Bruno Rossi (Image: Wikimedia Commons)

Rossi’s coincidence circuits form the basis of all modern electronic-counter experiments. In 1930, Bruno Rossi used electronic valves to register coincident pulses from the Geiger counters. He arranged the detectors in a triangle so that the cosmic rays could not transverse all three counters. In 1932 he found that 60% of the cosmic rays that pass through the 25 cm piece of lead could also traverse a full metre of lead. This was the first demonstration of the production of showers of secondary particles. Rossi also demonstrated that the cosmic ray flux contains a soft component easily absorbed in a few millimeters of lead and a hard component of charged particles with energies above 1 GeV. This ended Millikan’s theory that the cosmic rays consisted of gamma rays.

Rossi demonstrated that the Earth’s magnetic field bends incoming charged particle showers so that if they are more negative, more come from the east than from the west and vice-versa. In 1933, Rossi and others demonstrated an east-west effect that showed that the majority of cosmic rays were positive. Rossi noted coincidences between several counters placed in a horizontal plane, far in excess of chance coincidences. "It would seem that occasionally very extensive groups of particles arrive on the equipment," he noted in one of his papers.