Which effect co-discovered by Einstein demonstrates the link between magnetism and rotation?

The Einstein–de Haas effect is the phenomenon co-discovered by Albert Einstein and Wander Johannes de Haas that links magnetism with rotation. When the magnetization of a freely suspended ferromagnetic body changes, the body rotates because angular momentum must be conserved.

In the classic experiment, a magnetic cylinder hangs from a thin torsion wire inside a coil. Changing the coil’s current changes the cylinder’s magnetization. The associated change in the angular momentum of electrons produces a compensating mechanical rotation of the cylinder, which can be detected through the wire’s twist.

Owen Willans Richardson predicted the effect in 1908, and Einstein and de Haas reported experimental work in 1915. Their results helped establish a connection between magnetic moments and angular momentum, an idea that later became especially important after the discovery of electron spin.

The effect is often confused with the Barnett effect, which is the reverse relationship: rotating a body can magnetize it. The Hall effect concerns voltage generated across a current-carrying conductor in a magnetic field, while the Zeeman effect concerns the splitting of spectral lines.