To measure ionizing radiation away from the earth’s surface, several researchers took to the air in balloon flights in the first decade of the 20th century. One of these pioneers, Albert Gockel, measured the levels of ionizing radiation up to a height of 3000 metres. He concluded that the ionization did not decrease with height and consequently could not have a purely terrestrial origin. He also introduced the term “kosmische Strahlung” – cosmic radiation.
Later calculations by Schrödinger showed that the radioactivity came in part from above and in part from the Earth’s crust and that the decrease in the radioactivity from the Earth’s crust could be offset by the growth of radioactivity from extraterrestrial sources up to 3000 m.