Hans Friedrich Geitel (16.07.1855—15.08.1923) was a German experimental physicist. He was born in Brunswick. He studied at the universities of Heidelberg (1875 — 77) and Berlin (1877-79). In 1879-1920 he taught at a gymnasium in Wolfenbüttel (professor from 1896).
The research that he carried out together with J. Elster concerned the conductivity of gases, thermionic emission, ionisation in the atmosphere, the photoelectric effect, radioactivity, photoelectric methods of measurement and cathode rays. In 1899 they put forward the idea of the transmutation of elements in radioactive processes, detected the radioactivity of the Earth (1903), determined the value of the half-life of polonium (1906) and studied the radioactivity of various bodies (1907). In 1903, independently of W. Crookes, they detected the scintillation effect, studied the one-way conductivity of a vacuum space and in 1910 introduced photocells into technology, having built an industrial model of the vacuum photocell. He built a cathode tube. In 1903, together with J. Elster, he concluded that there exists radiation capable of ionising the air continuously.