The priest, the Eiffel Tower and the cosmic rays

Experimental evidence for the non-terrestrial origin of some of the radiation detected on Earth (cosmic rays) was discovered in 1910 by a Jesuit priest, Father Theodor Wulf. Wulf had taken readings with an electroscope in a range of places including at high altitude in Zermat, Switzerland, and underground in chalk mines and caves. When he took his detector to the top of the Eiffel Tower, he noted that on four days between 11 am and 5 pm the device discharged less than he had expected, an indication that ionising radiation was striking the device. It had been hypothesised that the Earth was the source of all background radiation and hence it was expected that radiation readings should fall to a few percent of their value at ground level at an altitude of 300 m above the Earth. To account for his surprising result, Wulf first checked that the tower itself was not radioactive and then tentatively suggested a source of gamma rays in the atmosphere.

His data received little attention at the time. Two years after Wulf’s experiments, Victor Hess reported elevated levels of radiation in the atmosphere from highaltitude balloon observations. Hess noted that the readings remained constant during a solar eclipse, eliminating the Sun as the source of the radiation. Hess, with Carl Anderson, would win the 1936 Nobel Prize for the discovery of cosmic radiation.