Late in 1931, Rossi arranged for Occhialini to work in the Cavendish Laboratory at the University of Cambridge with Patrick Blackett, whom he had met in Berlin. With the aid of the new technique of electronic coincidence, Occhialini helped Blackett develop the first counter-controlled cloud chamber, with which they confirmed Carl Anderson 's discovery of the positron and deduced that the positive electrons are produced in association with negative ones by pair production. Up to 23 positive and negative electrons were observed in some events, which were clearly related to the showers of Rossi's soft component.