The current density

(in MKS), where is the permittivity of free space and E is the electric field, having units of amps per square meter, and arising in Maxwell's generalization of Ampère's law.

where B is the magnetic field, is the permeability of free space,and J is the physical current density.

The term is an important component of the Maxwell equations, and represents magnetic effects caused by varying electric fields that were unknown at Ampère's time. In a way, is a fictitious quantity, since it does not correspond to an actual current, but inclusion of this term is mandatory to successfully explain observed physical phenomena accompanying time-varying electric fields.