At the time, an improved tubular version of the original Geiger counter, invented by Hans Geiger in 1908, had just been developed by his student Walther Müller. These Geiger–Müller tubes (GM tubes or counters) made possible Bothe's investigations. With Occhialini's help in the construction of GM tubes, and with the aid of a practical coincidence circuit, Rossi confirmed and extended the results of Bothe, who invited him to visit Berlin in the summer of 1930. Here, with financial support arranged by Garbasso, Rossi collaborated on further investigations of cosmic ray penetration. He also studied Carl Størmer's mathematical description of the trajectories of charged particles in the Earth's magnetic field. On the basis of these studies, he realised that the intensity of cosmic rays coming from eastward directions might be different from that of westward ones. From Berlin, he submitted the first paper suggesting that observations of this east–west effect could not only confirm that cosmic rays are charged particles, but also determine the sign of their charge.