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Foucault interrupter. 19th-century illustration of the interrupter invented by French physicist Leon Foucault (1819-1868). The purpose of such interrupters was to rapidly connect and disconnect a direct electric current to create the changing magnetic field needed for induction coils. This is a large design suitable for use with powerful induction coils (right). The interrupting mechanism is at upper centre and consists of contacts dipping into and out of a layer of alcohol over mercury. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/SCIENCE PHOTO LIBRARY)