This dissertation presents multiple results of our experiments with ring-shaped

23Na Bose-Einstein condensates. First, we measure the effect of temperature on the

lifetime of quantized, persistent currents. We find that the persistent current lifetime

decreases when the temperature is increased. We also extract the critical velocity by

measuring the size of hysteresis loops. The critical velocity is found to be a strong

function of temperature. Second, we implement a new technique of measuring the

circulation state of a persistent current in-situ, which is minimally-destructive. This

technique uses the Doppler effect. We then measure the current-phase relation of a

weak link inserted in the ring-shaped superfluid and measure the excitation spectrum

of the ring-shaped superfluid. Finally, we study the dynamics of rapidly expanding

rings, and explore the analogy between our experimental system and the expansion

of the universe.