The Barkhausen Effect: Hearing Magnets Are Born

The Barkhausen effect - named after German physicist Heinrich Barkhausen - is the term given to the noise produced by a ferromagnetic material due to the change in size and orientation of its discrete magnetic domains under the influence of a magnetic field. external magnet. The domains are small: smaller than the microcrystalline grains that form the magnetic material, but larger than the atomic scale. Barkausen found that as a magnetic field was brought closer to a ferrous material, the local magnetic field flipped randomly, as the magnetic domains rearranged into a minimum energy configuration and this magnetic field noise could be detected with a properly placed search coil. and an amplifier. In the short demonstration video below, this Barkhausen noise can be fed into an audio amplifier, producing a very illustrative example of the effect.

An example of the practical use of this effect is the non-destructive testing and qualification of magnetic structures that are susceptible to damage during use, such as in the nuclear industry. Crystalline discontinuities or impurities in an examined part cause localized mechanical stress to increase, which can lead to unexpected failure. The Barkhausen noise effect can be easily exploited to detect such discontinuities and give the assessor an idea of ​​the condition of the part in question. In short, a useful technique to know!

If you thought Barkhausen was a household name, you might be thinking of the Barkhausen Stability Criterion, which is fundamental to describing some of the conditions necessary for a linear feedback circuit to oscillate. We've covered such circuits before, like this dive into bridge oscillators.

Thanks to {Keith] for the tip!