"(1907) was intrigued by the fact that iron and other ‘soft’ ferromagnetic materials with small permanent magnetization become strongly magnetized when exposed to quite weak magnetic fields. Weiss proposed that external fields play a minor role compared to a hypothesized internal ‘molecular field’ which aligns the magnetic moments of individual atoms, producing a spontaneous magnetization, Ms. In the absence of any external field, the magnetic moments of regions with different directions of Ms (now called domains) cancel almost perfectly, but even a small applied field will either rotate domains or enlarge some at the expense of others. Weiss’ theory is the starting point for modern ideas about ferromagnetism ... and ferromagnetic domains ..."