Summary

Victor Francis Hess's 1928 monograph argues that the electrical conductivity of the atmosphere is primarily caused by a penetrating radiation from outer space, a phenomenon he had discovered and named "cosmic radiation" in 1912. The book synthesizes his balloon-flight experiments and ground-based measurements to demonstrate that this radiation, not terrestrial sources like radioactive soil or air, accounts for the steady ionization of the lower atmosphere. Hess systematically rules out alternative explanations—such as gamma rays from the Earth's crust or atmospheric radioactivity—by showing that ionization increases with altitude, peaking at heights where terrestrial influences are negligible. He also details the diurnal and seasonal variations of conductivity, linking them to cosmic ray intensity and meteorological conditions. The reader gains a foundational understanding of early cosmic ray physics, including the experimental methods (electroscopes on balloons) and the evidence that overturned the prevailing view of atmospheric electricity as solely Earth-bound. Hess's work established cosmic rays as a key factor in atmospheric science, paving the way for later studies in particle physics and geophysics.

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Key concepts

Cosmic radiation — A penetrating, ionizing radiation originating from outside Earth's atmosphere, discovered by Hess via balloon ascents showing increased ionization at high altitudes.
Atmospheric ionization — The process by which neutral air molecules become charged particles (ions), primarily driven by cosmic rays in the upper atmosphere according to Hess.
Electroscope — A device used by Hess to measure electrical conductivity by detecting the rate of charge leakage, calibrated for altitude and weather conditions.
Balloon ascents — Experimental flights (up to 5.3 km) where Hess carried electroscopes to measure ionization gradients, proving cosmic radiation's extraterrestrial origin.
Terrestrial radioactivity — The emission of ionizing particles from radioactive elements in soil and air, which Hess showed could not explain the observed high-altitude conductivity.
Diurnal variation — Daily fluctuations in atmospheric conductivity, which Hess correlated with cosmic ray intensity and solar influences, ruling out purely local causes.