Theodor Wulf became a Jesuit priest at the age of 20, before studying physics with Walther Nernst at the University of Göttingen. He taught physics at Valkenburg, a Jesuit University from 1904 to 1914 and 1918-1935. He designed and built an electrometer which could detect the presence of energetic charged particles (or electromagnetic waves). Since natural radiation sources on the ground were detected by his electrometer, he predicted that if he moved far enough away from those sources he would detect less radiation.
To test his hypothesis, in 1910 he compared the radiation at the bottom and the top of the Eiffel Tower. He found that the ionisation fell from 6 ions cm−3 to 3.5 ions cm−3 as he ascended the Eiffel Tower (330m). If the ionisation had been due to γ-rays originating at the surface of the Earth, the intensity of ions should have halved in 80m. Energy was coming from outside the Earth's atmosphere and being detected by his device; this radiation was from cosmic rays. He published a paper in Physikalische Zeitschrift detailing the results of his four days of observation on the Eiffel Tower. His results were not initially accepted.