Critical Velocities in SuperfluidHe3

The oscillatory flow of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ through an 18-μm-diam orifice has been studied. A clearly defined critical velocity is seen in both the $A$ and $B$ phases. For velocities greater than the critical value, the superflow involves an excess dissipation. The temperature dependence of the critical velocity is reported for temperatures near the superfluid transition ($\frac{T}{T_c}>0.9\mathrm{}$).