  Experiment and analytical calculations show that the demagnetizing field of a
superconductor is a sensitive probe of quantities otherwise difficult to
measure, such as the sample-probe distance in flux-density imaging experiments,
and the field of first flux penetration Hp. In particular, the ratio of the
maximum field measured above the superconductor edge and the applied field can
be determined unambiguously so as to define a linear "geometric"
susceptibility. The evolution of this susceptibility with field depends on the
regime of flux penetration, and can be used as a means to determine Hp and the
effect of a parallel field component in magneto-optical imaging experiments.
