THE VELOCITY OF 2ND SOUND NEAR THE LAMBDA-TRANSITION IN SUPERFLUID HE-4

We report new high precision data on the velocity of second sound clos e to the lambda transition of He-4 at the saturated vapor pressure. Th e second sound wave is detected by a powdered salt pill of copper ammo nium bromide in direct contact with the liquid helium. With a thermome ter noise of less-than-or-equal-to 10(-10)K/square-rootHz we are able to make measurements in the linear regime to within 20 nanokelvin of t he lambda point. We have obtained reasonable agreement with renormaliz ation group theory predictions after allowance for the effect of gravi ty. The results are also compared with previous experiments.