Walther Meißner

Walther Meißner was born in Berlin in 1882, the son of a middle-class family of civil servants. He studied mechanical engineering and then physics at the Technische Hochschule Berlin and at the University of Munich, where he wrote his doctorate under Max Planck on the interaction of radiation and matter. He joined the Physikalisch-Technische Reichsanstalt (PTR) — Germany's national metrology laboratory in Charlottenburg, the equivalent of Britain's NPL or America's NIST — in 1908 and stayed for a quarter-century. His assignment: build a low-temperature capability that could rival the Dutch. Kamerlingh Onnes at Leiden had been the world's only source of liquid helium since 1908, and Germany needed its own.

It took Meißner fourteen years, but in 1925 the PTR brought its first helium liquefier online — the third in the world after Leiden and Toronto. With helium in hand, Meißner spent the late 1920s systematically measuring the superconducting transitions of metal after metal: niobium, tantalum, thorium. He found five new elemental superconductors by 1930, doubling the known list. Then, in October 1933, he and his postdoc Robert Ochsenfeld ran an experiment no one at Leiden had thought to try: they cooled tin and lead cylinders through T_c *in the presence of* an applied magnetic field, and measured the field just outside the sample. What they found shocked the community. As the sample crossed T_c, the external magnetic field redistributed as if the cylinder had suddenly become a perfect diamagnet — the flux was expelled from the interior, not merely frozen in place. Before this result, superconductivity was understood as "perfect conduction": infinite electrical conductivity that, by Lenz's law, would freeze in whatever flux was present at the moment of transition. After the Meißner–Ochsenfeld paper, superconductivity was understood as a distinct thermodynamic phase of matter with the active expulsion of magnetic flux as its signature. The Londons' 1935 phenomenological theory and BCS theory in 1957 built on this foundation; neither would have been possible without the Meißner effect as a boundary condition.

Meißner became professor of technical physics at the Technische Hochschule München in 1934 and stayed in Munich through the war, through the destruction of his laboratory in an Allied air raid in 1944, and through the lean years of rebuilding. In 1946 he was elected president of the Bavarian Academy of Sciences, a post he held for fifteen years. He established a low-temperature institute in Herrsching on Lake Ammersee that later moved to Garching and, in 2002, was renamed the Walther-Meißner-Institut in his honour — today one of the world's leading superconductivity laboratories, specialising in the study of high-T_c materials. Meißner himself lived to see the first BCS-era experiments confirming his 1933 work in detail, and died in 1974 at the age of ninety-one, still publishing review articles on the state of low-temperature physics.