The familiar relation between the potentials in electromagnetic theory is regarded as a consequence of the principles of angular momentum and center of mass expressed by the symmetry of the stress-energy tensor. The electromagnetic equations are derived from a Lagrangian function $L$ equal to an arbitrary function of the invariants ${E}^{2}\ensuremath{-}{H}^{2}$ and $E\ifmmode\cdot\else\textperiodcentered\fi{}H$ of the electromagnetic field and rules different from that of Schr\"odinger are given for the construction of a stress-energy-tensor.