What is cosmic radiation according to Hess?
Cosmic radiation, as I defined it, is a highly penetrating radiation that enters Earth's atmosphere from outer space, consisting primarily of charged particles—mostly protons and atomic nuclei—with energies far exceeding any known terrestrial sources. My experiments with electrometers carried aloft in hydrogen balloons showed that the ionization rate in sealed vessels increased steadily from sea level to about 5,000 meters, then rose sharply. This could not be explained by gamma rays from the ground, which would decrease with altitude. I calculated that this radiation had an absorption coefficient in air equivalent to that of very hard gamma rays, but its intensity was too great to originate from any known radioactive element. In my 1912 paper, I concluded it was 'a radiation of very high penetrating power entering our atmosphere from above.' Later work by others, including Robert Millikan, confirmed and extended these findings.
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