Einstein–de Haas effect

The Einstein-de Haas effect was predicted in 1915 by Albert Einstein and discovered with hiking Johannes de Haas together. The effect is that the magnetism is due to the angular momentum of electrons, and a macroscopic detection of the electron spin.

Experiment

A thin magnetizable rod ( for example, iron, nickel ) hangs vertically on a torsion in a direction perpendicular magnetic coil. Since the magnetic field parallel to the rod and the entire assembly is rotationally symmetric, the magnetic field can not exert any torque on the rod. Nevertheless, he starts to turn around the direction of suspension when turning or reversing the polarity of the magnetic field.

Interpretation

The total angular momentum of the rod is composed of two parts: a portion corresponding to the externally visible rotary motion, the other portion is the sum of permanent angular momenta of the electrons. These are usually disordered, their sum is therefore zero. Through their parallel position but there is the macroscopic magnetization. If before switching on the magnetic field, the rod at rest and is not magnetized, the total angular momentum is zero, because both components are zero. Since nothing can change by the turn of the total angular momentum is zero, both angular momentum components must always be opposite of equal size. Therefore shows the apparent rotational movement, the magnetization of the rod is associated with a specific impulse of torque can be measured by observing the visible rotation. The measured ratio of the magnetic moment of the rod to this angular momentum ( the gyromagnetic ratio) is consistent with the observed electron gyromagnetic ratio of each.

The reversal of this effect is the Barnett effect.

Historical

The observed effect is only small, so using a resonant vibration in order to make it easier to measure. To the rod is constructed as a torsion pendulum with a certain resonance frequency and is applied a variable frequency alternating magnetic field in the coil. When tuning the frequency is reflected in the amplitude of the torsional oscillation a significant resonance curve. However, the quantitative evaluation is not simply in terms of the gyromagnetic ratio. For example, formed by the ferromagnetic hysteresis in the rod, the complication that does not linearly change the magnetization and hence of the angular momentum to the outer box. Therefore, the results obtained are subject to relatively large uncertainties, which were estimated by Einstein and de Haas to 10%. However, they were wrong by a factor of two, as other authors found out by repeating the experiments with improved structures in the coming years.

Einstein and de Haas had determined the result, which, theoretically, it seemed clear to the idea that charged electrons move in circular orbits. The twice as large in relation to magnetic moment, as it is apparent from the more accurate measurements are to be declared only by the intrinsic angular momentum or spin of the electrons, for its part, it is no mechanical explanation. The electron spin and its anomalous magnetic moment were not introduced until 1925.