Magnetic Flux Density
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Magnetic-flux-density sensors provide an electrical output proportional to the flux density. The most popular device for measuring the flux density is the Hall-effect sensor, in which the excitation current is held constant and the semiconductor crystal placed in the field generates voltage along the axis perpendicular to the direction of the field and current flow. This voltage is proportional to the flux density. The devices are designed as small-size probes, which can contain in their tip one, two, or three crystals for measuring the field in one, two, or three mutually orthogonal transverse directions.An inductor probe is made of an air-core inductor. The voltage induced in a coil placed in an alternating magnetic field is proportional to the measured flux density. Steady-state magnetic fields can be measured by spinning the coil in the field at a constant speed. The ac voltage developed in the coil due to crossing the lines of the field is a function of the flux density.
A nuclear magnetic resonance magnetometer is based on the effect of the sensitivity of the nuclear resonance frequency of certain substances placed in the magnetic field to the strength of the field. The transduction element consists of a small container with a substance in it such as deuterium or lithium. A coil, excited by a radio-frequency current, is wound around the container. The resonant frequency is identified by a sharp increase in the power consumed by the substance because of the characteristic energy absorption at resonance. Counting the frequency allows measurement of the flux density.
A flux-gate magnetometer, used for measuring extremely small flux densities, is composed of several saturable reactors that are excited by an ac voltage. This voltage keeps the induction in the cores of the reactors close to saturation. When the outside steady field acts against the core, the component of a second-harmonic current is produced in the reactor's circuit. It is noteworthy that this component does not manifest itself at all in the absence of the outside field. With the increase of the field intensity, the second-harmonic voltage can be detected, giving a measure of the flux density Three mutually orthogonal reactors allow measurement of the flux density along the three special axes.
Magnetic-flux-density sensors, a = one-directional-Hall-element sensor, b = three-directional-Hall-element sensor, c = inductor probe for alternating field, d = spinning-coil probe for steady-state magnetic field, e = nuclear-resonance-magnetometer sensor, f = flux-gate magnetometer sensor; B = flux density, B1, B2, and B3 = flux densities along three mutually orthogonal axes, B= = flux density of a steady-state field, B_ = flux density of an alternating field, lex= excitation current, Uex = excitation voltage, Uo, U1, U2, and U3 = output voltages, Uo (f) = output voltage with frequency depending on B, 1, 2, 3, and 4 = semiconductor crystals, 5 and 6 = coils, 7 = motor, 8 = sliding rings and brushes, 9 = capsule with substance containing lithium or hydrogen, 10 = coil around capsule, 11 = saturable reactors.
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