  A slab in parallel field is submitted to a step-like magnetic-field
excitation, of amplitude 1--10 G, in the same direction as the applied field.
The slab has been rolled or spark cut, without special care, from a
conventional superconductor ingot (lead-indium alloy), and its critical
currents have standard values. The transitory electric field, induced on a face
of the slab, has been measured. Thus, we obtain information about the magnetic
flux penetrating and vortices entering the sample through this face. The only
observed deviation from the ideal response is that magnetic-field diffusion
being limited by surface screening, associated with a superficial
critical-current density. Otherwise, the vortex array inside the slab responds
exactly as if bulk pinning were ineffective.
