On the Nonrelativistic Quantum-Mechanical Hamiltonian with $1/c^2$ terms. Transverse current-current interaction.

We extend the standard solid-state quantum mechanical Hamiltonian containing only Coulomb interactions between the charged particles by inclusion of $1/c^2$ terms representing (transverse) current-current interaction. For its derivation we use the classical formulation of Landau-Lifshitz, however consequently in the Coulomb gauge. Our Hamiltonian does not coincide with the Darwin Hamiltonian and we emphasize the mathematical inconsistency in its derivation. We show, that the quantized version of our Hamiltonian is equivalent to the non-relativistic QED considering only states without photons and retaining only terms of order $1/c^2$. The importance of this extended Hamiltonian lies in the possibility to distinguish external from internal magnetic fields. This aspect may be relevant for theories of the Meissner effect.