# The Einstein de Haas Experiment

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The Einstein de Haas Experiment

In 1915, well before the discovery of quantum physics, Albert Einstein and Dutch physicist W. J. de Haas carried out a clever experiment designed to show that the angular momentum and magnetic moment of individual atoms are coupled.  Einstein and de Haas suspended an iron cylinder from a thin timber, as shown in. A solenoid was placed around the cylinder but not touching it. Initially

the magnetic dipole moments jI of the atoms of the cylinder point in random directions, so their external magnetic  effects cancel . However, when a current is switched on in the solenoid  so that a magnetic field 8is set up parallel
to the axis of the cylinder, the magnetic dipole moments of the atoms of the cylinder reorient themselves, lining up with that field. If the angular momentum L of each  tom is coupled to its magnetic moment jI, then this alignment of the atomic magnetic moments must cause an alignment of the atomic angular momenta opposite the magnetic field.   No external torques initially act on the cylinder; thus, its angular momentum must remain at its initial zero value. However, when 8is turned on and the atomic angular momenta line up antiparallel to 8, they tend to give a net angular momentum  Lnel to the cylinder as a whole (directed downward in Fig. 41-4b). To maintain zero angular momentum, the cylinder begins to rotate around its central axis to produce an angular momentum L~in the opposite direction (upward in Fig. 41-4b). Were it net for the fiber, the cylinder would continue to rotate for as long as  the magnetic field is present. However, the twisting of the fiber quickly produces a torque that momentarily stops the cylinder’s rotation and then rotates the cylinder in the opposite direction as the twisting is undone. Thereafter, the fiber will twist and untwist as the cylinder oscillates about its initial orientation in angular simple harmonic motion.  Observation of the cylinder’s rotation verified that the-angular momentum and the magnetic dipole moment of an atom are coupled in opposite directions. Moreover, it dramatically demonstrated that the angular momenta associated with quantum states of atoms can result in visible rotation of an object of everyday size

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