Cathode rays (also called an
electron beam or
e-beam) are streams of
electrons observed in
vacuum tubes. If an evacuated glass tube is equipped with two
electrodes and a
voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the
cathode (the electrode connected to the negative terminal of the voltage supply). They were first observed in 1869 by German physicist
Johann Hittorf, and were named in 1876 by
Eugen Goldstein Kathodenstrahlen, or cathode rays.