  The raising of both indices in the components of the Minkowski
electromagnetic field strength 2-form to give the components of the
electromagnetic excitation bivector field can be regarded as being equivalent
to an electromagnetic constitutive law, as well as being defined by the
components of the spacetime metric. This notion is clarified, and the nature of
the equivalent dielectric tensors and magnetic permeability tensors that are
defined by some common spacetime metrics is discussed. The relationship of the
basic construction to effective metrics is discussed, and, in particular, the
fact that this effective metric is more general than the Gordon metric.
