  We describe an experiment in superconductivity suitable for an advanced
undergraduate laboratory. Point-contact spectroscopy is performed by measuring
the differential conductance between an electrochemically etched gold tip and a
100-nm superconducting niobium film with a transition temperature Tc ~= 7 K. By
fitting the results to Blonder-Tinkham-Klapwijk theory using a finite lifetime
of quasiparticles, we obtain a superconducting gap energy \delta ~= 1.53 meV, a
lower bound to the Fermi velocity vF >= 3.1 x 10^-7 cm/s, and a BCS coherence
length \zeta ~= 43 nm for niobium. These results are in good agreement with
previous measurements.
