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Pyotr Leonidovich Kapitza's 1938 test compared helium flow through optically flat glass disks. Suspension set the tube-to-bath level difference; a cathetometer read levels. The gap was estimated as $0.5\mu \mathrm{m}$. Levels barely moved over minutes above the transition, but equalized in seconds below. A laminar flow viscosity calculation gave only an upper bound.^[ref]

The essential control is to compare the same restriction and level readout at different temperatures. A changed pressure head is not itself evidence for a changed material response: the driving head must be included when interpreting the flow. For an idealized comparison at equal vapor pressure, negligible surface tension corrections and nearly constant mass density $\rho$, hydrostatic equilibrium givesHere $\Delta h$ is the difference in free-surface heights and $g$ is gravitational acceleration. This explains why following a liquid level provides both a driving pressure and a transported volume; it does not turn a finite-resolution observation into proof of zero viscosity.

Pyotr Kapitsa measured extraordinarily small flow resistance in liquid helium below its lambda transition. Confining the flow to narrow passages made the failure of ordinary viscous-flow expectations experimentally striking. The observation helped establish superfluidity as a distinct quantum-fluid phenomenon. In a modern two-component interpretation, the low-resistance response does not mean that every component of the liquid has lost all viscous behavior; it motivates separating normal and superfluid contributions.^[ref]

A physicist.^[ref]