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Fuchs’s transfer of scientific secrets to the Soviet Union between 1941 and 1943 was one of the most concentrated spy hauls in history, some 570 pages of copied reports, calculations, drawings, formulae and diagrams, the designs for uranium enrichment, a step-by-step guide to the fast-moving development of the atomic weapon. Much of this material was too complex and technical to be coded and sent by radio, and so Ursula passed the documents to Sergei through a “brush contact,” a surreptitious handover imperceptible to a casual observer. If Ursula needed to pass on urgent information, or bulky files, she alerted Aptekar by means of an agreed “signal site”: “I had to travel to London and, at a certain time and in a certain place, drop a small piece of chalk and tread on it.” Two days later she would cycle to the rendezvous site, a side road six miles beyond the junction of the A40 and A34 on the road from Oxford to Cheltenham; Aptekar would drive from London in the military attaché’s car and arrive at the pickup site at an appointed time for a swift handover. At one of these meetings, the Soviet officer presented her with a new Minox camera for making microdots and copying documents, and a small but powerful transmitter measuring just six by eight inches, a sixth of the size of her homemade radio and easier to conceal. She dismantled her own equipment, but kept it in reserve “for emergency use.” Fuchs was privy to the innermost workings of the atomic project and he held nothing back. In the first year, he and Peierls wrote no fewer than eleven reports together, including seminal papers on isotope separation and calculating the destructive power of
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