Application of fourth sound to study the superfluid helium‐4 and the recently discovered superfluid helium‐3

Fourth sound is propagated in a superfluid placed in the tiny pores of tightly packed fine powder. Since it is a type of usual sound (pressure‐density) propagation, fourth sound may be excited and detected by condenser transducers. In superfluid helium‐4 fourth sound has been used (1) to determine the superfluid component density by measuring the sound velocity, and (2) to study the nature of persistent currents using Doppler‐shifted fourth sound. In superfluid helium‐3 propagation of fourth sound provided conclusive evidence of superfluidity of liquid helium below 2.6 mK. Superfluid component density and some anisotropic properties of superfluid helium‐3 have been investigated using fourth sound.