A ring-shaped channel of gallium arsenide is like a round swimming pool with no rim: electrons flow in through the inner bank and out through the outer one. In an ultraclean material, they collide with one another more often than with lattice defects, and they start to behave like a liquid. Switch on a magnetic field and it sets this flow swirling, so the resistance of the channel grows. What is more, it grows in proportion to the square of the field, and that growth tells you the viscosity of the electron liquid.

The researchers fabricated several rings of different sizes and measured the resistance at different temperatures. It turned out that the coefficient in front of the square of the field grows with the size of the ring and falls with temperature. This is exactly what theory predicts: in a larger ring the liquid has more room to show its viscosity, while heating makes the electrons collide more often and the viscosity drop.

So the ring channel has become a new tool for measuring the viscosity of the electron liquid. Previously this was done mainly with Hall bars, but there the result depended on how the electrons interact with the side walls. In a ring that source of uncertainty disappears, and only the bulk flow remains. Physicists can now test theories of electron hydrodynamics on a cleaner platform.