl alcohol exhibits three peaks, with relative weights or intensities of 3:2:1. In more complicated molecules such spectra contain much chemical information and can help in the determination of unknown molecular structures. The multiplicity of lines is further increased by the interaction between nuclear spins. As already mentioned in connection with motion narrowing in liquids, the usual magnetic dipolar interactions are averaged out by molecular motion and do not split the NMR spectra. There exists, however, an indirect interaction between nuclear spins, caused by the electrons, that splits the resonance line of a specific nuclear spin into many components. High resolution nuclear magnetic resonance has become one of the most prized tools in the fields of organic chemistry and biochemistry. On the experimental side, the requirements to be met by the equipment are severe. In order to match natural line widths of a fraction of a cycle, the applied magnetic fields must have a relative stability and homogeneity throughout the sample better than one part in 10. Special magnets that give uniform fields and are stabilized, devices that twirl samples in order to smooth out the magnetic inhomogeneity, and sophisticated radio- frequency detection equipment are commercially available. The trend toward higher fields (over 100 kilogauss), resulting from super conducting solenoids, improves the resolution by increasing the chemical shift splittings and the signal-to-noise ratio.

    The measurement of the precession frequency of proton spins in a magnetic field can give the value of the field with high accuracy and is widely used for that purpose. In low fields, such as the Earth's magnetic field, the NMR signal is expected to be weak because the nuclear magnetization is small, but special devices can enhance the signal 100 or 1000 times. Incorporated in existing portable magnetometers, these devices make them capable of measuring fields to an absolute accuracy of about one part in 1,000,000 and detecting field variations of about 10 gauss. Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful whenever a phenomenon is linked with variations of magnetic field in space or in time, such as anomalies arising from submarines, skiers buried under snow, archaeological remains, and mineral deposits

    Fluxgate magnetometer A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily magnetised in alignment with that field and less easily magnetised in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field.

    Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospection. In Britain the commonest such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).

    Overhauser magnetometer The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This NMR effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source).

    RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's direct current magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

    The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

    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. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant.

    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. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

    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. When a cesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this stage the device can be said to be perfectly calibrated.

    Detection Given that this theoretically perfect magnetometer is now calibrated it can be exposed to the environment. It is easy to imagine that the environment is constantly emitting quanta of energy and that some of these will pass through the chamber. When they do, they may hit one of our cesium atoms and cause it to jump into a new energy state, which may in turn be one in which it can absorb a photon from our cesium emitter. If this is the case it will cause a decrease in the number of photons reaching our detector and this can be easily recorded. Scaling from this simple example to account for the vast number of energy transactions occurring within the cesium vapour, it is easy to see how the system works.

    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. However, by the application of feedback systems and an averaging of the detection rates seen in a benign environment, the instrument can be calibrated sufficiently well in a real-world environment to make it accurate and useful for detection.

    Spin-exchange-relaxation-free (SERF) atomic magnetometers At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free atomic magnetometers containing potassium vapor operate similarly to the cesium magnetometers described above yet can reach sensitivities lower than 1 fT/√Hz . Large volume detectors have achieved 200 aT/√Hz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.

    SQUID magnetometer SQUIDs, or Superconducting Quantum Interference Devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.

    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. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).

    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". It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer) . It consisted of a permanent bar magnet suspended horizontally from a gold fibre . A magnetometer is also called a gaussmeter.

    See also

    External links



    Magnetometer - Wikipedia, the free encyclopedia
    A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.

    Definition: magnetometer from Online Medical Dictionary
    The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

    Magnetometer - Measuring the magnetic field of Earth
    Join the Magnetometer-Group now! Click Here! Discussions about Magnetometers, how to build one, recording magnetic field disturbances of Earth's magnetic field and much more..

    magnetometer - Hutchinson encyclopedia article about magnetometer
    Device for measuring the intensity and orientation of the magnetic field of a particular rock or of a certain area. In geology, magnetometers are used to determine the original ...

    magnetometer - definition of magnetometer by the Free Online ...
    mag·ne·tom·e·ter   (m g n-t m-t r) n. An instrument for measuring the magnitude and direction of a magnetic field. mag ne·to·met ric (-t-m t r k) adj. mag ne·tom e·try n.

    magnetometer - What does MAG stand for? Acronyms and abbreviations by ...
    Acronym Definition; MAG: Metal Active Gas (welding) MAG: Magazine: MAG: Magnetic: MAG: Magnitude (astronomy) MAG: Magnesium: MAG: Magician (Everquest) MAG: MagLite (Flashlight)

    Fluxgate Magnetometer
    Projects built by Richard ... Fluxgate Magnetometer. This was a 'research project' based closely on the renowned September 1991 Wireless World (WW) article written by Dr.

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    Unexploded ordnance When developing new or existing building sites, it can be necessary to survey the site for unexploded bombs (UXB’s). This depends on the site's location and ...

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    WELCOME TO THE SPACE MAGNETOMETER LABORATORY ... Welcome to the Space Magnetometer Laboratory. The Magnetometer lab is part of the  Space and Atmospheric Physics Group  at ...

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    The Magnetometer. Adapted from a TOPS Terra Bagga activity. Type of Lesson: Hands-on activity . Time Needed: 25 minutes . National Standards Addressed

     

    Magnetometer



     
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