  For a particle in a box, the operator $- i \partial_x$ is not Hermitean. We
provide an alternative construction of a momentum operator $p = p_R + i p_I$,
which has a Hermitean component $p_R$ that can be extended to a self-adjoint
operator, as well as an anti-Hermitean component $i p_I$. This leads to a
description of momentum measurements performed on a particle that is strictly
limited to the interior of a box.
