An effect observed in ferromagnetic materials whereby the magnetization of the specimen proceeds as a series of finite jumps when the magnetizing flux is increased steadily. The effect supports the domain theory of ferromagnetism: the spin magnetic moments present in the material can only have certain allowed orientations; the minute jumps correspond to the spins changing from one allowed orientation to the next. If all possible directions were allowed the magnetization would proceed smoothly.

The effect can be demonstrated by winding the specimen with two coils. When the current in the primary coil is increased steadily to produce a smoothly increasing magnetic flux density, the fluctuations in the magnetization can be shown by connecting the secondary coil to a sensitive cathode-ray oscillograph.

https://www.youtube.com/watch?v=BLXVLDysroY A video demonstration of the Barkhausen effect