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If the resistances of the pure metals are measured for the same amplitude of thermal oscillation of the atoms they show a marked periodic behaviour, being always two or three times greater for a monovalent element than for the divalent metal next to it in the periodic table. This fact is discussed on the basis of wave mechanics, and it is suggested that the periodic variation is due to a smaller effective number of free electrons in the divalent metals. Evidence in support of this view can be obtained from the optical constants of metals (§ 3) and from the increase of resistance due to foreign metals in solid solution (§ 4).
N. F. Mott (Sat,) studied this question.