A Poincar\'e Covariant Current Operator for Low and High Energy Phenomena

Within front-form dynamics and in the Breit frame where initial and final three-momenta of the system are directed along the $z$ axis, Poincar\'e covariance constrains the current operator only through kinematical rotations around the $z$ axis. Therefore, in this frame the current can be taken in the one-body form. Applications to deep inelastic structure functions in an exactly solvable model and to the deuteron magnetic form factor are presented.