The Barnett effect is the

magnetization In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Di ...

of an uncharged body when spun on its axis. It was discovered by American physicist Samuel Barnett in 1915. An uncharged object rotating with angular velocity tends to spontaneously magnetize, with a magnetization given by :

M = \chi \omega / \gamma,

where is the

gyromagnetic ratio In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol , gamma. Its SI u ...

for the material, is the

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

. The magnetization occurs parallel to the axis of spin. Barnett was motivated by a prediction by

Owen Richardson Sir Owen Willans Richardson, FRS (26 April 1879 – 15 February 1959) was a British physicist who won the Nobel Prize in Physics in 1928 for his work on thermionic emission, which led to Richardson's law. Biography Richardson was born in Dew ...

in 1908, later named the

Einstein–de Haas effect The Einstein–de Haas effect is a physical phenomenon in which a change in the magnetic moment of a free body causes this body to rotate. The effect is a consequence of the conservation of angular momentum. It is strong enough to be observable in f ...

, 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.

See also

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Further reading