Field Intensity, Magnetic

a physical vector quantity (H) that is a quantitative characteristic of a magnetic field. The magnetic field intensity is independent of the magnetic properties of the medium and in free space is the same as the magnetic induction B; numerically, H = B in the centimeter-gram-second (cgs) system of units and H = B/μ₀ in the International System of Units (SI), where μ₀ is the magnetic constant. In a medium, the magnetic field intensity H determines the contribution to the magnetic induction B made by external field sources: H = B —4πJ (in the cgs system) or H = (B₀/μ₀) —J (in SI units), where J is the intensity of magnetization of the medium. If we introduce the relative magnetic permeability of the medium μ, then for an isotropic medium we have H = B/(μ₀ μ) (in SI units). The units of magnetic field intensity in the SI system are amperes per meter (A/m); in the cgs system this unit is called an oersted; 1 A/m = 4π . 10^-3 oersted ≃ 1.256 • 10^-2 oersted.

The magnetic field intensity of a rectilinear conductor carrying a current I (in SI units) is H = μ₀I/2πa, where a is the distance from the conductor. In the center of a ring current, H = μ₀I/2R, where R is the radius of a loop carrying the current I. Along the axis of a solenoid, H = μ₀ni, where n is the number of turns per unit length of the solenoid. The practical determination of H in ferromagnetic media (in magnetic materials) is based on the constancy of the tangential component of H during a transition from one medium to another. If the intensity of magnetization of a body is homogeneous, the field intensity measured on the parts of its surface that are parallel to the magnetization vector will correspond to the field intensity within the body.