 The basic distinction between metals and insulators was proposed by Bethe, Sommerfeld and Bloch in 1928/1929.It distinguished between conducting metals (with partially filled bands) and nonconducting insulators.However, in 1937 de Boer and Evert Verwey reported that many transition-metal oxides (such as NiO) with a partially filled d-band were poor conductors, often insulating.