# magnetometer

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## magnetometer

(mag-nĕ-**tom**-ĕ-ter) Any of a variety of instruments used to measure the strength and direction of a magnetic field.

## Magnetometer

an instrument for measuring the characteristics of magnetic fields and the magnetic properties of substances (magnetic materials). A distinction is made among instruments for measuring field intensity (oerstedmeters), field direction (inclinometers and declinometers), field gradient (gradiometers), magnetic induction (teslameters), magnetic flux (webermeters, or fluxmeters), coercive force (coercimeters), magnetic permeability (mu-meters), magnetic susceptibility (kappa-meters), and magnetic moment.

In a narrower sense, magnetometers are instruments for measurement of the intensity, direction, and gradient of a magnetic field. Modern magnetometers use the following methods for readout of the quantity being measured: visual readout from a scale, recording in digital or analog form, photographic recording, and recording on magnetic tape and punched tape and cards. Magnetometer scales are calibrated in units of magnetic field intensity in the cgs system of units (oersted, millioersted, microoersted, and gamma = 10^{5} oersteds) and in units of magnetic induction in the International System of Units (tesla [T], microtesla [_{μ}T], and nanotesla [nT]).

A distinction is made between magnetometers for measuring the absolute values of the field characteristics and those for measuring the relative changes of the field in space or time. The latter are called magnetic variometers. Magnetometers are also classified according to the conditions under which they are used (stationary, on mobile platforms, and so on) and, finally, by the physical phenomena that are the basis for their operation.

Magnetostatic magnetometers are based on measurements of the mechanical moment J acting on the indicator magnet of the instrument in the field H_{meas} being measured: J = [M, H_{meas}], where M is the magnetic moment of the indicator magnet. In various types of magnetometers the moment J is compared with the torsional moment of a quartz filament (quartz and universal magnetometers, which are based on this principle, have a sensitivity *G* ~ 1 nT), the gravitational force moment (magnetic balances with *G* ~ 10-15 nT), or the moment acting on an auxiliary standard magnet mounted in a fixed position (at equilibrium, the axes of the indicator and auxiliary magnets are perpendicular). In this case, determination of the period of oscillation of the auxiliary magnet in the field H_{meas} also makes possible measurement of the absolute magnitude of H_{meas} (the Gauss absolute method). Magnetostatic magnetometers are used mainly to determine the components and absolute magnitude of the geomagnetic field intensity (Figure 1) and gradient, as well as the magnetic properties of materials.

Electric magnetometers are based on the comparison of H_{meas} with the field of a reference solenoid *H = kl*, where *k* is the solenoid constant, determined by its geometric and design parameters, and I is the current being measured. Electromagnetic magnetometers consist of a comparator for measuring the dimensions of the solenoid and the winding, a theodolite for exact orientation of the axis of the solenoid in the direction of the field component being measured, a potentiometer system for measuring the current I, and a sensitive indicator of the equality of the two fields. The sensitivity of magnetometers of this type is of the order of 1 microoersted; they are used in determining the horizontal and vertical components of the geomagnetic field.

Induction magnetometers are based on the phenomenon of electromagnetic induction—that is, the generation of an electromotive force in a measuring coil when the magnetic flux Φ through the coil changes. The change in the flux ΔΦ in the coil may be related to the following factors:

(1) A change over time in the magnitude or direction of the field being measured (examples are induction variometers and fluxmeters). The simplest fluxmeter (webermeter) consists of a ballistic galvanometer operating in a strongly overdamped mode ( *G* ~ 10 ^{-4} weber per division). Magnetoelectric webermeters with *G ~* 10 ^{-6} weber per division and photoelectric webermeters with *G* ~ 10^{-8} weber division are widely used. (2) A periodic change in the position (rotation or vibration) of the measuring coil in the field being measured (Figure

(2) The simplest teslameters with a coil mounted on the shaft of a synchronous motor have *G ~* 10^{-4} T. For the most sensitive vibrational magnetometers, *G* ~ 0.1-1.0 nT.

