The source: Earth, atmosphere or outer space?
In studying electrical conduction through air in 1899, Julius Elster and Hans Geitel designed a key experiment where they found that surrounding a gold leaf electroscope with a thick metal box would decrease its spontaneous discharge. From this observation, they concluded that the discharge was due to highly penetrating ionizing agents outside of the container. In a similar experiment at about the same time, Charles Thomson Rees Wilson in Cambridge came to the same conclusion.
To test whether the ionizing radiation originated beyond the atmosphere, in 1901 Charles Thomson Rees Wilson took measurements of natural radioactivity using an electroscope inside an old railway tunnel in Scotland. If the radiation were coming from outer space, Wilson could have expected to measure a signification reduction in the tunnel compared to outside on the surface. But he saw no such reduction. Following Wilson’s observations, the scientific community largely dismissed the extra-terrestrial theory.
Since some of the radiation was found to be too penetrating and perhaps too abundant to originate from known sources, altitude-dependent studies were carried out to test the idea of an extraterrestrial source – although at first the results were contradictory.