  The quantum phase difference across a superfluid Josephson-like weak link may
exhibit periodic motion describable by a nonlinear equation identical to that
of a rigid pendulum. We have directly observed this mode and find that the
phase oscillation frequency decreases toward zero as the phase amplitude
increases toward $\pi$. We also find that the low amplitude frequency is a
direct measure of the weak link's critical current in quantitative agreement
with theory.
