# What quantity does magnetic measurement mainly refer to?

Magnetic measurement mainly refers to the measurement of magnetic field intensity, magnetic induction intensity, magnetic flux, permeability, hysteresis loss and eddy current loss of medium and other basic magnetic parameters of medium.
The magnetic flux density in space is proportional to the magnetic field intensity. The measurement of the magnetic field intensity in space is also the measurement of the magnetic flux density. Therefore, the magnetic flux density is actually measured by magnetometer. Magnetic field measurement is mainly carried out by magnetic measuring instruments. According to the nature of the measured magnetic field, magnetic field measurement is divided into constant magnetic field measurement and variable magnetic field measurement.
Constant magnetic field measurement
For the measurement of DC magnetic field which does not change with time. There are seven common measuring instruments.

• ① Torque magnetometer: magnetometer for short. The force effect of magnetic field is used to measure the intensity of magnetic field or the magnetization of material.
• ② Fluxmeter and impact galvanometer (see galvanometer): used to measure magnetic flux and flux density in impact method. During the measurement, 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 uniform 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 Mt. The error is 0.1-1%. The magnetic flux density can also be measured according to the principle of impact method.
• ④ Fluxgate magnetometer: the core made of high permeability soft magnetic material is subject to both alternating and constant magnetic fields. Due to the nonlinearity of magnetization curve and the core working in the asymmetric region of the curve, the voltage induced by the detection coil wound on the core contains even harmonics, especially the second harmonics. The harmonic voltage is proportional to the constant magnetic field strength. By measuring the harmonic voltage of the detection coil, the magnetic field intensity is calculated. The principle structure of fluxgate magnetometer is shown in the figure. The two cores in the probe are made of high permeability soft magnetic alloy. Each core is wound with AC excitation coil, and the detection coil is wound on two cores. After two AC excitation coils are connected in series, they are powered by the oscillator. The magnetic field intensity generated in the two cores is H ~, but the direction is opposite. In this way, the fundamental and odd harmonic voltages induced in the detection coil cancel each other. When the probe is in the constant magnetic field with the strength of H0, the two cores are subjected to the superposition of H0 + H ~ and h0-h ~ respectively, that is, the alternating and constant magnetic fields, so as to generate even harmonic voltage in the detection coil. The second harmonic voltage is taken through the frequency selection amplification and synchronous detection link, and its reading is proportional to the constant magnetic field strength H0. The sensitivity of fluxgate magnetometer is very high and the resolution is up to 100%. It is mainly used to measure weak magnetic field. It is widely used in geology, ocean and space technology. The optical pump magnetometer developed in 1960s and 1970s and the superconducting quantum magnetometer based on squid have higher sensitivity and resolution of 10-7 and 10-9 A / m, respectively.
• ⑤ Hall effect magnetometer: a semiconductor rectangular sheet is placed in a magnetic field perpendicular to the plane of the sheet (magnetic flux density is b). If DC current I is applied between the two opposite ends of the sheet, electromotive force E (Hall effect) will be generated between the corresponding points of the other two ends. When I is a constant, e is proportional to B. the proportional coefficient is related to the width b, length L, thickness D and the material used. This property of material is also called magnetic sensitive property. The magnetometer made of Hall effect can measure the magnetic flux density in the range of 1 μ t to 10 t. The error is 0.1-5%. Hall plate can be made thin and small. It can be used to measure in narrow gap or non-uniform magnetic field. In addition to Hall plate, there are bismuth helix, magnetic sensitive diode and so on.
• ⑥ Nuclear magnetic resonance magnetometer: under the effect of magnetic flux density B, the magnetic moment of the nucleus will precession around the magnetic field direction, and its precession frequency ƒ 0 = γ B (γ is the rotation magnetic ratio, which is a constant for certain substances). If a small alternating magnetic field is applied in the direction perpendicular to B, when its frequency is equal to ƒ 0, the resonance absorption phenomenon will occur, namely nuclear magnetic resonance. The magnetic flux density or magnetic field intensity can be calculated accurately 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 ~ 10-5. It is often used to provide the standard magnetic field and as the calibration standard.
• ⑦ Magnetic potential meter: it is used to measure the magnetic potential difference between two points a and B in space. If it is a uniform magnetic field, the magnetic field intensity can be calculated. The magnetometer can also be used to measure the magnetic field strength inside the material. Because the tangent components of the magnetic field intensity at the interface of the magnetic material are equal, the magnetic field intensity measured by the magnetometer along the surface space of the material is the tangent component of the magnetic field intensity inside the material. The structure of magnetic potential meter is that the thin insulated wire is evenly wound on the non-magnetic soft tape or hard piece, the former is called soft magnetic potential meter, the latter is called hard magnetic potential meter. The measuring instrument is impact galvanometer or fluxmeter. For a constant magnetic field, the flux of the magnetometer must be changed during the measurement. If the measured magnetic field is uniform, the magnetic field intensity is converted from the magnetic potential difference. The magnetometer can be calibrated in the standard uniform magnetic field and calibrated according to the magnetic field intensity.

## Measurement of variable magnetic field

For the measurement of alternating magnetic field changing with time. Usually, the electromagnetic induction effect is used to convert the magnetic quantity of magnetic field into electromotive force. Taking the alternating magnetic field with periodic monotonous rise and fall as an example, when measuring the magnetic flux density, it is only necessary to connect the detection coil to the average value voltmeter. From the reading of the voltmeter, the maximum magnetic flux density BM,, is the frequency, s is the effective cross section of the core, and N2 is the number of turns of the measuring coil. The magnetic flux density can be directly measured by Hall plate. If I is kept as DC, the waveform of output electromotive force E is the same as that of magnetic flux density. The magnetic flux density can be calculated from E. When measuring the magnetic field strength, if the average value voltmeter is used as the measuring instrument of the magnetometer, the maximum value of the magnetic field strength can be converted according to the reading of the voltmeter, and it can also be calibrated in the uniform standard magnetic field.

## Other magnetic field measurements

• 1. Measurement of magnetic induction B with ampere force formula F = bil.
• 2Using induced electromotive force to measure magnetic induction B.
• 3The magnetic induction B is measured by the relationship between the electric quantity of the loop and the magnetic induction when the induced electromotive force is generated.

Helmholtz coil: two identical coils are parallel to each other and connected with current (I) in the same direction. The theoretical calculation shows that when the coil spacing a is equal to the coil radius r, the combined magnetic field of the two coils is parallel to Canada in a large range near the axis (the connecting line between the centers of the two coils). This kind of magnetic field is widely used.

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