# Einstein-de Haas Effect

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The only experimental work ever published by Albert Einstein was carried out in 1915 in collaboration with Wander Johannes de Haas, who was the son-in-law of H. A. Lorentz. Einstein had long contemplated Ampère's conjecture in 1820 that magnetism is caused by circulation of electric charge. This Demonstration describes a technologically updated version of the original Einstein–de Haas experiment. A cylindrical soft-iron magnet is supported by a thin quartz fiber attached to a mirror. The magnet is suspended within a solenoid connected to a reversible DC power source. The current is large enough to create a magnetic field strong enough to saturate the cylinder's magnetization in either direction. If Ampère was right, this should create an angular displacement of the magnet, which can be detected by deflection of a laser beam directed at the rotating mirror.

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Contributed by: S. M. Blinder (March 2011)

Open content licensed under CC BY-NC-SA

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## Details

An iron cylinder with a saturation magnetization of gauss has a net magnetic dipole moment of directed along the axis of the solenoid. If this magnetic dipole is associated with charge circulation, then , where is the angular momentum of the magnet and is the magnetogyric ratio. The maximum kinetic energy of rotation equals , where is the moment of inertia of the magnet. This is converted into potential energy of the torsional balance at its maximum angular displacement, given by . From the preceding formulas, the -factor is determined by , where (expressed in degrees) is the maximum deflection of the reflected laser beam. Reversing the magnetization of the iron should create an angular displacement in the opposite direction as the angular momentum of the charge carriers is reversed.

Reference: Einstein–de Haas effecton Wikipedia.

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