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 expansion 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.
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*Support received in part by CONACyT M~xico under grant I27212-E, by the U.S. National Science Foundation under grant NSF PHY94-21309 and by the U.S. Department of Energy under grants DE-FG02-87ER40328, DE-AC0376SF00098 and DE-FG03-93ER40792.