  Off-axis electron holography can measure the mean inner electric potential of
materials. The theory of hole superconductivity predicts that when a material
is cooled into the superconducting state it expels electrons from its interior
to the surface, giving rise to a mean inner potential that increases with
sample thickness. Instead, in a normal metal and in a conventional BCS
superconductor the mean inner potential is expected to be independent of sample
thickness and temperature. Thus, this experiment can provide a definitive test
of the validity of the theory of hole superconductivity.
