A SUPERFLUID ANALOGUE OF A JOSEPHSON JUNCTION has been devised by Richard Packard at UC Berkeley. One of the peculiar properties of superconductors is that the amount of magnetic flux penetrating a sample can only be a multiple of a basic flux unit. At the heart of a superconducting quantum interference device (SQUID) is a pair of insulating barriers which interrupt a ring-shaped superconducting sample. Electron pairs tunneling through the barriers interfere with each other in a way that depends on the amount of flux threading the superconducting circuit; thus the quantization of flux can be exploited to measure tiny magnetic fields. In a superfluid, by contrast, fluid circulation is quantized, and this property can be exploited to measure very tiny rotations. In the Berkeley experiment, the flow of superfluid helium through a ring-shaped vessel is interrupted by a barrier containing a micron-sized pinhole. When the vessel is rotated, the helium must squirt back through the hole to maintain its place in space (like an icecube in your drink wanting to stay where it is when you turn the glass). With this scheme the rotation of the earth can be detected to a precision of 0.5%. (Nature, 10 April 1997.)