 Of seven tests, four produced a measured force in the intended direction and three produced thrust in the opposite direction.Furthermore, in one test, thrust could be produced in either direction by varying the spring constants in the measuring apparatus.In 2008, a team of Chinese researchers led by Juan Yang (杨涓), professor of propulsion theory and engineering of aeronautics and astronautics at Northwestern Polytechnical University (NWPU) in Xi'an, China, said that they had developed a valid electro-magnetic theory behind a microwave resonant cavity thruster.