  Using the Covariant Spectator Theory (CST), one boson exchange (OBE) models
have been found that give precision fits to low energy np scattering and the
deuteron binding energy. The boson-nucleon vertices used in these models
contain a momentum dependence that requires a new class of interaction currents
for use with electromagnetic interactions. Current conservation requires that
these new interaction currents satisfy a two-body Ward-Takahashi identity, and
using principals of simplicity and picture independence, these currents can be
uniquely determined. The results lead to general formulae for a two-body
current that can be expressed in terms of relativistic np wave functions, Psi,
and two convenient truncated wave functions, ${\it \Psi}^{(2)}$ and $\widehat
{\it \Psi}$, which contain all of the information needed for the explicit
evaluation of the contributions from the interaction current. These three wave
functions can be calculated from the CST bound or scattering state equations
(and their off-shell extrapolations). A companion paper uses this formalism to
evaluate the deuteron magnetic moment.
