On the relationship between the deuteron form factors and the physical $S$-matrix. I

A relativistic parametrlzatlon is obtained for the matrix element of the electromagnetic current of the neutron-proton system. Allowance is made for the spins of the nucleons, the partial $s$ and $d$ waves of the realtive motion, and the relativistic spin rotation. The two-particle form factors of the free current are expressed in terms of the single-particle form factors. Because of the relativistic spin rotation the charge form factor of the $n$-psystem contains an admixture of the magnetic form factors of the neutron and proton. The results obtained are needed to calculate the electromagnetic form factor of the deuteron with exact allowance for the effects of the neutron-proton interaction.