Onset for Superfluid Flow inHe4Films on a Variety of Substrates

The onset of superfluid flow in ${\mathrm{He}}^4$ films was measured between 1.37 and 2.13 K on substrates of NaF, Ca${\mathrm{F}}_2$, Si${\mathrm{O}}_2$, Si, and Grafoil. Values of the Van der Waals constant of 19, 26, and 65 K were obtained for NaF, Si${\mathrm{O}}_2$, and Grafoil, respectively. In addition, a precise measurement of the critical thickness of the liquid-helium film was obtained, taking into account a small correction for the influence of the substrate.