Magnetic field 'magnetic field' is used to describe as well as or in place of . There are many alternative names for both (see sidebar). In SI units, is measured in teslas (symbol: T) and correspondingly (magnetic flux) is measured in webers (symbol: Wb) so that a flux density of 1 Wb/m is 1 tesla. The SI unit of tesla is equivalent to (newton·second)/(coulomb·metre). In Gaussian-cgs units, is measured in gauss (symbol: G). (The conversion is 1 T = 10000 G.) One nanotesla is equivalent to 1 gamma (symbol: γ). The -field is measured in amperes per metre (A/m) in SI units,
Spectral flux density approach defines flux density as a vector at a point of space and time prescribed by the investigator. To distinguish this approach, one might speak of the 'full spherical flux density'. In this case, nature tells the investigator what is the magnitude, direction, and sense of the flux density at the prescribed point. For the flux density vector, one may write where formula_2 denotes the spectral radiance (or specific intensity) at the point formula_3 at time formula_4 and frequency formula_5, formula_6 denotes a variable unit vector with origin at the point formula_3, formula_8 denotes an element of solid angle around