Wilhelm Lenz

Wilhelm Lenz was born in Frankfurt in 1888 and took his doctorate under Arnold Sommerfeld in Munich in 1911. After service in the First World War he worked in Munich and Rostock before being appointed professor of theoretical physics at the University of Hamburg in 1921, where he built one of Germany's strongest theory groups and remained until his retirement.

In 1920 Lenz proposed a model of ferromagnetism stripped to its essentials: a lattice of elements each able to point one of two ways, interacting only with their nearest neighbours, with agreement energetically favoured. The question was whether so simple a rule could produce the sharp Curie transition that real magnets show. He handed the one-dimensional case to his doctoral student Ernst Ising, who solved it in 1924 and found no transition at all — an answer that led both men to set the model aside.

The judgement was premature by two decades. Onsager's exact solution of the two-dimensional case in 1944 found the transition Lenz had been looking for, and the model became the most-studied in statistical physics. It is now universally called the Ising model, though Lenz-Ising model is occasionally used and is arguably the fairer name: the formulation was Lenz's, the solution Ising's.

Lenz's own career centred on quantum mechanics — the Laplace–Runge–Lenz vector, which he applied to the old quantum theory of the hydrogen atom, still bears his name — and on his role as a teacher and department builder. Wolfgang Pauli was his assistant in Hamburg, as were Hans Jensen and Ernst Ising. He died in Hamburg in 1957, by which time the model he had discarded was being worked on around the world.