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Elephter Andronikashvili tested the two-fluid model of superfluidity by oscillating a stack of closely spaced disks in liquid helium-4. Viscous coupling makes the normal component follow the disks, adding to their effective moment of inertia; the superfluid component need not rotate with them. Measuring the oscillation period as temperature changes therefore estimates the normal-fluid fraction under the apparatus's entrainment conditions. The experiment turns an apparently unusual flow law into a mechanical measurement of two distinct contributions to one liquid.^[ref]