The discovery of fission raised the possibility that an extremely powerful atomic bomb could be created. The term was already familiar to the British public through the writings of H. G. Wells, with a continuously-exploding bomb in his 1913 novel "The World Set Free". George Paget Thomson, at Imperial College London, and Mark Oliphant, an Australian physicist at the University of Birmingham, were tasked with carrying out a series of experiments on uranium. Oliphant delegated the task to two German refugee scientists, Rudolf Peierls and Frisch, who ironically could not work on the university's secret projects like radar because they were enemy aliens and therefore lacked the necessary security clearance. In March 1940 they calculated the critical mass of a metallic sphere of pure uranium-235, and found that instead of tons, as everyone had assumed, as little as would suffice, which would explode with the power of thousands of tons of dynamite.