  We report on the realization and characterization of a magnetic microtrap for
ultra cold atoms near a straight superconducting Nb wire with circular cross
section. The trapped atoms are used to probe the magnetic field outside the
superconducting wire. The Meissner effect shortens the distance between the
trap and the wire, reduces the radial magnetic-field gradients and lowers the
trap depth. Measurements of the trap position reveal a complete exclusion of
the magnetic field from the superconducting wire for temperatures lower than
6K. As the temperature is further increased, the magnetic field partially
penetrates the superconducting wire; hence the microtrap position is shifted
towards the position expected for a normal-conducting wire.
