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 the iron 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 within the iron gradually realign with
the field of the magnet. Due to electromagnetic induction, the
shifting of a domain creates