Julius Elster (24.12.1854-6.04.1920) was a German experimental physicist. He was born in Blankenburg. In 1879 he received the degree of Doctor of Philosophy at the University of Heidelberg. From 1881 he worked at the gymnasium in Wolfenbüttel, where he carried out joint research with H. Geitel.
His work concerned electrical phenomena in gases and in the atmosphere, the photoelectric effect, radioactivity, thermionic emission and cathode rays. Many of these investigations were pioneering. He built a production model of the vacuum photocell. In 1899 he 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), investigated the radioactivity of various bodies (1907), and detected the scintillation effect independently of W. Crookes (1903).
In 1887 he rediscovered thermionic emission, and in 1903, in a study of atmospheric electricity, he noticed an unknown source of ions in the air and came to the conclusion that there exists a radiation capable of ionising a gas continuously.