English: A replica of the experimental apparatus used by German physicist Heinrich Barkhausen in 1919 to detect the Barkhausen effect. This is a hissing or series of crackling sounds heard in an earphone attached to a coil of wire around an iron core when the magnetic field through it changes. When a magnet is brought near a piece of iron, the domain walls of the magnetic domains move, reorienting the domains so the , The sound is caused by the domain walls getting hung up on defects in the crystal lattice and then "snapping" past them. This effect is responsible for the magnetization hysteresis curve of the magnetic material.
The apparatus consists of a coil of wire wound around an iron core (center) with the coil attached to a vacuum tube amplifier and a pair of earphones. A horseshoe magnet (right) that can be rotated by a knob is next to the iron. When the magnet is rotated by 180°, its magnetic field which passes through the iron core changes from one direction to the opposite direction. The Barkhausen noise is heard in the earphone while the magnet is being turned.