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Current conservation and interaction currents with relativistic separable interactions.

A method of minimal substitution is used to obtain the electromagnetic two-body interaction current operator associated with a relativistic separable interaction. The result is shown to satisfy a general restriction obtained from current conservation using the Ward-Takahashi identity within the Bethe-Salpeter formalism. It is shown that this two-body relativistic current operator can be reduced to an effective one-body form. Applications of this method to nuclear few-body systems, relativistic quark systems, and hadronic resonance processes are discussed. The numerical size of the interaction current for the pion charge form factor is estimated.