(Other system units are discussed e.g. in [Jackson75].)
is the electric field, 
is the displacement, 
the permittivity of free space and 
the polarization. 
is the magnetic induction (magnetic flux density), 
the magnetic field, 
the permeability of free space and 
the magnetization. 
is the density of electric charge and 
is the current density. The relations between and 
, and between and 
, are called constitutive equations; they describe the medium. In a linear, isotropic medium 
and 
, where 
and 
are constants. In general (or ) is not even a unique function of (or ), but depends upon the earlier time evolution (hysteresis).
where c is the speed of light in vacuum and by definition
The continuity equation
follows from Maxwell's equations and expresses the conservation of electric charge.
Defining the scalar potential 
and vector potential 
by
explicitly solves half of Maxwell's equations. The potentials are not unique, since any gauge transformation
leaves the physical fields and unchanged, 
being an arbitrary function.
Other types of potentials may be useful in certain cases (e.g. a scalar potential for ).