What are the constant magnetic field measuring instruments

For measurements of DC magnetic fields that do not vary with time. There are the following seven types of commonly used measuring instruments.

①Torque magnetometer: referred to as magnetometer. Use the force effect of a magnetic field to measure the strength of a magnetic field or the magnetization of a material.

② Fluxmeter and impact galvanometer (see galvanometer): used to measure magnetic flux and magnetic flux density in impact method (see soft magnetic material measurement). When measuring, the magnetic flux in the detection coil must be changed artificially.

③ Rotating coil magnetometer: In the constant magnetic field to be measured, place a small detection coil and make it rotate at a constant speed. By measuring the electromotive force of the coil, the magnetic flux density or magnetic field strength can be calculated. The measurement range is 0.1 mT to 10 tex. The error is 0.1 to 1%. The detection coil can also be turned over suddenly or quickly moved to the no-field area, and the magnetic flux density can be measured according to the principle of the impact method.

④ Fluxgate magnetometer: The iron core made of soft magnetic material with high magnetic permeability is subjected to alternating and constant magnetic fields at the same time. Due to the nonlinearity of the magnetization curve and the iron core working in the asymmetric region of the curve, the winding is The voltage induced by the detection coil on the iron core contains even harmonic components, especially the second harmonic. This harmonic voltage is proportional to the constant magnetic field strength. By measuring the harmonic voltage of the detection coil, the magnetic field strength is calculated. The principle structure of the fluxgate magnetometer is shown in the figure. The two iron cores in the probe are made of high permeability soft magnetic alloy. An AC excitation coil is wound around each iron core, and a detection coil is wound around the two iron cores. After the two AC excitation coils are connected in series, they are powered by the oscillator, and the magnetic field strength generated in the two iron cores is H~, but in opposite directions. In this way, the fundamental and odd harmonic voltages induced in the detection coil cancel each other out. When the probe is in the measured constant magnetic field with a strength of H0, the two iron cores are respectively subjected to the superposition of H0+H~ and H0-H~, that is, the alternating and constant magnetic fields, thereby generating even harmonic voltages in the detection coil. After frequency selective amplification and synchronous detection, the second harmonic voltage is taken, and its reading is proportional to the measured constant magnetic field strength H0. Fluxgate magnetometers are highly sensitive, with a resolution of 100 pett. Mainly used to measure weak magnetic fields. Widely used in geological, marine and space technology. The optical pump magnetometer developed in the 1960s and 1970s and the superconducting quantum magnetometer made of superconducting quantum interference devices (squid) have higher sensitivity and resolutions of 10-7 and 10-9 amperes, respectively. /Meter.

⑤ Hall effect magnetometer: The rectangular semiconductor sheet is placed in a magnetic field perpendicular to the plane of the sheet (the magnetic flux density is B). An electromotive force E is generated between them (ie, the Hall effect). When I is constant, E is proportional to B, and the proportionality factor is related to the width b, length l and thickness d of the sheet and the material used. This property of the material is also known as the magneto-sensitive property. A magnetometer made of the Hall effect can measure the magnetic flux density value in the range of 1 microTer to 10 Tera. The error is 0.1 to 5%. The Hall sheet can be made thin and small, and can be extended into narrow gaps for measurement, and can also be used to measure non-uniform magnetic fields. Devices with magneto-sensitive properties include bismuth spirals and magneto-sensitive diodes in addition to Hall chips.

⑥ NMR magnetometer: The magnetic moment of the atomic nucleus will precess around the direction of the magnetic field under the action of the magnetic flux density B, and its precession frequency ƒ0=γB (γ is the gyromagnetic ratio, which is a constant for a certain substance ), if a small alternating magnetic field is applied in the direction perpendicular to B, when its frequency is equal to ƒ0, a resonance absorption phenomenon will occur, that is, nuclear magnetic resonance. The magnetic flux density or magnetic field strength can be accurately calculated from the resonance frequency. The measurement range of this magnetometer is 0.1 mT to 10 T. The accuracy is very high, and the error is less than 10-4 to 10-5. It is often used to provide a standard magnetic field and as a calibration standard.

⑦Magnetic Potentiometer: It is used to measure the magnetic potential difference between two points in space a and b. If it is a uniform magnetic field, the magnetic field strength at that place can be calculated. Magnetic potentiometers can also be used to measure the strength of the magnetic field inside a material. Since the tangent components of the magnetic field strength at the interface of the magnetic material are equal, the magnetic field strength measured by the magnetometer at the space along the surface of the material is the tangent component of the magnetic field strength inside the material. The structure of the magnetic potentiometer is to evenly wind the thin insulated wire on the non-magnetic soft tape or hard sheet. The former is called a soft magnetic potentiometer; the latter is called a hard magnetic potentiometer. The measuring instrument adopts an impact galvanometer or a flux meter. For a constant magnetic field, the magnetic flux linked by the magnetic potentiometer must change during the measurement. If the measured is a uniform magnetic field, the magnetic field strength is converted from the magnetic potential difference. The magnetic potentiometer can be calibrated in a standard uniform magnetic field, and is scaled according to the magnetic field strength value.