The Barnett effect is the magnetization of an uncharged body when spun on its axis.^[1] It was discovered by American physicist Samuel Barnett in 1915.^[2]

An uncharged object rotating with angular velocity ω tends to spontaneously magnetize, with a magnetization given by

${\displaystyle M=\chi \omega /\gamma ,}$

where γ is the gyromagnetic ratio for the material, χ is the magnetic susceptibility.

The magnetization occurs parallel to the axis of spin. Barnett was motivated by a prediction by Owen Richardson in 1908, later named the Einstein–de Haas effect, that magnetizing a ferromagnet can induce a mechanical rotation. He instead looked for the opposite effect, that is, that spinning a ferromagnet could change its magnetization. He established the effect with a long series of experiments between 1908 and 1915.