  The classical theory of electrodynamics is built upon Maxwell's equations and
the concepts of electromagnetic field, force, energy and momentum, which are
intimately tied together by Poynting's theorem and the Lorentz force law.
Whereas Maxwell's macroscopic equations relate the electric and magnetic fields
to their material sources (i.e., charge, current, polarization and
magnetization), Poynting's theorem governs the flow of electromagnetic energy
and its exchange between fields and material media, while the Lorentz law
regulates the back-and-forth transfer of momentum between the media and the
fields. The close association of momentum with energy thus demands that the
Poynting theorem and the Lorentz law remain consistent with each other, while,
at the same time, ensuring compliance with the conservation laws of energy,
linear momentum, and angular momentum. This paper shows how a consistent
application of the aforementioned laws of electrodynamics to moving permanent
dipoles (both electric and magnetic) brings into play the rest-mass of the
dipoles. The rest mass must vary in response to external electromagnetic fields
if the overall energy of the system is to be conserved. The physical basis for
the inferred variations of the rest-mass appears to be an interference between
the internal fields of the dipoles and the externally applied fields. We use
two different formulations of the classical theory in which energy and momentum
relate differently to the fields, yet we find identical behavior for the
rest-mass in both formulations.
