  Quantum mechanics of a particle in an infinite square well under the
influence of a time-dependent electric field is reconsidered. In some gauge,
the Hamiltonian depends linearly on the momentum operator which is symmetric
but not self-adjoint when defined on a finite interval. In spite of this
symmetric part, the Hamiltonian operator is shown to be self-adjoint. This
follows from a theorem by Kato and Rellich which guarantees the stability of a
self-adjoint operator under certain symmetric perturbations. The result, which
has been assumed tacitly by other authors, is important in order to establish
the equivalence of different Hamiltonian operators related to each other by
quantum gauge transformations. Implications for the quantization procedure of a
particle in a box are pointed out.
