  It was shown by F. Low in the 1950s that the subleading terms of soft photon
S-matrix elements obey a universal linear relation. In this paper we give a new
interpretation to this old relation, for the case of massless QED, as an
infinitesimal symmetry of the S-matrix. The symmetry is shown to be locally
generated by a vector field on the conformal sphere at null infinity. Explicit
expressions are constructed for the associated charges as integrals over null
infinity and shown to generate the symmetry. These charges are local
generalizations of electric and magnetic dipole charges.
