  In earlier papers on the loop variable approach to gauge invariant
interactions in string theory, a ``wave functional'' with some specific
properties was invoked.
  It had the purpose of converting the generalized momenta to space time
fields. In this paper we describe this object in detail and give some explicit
examples. We also work out the interacting equations of the massive mode of the
bosonic string, interacting with electromagnetism, and discuss in detail the
gauge invariance. This is naturally described in this approach as a massless
spin two field interacting with a massless spin one field in a higher
dimension. Dimensional reduction gives the massive system. We also show that in
addition to describing fields perturbatively, as is required for reproducing
the perturbative equations, the wave functional can be chosen to reproduce the
Born-Infeld equations, which are non-perturbative in field strengths. This
makes contact with the sigma model approach.
