Fountain Pressure Measurements in Liquid He II

Abstract

New observations of the fountain pressure ${P}_{f}$ are described for slits of width 0.28 \ensuremath{\mu} and 3.36 \ensuremath{\mu} and for temperature differences up to 1.0\ifmmode^\circ\else\textdegree\fi{}K. The measuring technique employs a commercially available strain gauge pressure transducer, the low-temperature characteristics of which are discussed. The results show that some of the fountain pressure phenomena previously observed by the authors and believed to be anomalous could be ascribed to experimental difficulties. In particular, in the present work no "excess ${P}_{f}$" (i.e., greater than given by the London equation) values were found and no hysteresis in ${P}_{f}$ at large heat currents in the wide slit was observed. However, an anomalous lowering of the $\ensuremath{\lambda}$ point with pressure and ideal ${P}_{f}'\mathrm{s}$ for large heat currents have been obtained as previously. The validity of associating the heat of transport ${Q}^{*}=\ensuremath{\rho}\mathrm{ST}$ in the thermomechanical effect with the calorimetric entropy is demonstrated up to 2.16\ifmmode^\circ\else\textdegree\fi{}K.