(3) A change in the magnetic resistance of the measuring coil, which is achieved by periodic changes in the magnetic permeability of a Permalloy core (it is periodically magnetized to saturation by an auxiliary alternating exciting field). Magnetometers of the ferroprobe type, which are based on this principle, have *G* ~ 0.2-1.0 nT. Induction magnetometers are used for measuring terrestrial and cosmic magnetic fields and industrial fields, and also in magnetobiology.

Quantum magnetometers are instruments based on nuclear magnetic resonance, electron spin resonance, free precession of the magnetic moments of nuclei or electrons in an external magnetic field, and other quantum effects. Observation of the dependence of the precession frequency ω of the magnetic moments of microscopic particles on the intensity H_{meas} of the field being measured (*ω = γ* H_{meas}, where *γ* is the gyromagnetic ratio) requires generation of a macroscopic magnetic moment of an assembly of the microscopic particles (nuclei or electrons). The methods of generating the macroscopic magnetic moment and detecting the signal are the basis for distinctions among proton magnetometers (of the free precession type, with dynamic polarization, and with synchronous polarization), electron and nuclear resonance magnetometers, and magnetometers with optical pumping. Quantum magnetometers are used to measure the intensity of weak magnetic fields (including the geomagnetic field and the magnetic field in space), in geological prospecting, and in magnetochemistry (*G* up to 10^{-5}-10^{-7} nT). Quantum magnetometers for measuring strong magnetic fields have a considerably lower sensitivity.

Superconducting quantum magnetometers are based on quantum effects in superconductors: expulsion of the magnetic field from a superconductor (the Meissner effect), quantization of the magnetic flux in a superconductor, and the dependence on H_{meas} of the critical current through the contact between two superconductors (the Josephson effect). Superconducting magnetometers are capable of measuring the components of the geomagnetic field; they are used in biophysics and magnetochemistry. The sensitivity of superconducting magnetometers may be as high as -10_{-3}nT.

Galvanomagnetic magnetometers are based on the bending of the trajectories of electric charges moving in a magnetic field H_{meas} under the influence of the Lorentz forces. This group of magnetometers includes magnetometers based on the Hall effect (generation of a potential difference between the faces of a conducting plate, which is proportional to the current flow and to H_{meas}); magnetometers based on the Gauss effect (change in the resistance of a conductor in a transverse magnetic field H_{meas}); and magnetometers based on the phenomenon of a decrease of the anodic current in vacuum magnetrons and cathode-ray tubes (caused by the deflection of electrons in a magnetic field). The operation of various types of teslameters for measurement of constant, variable, and pulsating magnetic fields (with a sensitivity of 10^{-4}-10^{-5} T; Figure 3), as well as gradiometers and instruments for the study of the magnetic properties of materials, is based on the Hall effect. The sensitivity of teslameters based on the Gauss effect may be as high as 10 microvolts per tesla; the sensitivity of electrovacuum magnetometers is of the order of 30 nT.

Magnetometers whose operation is based on rotation of the plane of polarization of light in a magnetic field and on changes in the length of a rod magnetized by an applied field are used to measure the intensity and study the topology of magnetic fields in various media. Magnetometers based on a variety of principles and having a large sensitivity range are widely used in geophysics, space physics, nuclear physics, magnetochemistry, biophysics, and flaw detection and as elements of automation and control mechanisms.

### REFERENCES

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*Pribory dlia izmereniia magnitnykh velichin*. Moscow, 1969.

Pomerantsev, N. M., V. M. Ryzhkov, and G. V. Skrotskii.

*Fizicheskie osnovy kvantovoi magnitometrii*. Moscow, 1972.

“Instrumenten und Massemnethoden.” In

*Geomagnetismus undAeronomie*, vol. 2. Berlin, 1960.

“Communications présentées en colloque international des champs magnétiques faibles d’Intéret géophysique et spatial, Paris, 20-23 mai 1969.” In

*Revue de physique appliquee*, 1970, vol. 5, no. 3.

SH. SH. DOLGINOV