Victor Hess investigated penetrating radiation by taking ionization instruments on balloon flights. If the dominant source were radiation from the ground, increasing the distance from that source should generally reduce its contribution.
At high altitude the measured ionization instead increased. Hess's 1912 report used this altitude dependence to argue that part of the radiation entered from above. The experiment inferred an external origin from a changing detector response; it did not by itself identify every component of what is now called cosmic rays.[ref]
Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten by Victor Franz Hess. Original report: seven flights, high-altitude increase and conclusion about radiation entering from above
The Hess cosmic-ray balloon experiment discusses the apparatus and the need to distinguish atmospheric ionization from instrument leakage and local background.
Portrait of Victor Hess
. Anonymous photographer; Nobel Foundation / NobelPrize.org source. Public domain claims for EU anonymous-work expiration and United States nonrenewal, as documented on the File. Source usage terms, via Wikimedia Commons. Source.
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