Cosmic rays were discovered in 1912 when Victor F. Hess (Austria) was flying ion chambers in a balloon to measure expected the decrease of ionization from terrestrial rock and water with the increase of height. The results were unexpected: above about a kilometer the ionization increased continuously with increasing altitude.

Hess proposed that a very penetrating radiation was entering our atmosphere from above. You can imagine that it was alarming, and perhaps a tabloid story, to discover that we are all constantly subjected to a powerful radiation coming from an unknown source in space.

Millikan, who was at first a skeptic, carried out (in 1923-26) an extensive series of measurements which gave conclusive evidence for the extraterrestrial origin of the radiation, generally believed to be energetic gamma rays, and he gave it the name cosmic rays.

Cloud Chamber
Spark Chamber

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