Function introduction

　　 Helmholtz coil, the uniform area is large, the use space is wide, and the operation is simple. It can realize one-dimensional, two-dimensional, and three-dimensional combined magnetic fields, and can provide AC and DC magnetic fields. The current has a good linear relationship with the magnetic field. It is suitable for various research institutes, universities and enterprises to do material magnetism or testing experiments, applied to materials, electronics, biology, medical treatment, aerospace, chemistry, applied physics and other disciplines. Its main purpose: to generate standard magnetic fields; Cancellation and compensation, geomagnetic environment simulation, determination of magnetic shielding effect, electromagnetic interference simulation experiment, calibration of Hall probes and various magnetometers, research on biological magnetic field and magnetic properties of materials.

　　The Helmholtz coil is composed of a pair of identical circular conductor coils. The central axes of the two circular coils with a radius of R are coaxial with the Z axis. The distance between two circular coils is h=R. The number of turns of the two circular coils is the same, and each conductor coil carries a current I in the same direction, and the current value has a good linear relationship with the value of the magnetic field.

　　As shown in the figure: the distribution of the magnetic field lines in the section at the center of the Helmholtz coil. The magnetic field between the two coils is approximately uniform (in this figure, the central axis of the coil is longitudinal).

　　The contour plot shows the magnitude of the magnetic field in the Helmholtz coil. In the central octagonal area, the difference between the magnetic field value and the magnetic field value H0 at the center position is no more than 1%. The magnetic field values of the five contours are 0.5H0, 0.8H0, 0.9H0, 0.99H0, respectively.

Basic magnetic terms

　　 magnetic field

　　 (the definition of the International Electrotechnical Commission IEC) The components of the electromagnetic field are characterized by the magnetic field strength H and the magnetic flux density B. (Chinese national standard definition) The magnetic field is a kind of field whose characteristics can be determined by the force of the charged particles moving in the field. This force comes from the movement of the particle and the charge it carries.

  Hysteresis loop

　　The ferromagnet is magnetized repeatedly from the forward direction to the reverse direction, and then to the forward direction until the technology is saturated for one week. The obtained closed relationship curve of B and H is called the hysteresis loop.

　　Remanence Br, UoMr or 4πMr

　　 After the permanent magnet is magnetized to technological saturation and the external magnetic field is removed, the remaining Mr, UoMr or 4πMr or Br are called residual magnetization, residual intrinsic magnetic induction and residual magnetic induction, and they are collectively referred to as residual magnetization.

  Coercivity

Hcb, Hcj reduce the B (magnetic induction intensity) of the permanent magnet that is magnetized to technical saturation to zero. The reverse magnetic field strength is called magnetic coercivity. Similarly, the intrinsic magnetic induction intensity UoM or Mr is reduced to zero. The required strength of the reverse magnetic field is called the intrinsic coercivity.

　　Maximum magnetic energy product

(BH)max The product of B and H at any point on the demagnetization curve, namely Bm, Hm and (BH) represents the magnetic energy density established by the magnet in the air gap space, that is, the magnetostatic energy per unit volume of the air gap. The energy is equal to the product of the magnet Bm and Hm, so it is called the magnetic energy product. The relationship curve of the magnetic energy product changing with B is called the magnetic energy curve, and the product of Bd and Hd corresponding to a point has the maximum value, which is called the maximum magnetic energy product.

　　 bending point Hk

　　The magnetic field corresponding to the point Bi=0.9Br on the intrinsic demagnetization curve is usually called the bending point magnetic field Hk. The larger the Hk, the better the squareness of the intrinsic demagnetization curve.

　　Remanence temperature coefficient (αBr)

　　 The ratio of the percentage of reversible change in residual magnetic induction to the degree of temperature change when the temperature changes within a certain range is called the residual magnetic temperature coefficient.

　　Temperature coefficient of magnetization and coercive force (βHcj)

　　The ratio of the percentage of the reversible change of the coercivity of the magnetization to the degree of temperature change when the temperature changes within a certain range.

　　Curie temperature

　　Tc The temperature at which the spontaneous magnetization disappears.