Fred L Crowson: Method and apparatus for electrically heating a subsurface formation. The Electrothermic Co Corpus Christi TX, Giles C Clegg Jr, November 16, 1971: US3620300 (69 worldwide citation)

A method and apparatus include providing one or more vertically spaced electrodes in the borehole in electrical contact with the formation. Two electrodes in a common borehole may be connected to a surface source of alternating current voltage to conductive paths provided respectively by a conductiv ...