In a uniform magnetic field of unit intensity formed in empty space the induction or magnetic flux across an area of I square centimetre normal to the direction of the field is arbitrarily taken as the unit of induction.
Hence if the induction per square centimetre at any point is denoted by B, then in empty space B is numerically equal to H; moreover in isotropic media both have the same direction, and for these reasons it is often said that in empty space (and practically in air and other nonmagnetic substances) B and H are identical.
The magnetic flux per square centimetre at any point (B, B, or 0) is briefly called the induction, or, especially by electrical engineers, the flux-density.
The induction may therefore be specified as B lines per square centimetre.
The total magnetic induction or flux corresponds to the current of electricity (practically measured in amperes); the induction or flux density B to the density of the current (number of amperes to the square centimetre of section); the magnetic permeability to the specific electric conductivity; and the line integral of the magnetic force, sometimes called the magnetomotive force, to the electro-motive force in the circuit.
