High-resolution second-sound velocity measurements near the<i>λ</i>point of helium

Abstract

We report measurements of the second-sound velocity in superfluid helium in the region very close to the \ensuremath{\lambda} point. The sound was detected using a powdered-paramagnetic-salt thermometer with a superconducting readout system. Large gravitational and power effects were observed near the transition. The corrected data yield the value 0.6740\ifmmode\pm\else\textpm\fi{}0.0005 for the exponent characterizing the divergence of the superfluid density at the transition.