“Fourth sound” in helium‐three confirms superfluidity

An experiment at the University of California at La Jolla, based on the propagation of fourth sound, appears to provide the first really direct evidence for superfluidity in $He3.$ Over the past two years (PHYSICS TODAY, October 1972, page 17 and July 1973, page 17), a variety of experiments have studied the curious behavior of $He3$ below a few millidegrees Kelvin, where it undergoes two transitions, both in the liquid—“A” near 2.6 mK and “B” near 2.0 mK (at a melting pressure near 34.4 bar). These experiments measured various properties characteristic of a superfluid, ranging from nuclear magnetic resonance to viscosity.