The figure illustrates a two-component medium. Consider the cross-hatched volume of conductivity , take it as a sphere of volume  and assume it is embedded in a uniform medium with an effective conductivity . If the electric field far from the inclusion is  then elementary considerations lead to a dipole moment associated with the volume
This polarization produces a deviation from . If the average deviation is to vanish, the total polarization summed over the two types of inclusion must vanish. Thus
where  and  are respectively the volume fraction of material 1 and 2. This can be easily extended to a system of dimension  that has an arbitrary number of components. All cases can be combined to yield Eq. (1).
Eq. (1) can also be obtained by requiring the deviation in current to vanish.
 It has been derived here from the assumption that the inclusions are spherical and it can be modified for shapes with other depolarization factors; leading to Eq. (2).
A more general derivation applicable to bianisotropic materials is also available.