  The subtle differences between positive and negative pion spectra can used be
used to study the nature of the nuclear interaction region in heavy-ion
collisions. Several large acceptance heavy ion experiments at facilities
ranging from SIS, the AGS, the SPS, to RHIC have measured mid-rapidity $\pi^+$
and $\pi^-$ spectra for central Au+Au or Pb+Pb collisions. From these spectra
one can create pion ratios as a function of $m_t-m_0$, which are used to
determine the Coulomb potential, $V_C$, and the initial pion ratio, $R_i$,
across a range of collision energies from 1 to 158 AGeV. The implications of
the $V_C$ and $R_i$ trends with collision energy will be discussed.
