Interaction of a Magnet and a Point Charge: Unrecognized Internal   Electromagnetic Momentum Eliminates the Myth of Hidden Mechanical Momentum
  A model calculation using the Darwin Lagrangian is carried out for a magnet consisting of two current-carrying charges constrained by centripetal forces to move in a circular path in the presence of the electric field from a distant external point charge. In the limit that the magnet's two charges are non-interacting, the calculation recovers the only valid calculation for hidden mechanical momentum. However, if the magnet's charges are mutually interacting, then there is internal electromagnetic linear momentum associated with the perturbed magnet's electrostatic charge distribution and the motion of the magnet's charges. This internal electromagnetic momentum does not seem to be recognized as distinct from the familiar external electromagnetic momentum associated with the electric field of the external charge and the magnetic field of the unperturbed magnet. In the multiparticle limit, the hidden mechanical momentum becomes negligible while the internal electromagnetic momentum provides the compensating linear momentum required by the relativistic conservation law connecting the total linear momentum to motion of the center of energy. Whereas the changes in the external electromagnetic momentum are often associated with magnetic forces of order $1/c^{2},$ changes in the internal electromagnetic momentum are associated with the electrical forces of order $1/c^{2}$. These electrical forces are relevant to the Shockley-James paradox and to the experimentally observed Aharonov-Bohm and Aharonov-Casher phase shifts. 
Timothy H. Boyer
physics.class-ph quant-ph