  In this paper I compare the Abraham and the Minkowski forms for the momentum
pertaining to an electromagnetic wave inside a dielectric or a magnetic
material. The discussion is based on a careful treatment of the surface charges
and currents and of the forces acting on them. While in the dielectric case the
Abraham momentum is certainly more appealing from the physical point of view,
for a magnetic material it suggests an interpretation in terms of magnetic
charges and re- lated magnetic currents. The Minkowski momentum for magnetic
non conducting materials, on the contrary, has a natural interpretation in
terms of an amperian model, in which the dynamics is determined by the Lorentz
force acting on bulk and surface electric currents.
