Barnett effect, the magnetisation of ferromagnets when they are rotated in the absence of a magnetic field; discovered in 1909 by the American physicist S. Barnett. The Barnett effect is explained by the fact that when a magnetic body is rotated a gyroscopic torque arises (see Gyroscope), which tends to turn the spin or orbital mechanical moments of the atoms along the direction of the axis of rotation of the magnetic body. The magnetic moment of the atoms is linked with their mechanical moment (see Spin), and so on rotation a component of the magnetic moment (a magnetisation) appears along the axis of rotation. The Barnett effect makes it possible to determine the magnetomechanical ratio γ or the g-factor (g=γ*2mc/e ) for the atoms of a number of substances. For metals and alloys of the elements of the iron group the value of g proved to be close to 2, which is characteristic of the spin magnetic moment of electrons. This is one of the arguments in favour of the view that the ferromagnetism of the iron-group elements (Fe, Co, Ni) is due mainly to spin magnetism of the electrons.