The ‘how’ and the ‘why’ of knowledge circulation is explored in a study of the encounter between American and British nuclear scientists and engineers who together developed a gas centrifuge to enrich uranium in the 1960s. A fine-grained analysis of the transnational encounter reveals that the ‘how’ engages a wide variety of sometimes mundane modes of exchange in a series of face-to-face interactions over several years. The ‘why’ is driven by the reciprocal wish to improve the performance of the centrifuge, (...) though this motive is embedded in the asymmetric field of the ‘special relationship’ in nuclear matters between the United Kingdom and the United States. The result of the encounter is co-produced, hybrid knowledge in which the national provenance of the contributions from each side of the Atlantic is at once diluted and a contested site for the affirmation of national power. (shrink)