Lower Bound to Limiting Fields in Nonlinear Electrodynamics

In view of new high-precision experiments in atomic physics it seems necessary to reexamine nonlinear theories of electrodynamics. The precise calculation of electronic and muonic atomic energies has been used to determine the possible size of the upper limit $E_{max}$ to the electric field strength, which has been assumed to be a parameter. This is opposed to Born's idea of a purely electromagnetic origin of the electron's mass which determines $E_{max}$. We find $E_{max}\ge 1.7\times {10}^{20}$ V/cm.