  We compute the Coulomb distortion produced by an expanding and highly charged
fireball on the spectra of low transverse momenta and mid rapidity pions
produced in central high energy heavy ion collisions. We compare to data on
Au+Au at 11.6A GeV from E866 at the BNL AGS and of Pb+Pb at 158A GeV from NA44
at the CERN SPS. We match the fireball expanison velocity with the average
transverse momentum of protons and find a best fit to the charged pion ratio
when the fireball radius is about 10 fm at freeze-out. This value is common to
both AGS and SPS data.
