During experiments carried out in 1919 involving magnetism and acoustics, German scientist Heinrich Barkhausen provided convincing evidence that iron and other ferromagnetic materials are magnetized in small, distinct intervals rather than in a smooth, continuous manner, as had been theorized. Barkhausen did so by connecting a wire coil surrounding an iron core to an amplifier, then bringing a magnet close to the coil.
Any signal picked up by the amplifier was sent to a speaker, which enabled Barkhausen to hear a progression of clicking noises whenever he moved the magnet.
The sound reflects the shifting of what are known as magnetic domains in the iron. Magnetic domains are microscopic areas in which the atoms – each a kind of tiny magnet with its own tiny magnetic field – are all aligned in the same direction.
When the bar magnet is moved near the core, those domains gradually realign with the field of the magnet. Due to electromagnetic induction, the shifting of a domain creates a change in the magnetic field around the iron, and that changing magnetic field induces a current in the surrounding coil detectable by the amplifier.
