[en] A dynamical theory of torsion is proposed in which the electromagnetic potential is dispensed from minimal coupling, and a ''semiminimal'' photon-torsion coupling is introduced and physically justified. Local gauge invariance of the total Lagrangian density is preserved in its usual form, provided that torsion is determined by the gradient of a propagating potential. From the field equations we deduce that a polarized macroscopic body is a source of a dipolar torsion field, and that a test body may feel torsion only if polarized. No disagreement is found with the Eoetvoes-Dicke-Braginskii tests of the equivalence principle