vector field, usually denoted by , defined as follows. The torque experienced by a magnetic dipole with magnetic dipole moment is Thus by measuring for oriented in two orthogonal directions, the magnetic induction components are obtained as torque components divided by the magnitude of . The fundamental relation linking electric field and magnetic induction to the force on a charge with velocity is the Lorentz force equation Magnetic induction is sometimes called magnetic field, a term usually applied to a different field , related to but different from it. In free space, and are proportional: where μ0, the permeability of free space, is a universal constant. is the primitive field, whereas is secondary, not strictly needed but convenient. Care must be exercised in deciding if, by magnetic field, or is meant. What is usually meant by the electric and magnetic fields (or the electromagnetic field) are and , although according to the Lorentz force equation and are the fundamental fields. Moreover, the Lorentz transformation preserves the (, ) structure but not the (, ) structure.
The magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and the resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.