High-resolution second-sound velocity measurements near theλpoint of helium

We report measurements of the second-sound velocity in superfluid helium in the region very close to the {λ} 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±{}0.0005 for the exponent characterizing the divergence of the superfluid density at the transition.