Cathode ray
![A beam of cathode rays bent into a circle by a magnetic field generated by a Helmholtz coil. Cathode rays are normally invisible; in this tube enough residual gas has been left that the gas atoms glow from fluorescence when struck by the fast moving electrons.](/Images/godic/202501/12/Cyclotron_motion_smaller_view0058.jpg")
![A diagramatic Crookes tube showing the connections for the high voltage supply. The Maltese cross has no external electrical connection.](/Images/godic/202501/12/Crookes_tube2_diagram.svg0058.png")
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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.