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Giuseppe Occhialini and Patrick Blackett combined electronic counters with a cloud chamber to photograph rare penetrating-radiation events. A coincidence signal selected moments when a particle crossed the apparatus, greatly improving the yield over unselected expansions. The resulting magnetic-field photographs recorded groups of tracks and evidence for light positive particles. The method connected fast electronic selection with a detailed image that could be examined after the event.^[ref]

A physicist.^[ref]

Patrick Blackett helped turn the cloud chamber into a selectively triggered particle detector. With Giuseppe Occhialini, he used coincident counter signals to operate the chamber when penetrating particles passed. Magnetic curvature and associated tracks then gave more information than a simple particle count, including evidence relevant to electron-Positron production. The experiment illustrates how triggering can make a slow, detailed recording device useful for rare events.^[ref]

A physicist.^[ref]