Walter Elsasser. Portrait of Walter Maurice Elsasser (born 1904), the German-American physicist who devised the theory explaining the Earth's magnetic field. Elsasser emigrated to America in 1936 and held professorships at the universities of Pennsylvania, Utah, Princeton and Maryland before retiring in 1976. In 1939 he proposed that the Earth's magnetic field was due to the planet's rotation causing eddy currents in its molten iron core. Photographed during the 1980s.