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The Barnett effect is the change of a body's magnetization caused by mechanical rotation. Turning the whole specimen can bias the internal angular momentum carrying its magnetic moments, even with zero applied magnetic field. It is the reciprocal physical question to the Einstein-de Haas effect: impose rotation and measure magnetization, rather than change magnetization and observe mechanical motion. The relation between the two becomes especially transparent when the same signed gyromagnetic ratio connects magnetic moment and internal angular momentum.

Samuel Jackson Barnett published Magnetization by Rotation on 1 October 1915 in Physical Review. The Barnett rotation magnetometry experiment below provides an inspected modern example. The important distinction is between an equivalent term in a Hamiltonian and the actual external magnetic field measured by a magnetometer: the first biases the material; its resulting magnetization produces the second.

Samuel Jackson Barnett demonstrated that mechanical rotation can magnetize a material, the Barnett effect. Rotation couples to the angular momentum associated with microscopic magnetic moments, producing a magnetic response without using an applied field as the driving cause. The effect connects mechanical angular momentum with magnetization and complements experiments in which a change of magnetization produces mechanical motion. It makes the magnetic moment's connection with microscopic angular momentum experimentally accessible.^[ref]

A physicist listed as professor of physics at the University of California, Los Angeles, in its 1927–1928 faculty announcement.^[ref]