In 1937 Jensen was Privatdozent (unpaid lecturer) at the University of Hamburg and began working with Paul Harteck, director of the university's physical chemistry department and advisor to the "Heereswaffenamt" (HWA, Army Ordnance Office) on explosives. Harteck and his teaching assistant Wilhelm Groth made contact with the "Reichskriegsministerium" (RKM, Reich Ministry of War) on 24 April 1939 to tell them of potential military applications of nuclear chain reactions. Military control of the German nuclear energy project, also known as the "Uranverein" (Uranium Club), began on 1 September 1939, the day that Nazi Germany initiated World War II by invading Poland. Harteck, one of the principals in the "Uranverein", brought Jensen into the project. Jensen's main thrust was on double centrifuges for separation of uranium isotopes (see the section below citing internal reports of the "Uranverein"). Harteck and Jensen developed a double centrifuge based on a rocking process ("Schaukelverfahren") to facilitate the separation effect.