Crookes Tube, device for the production of a beam of high-speed electrons. Invented by the British chemist and physicist Sir William Crookes, the Crookes tube is made of glass and contains air or some other gas reduced to a pressure below 0.1 torr (about 1/10,000 normal atmospheric pressure). When a high electric potential is created across the electrodes that protrude into opposite ends of the tube, a golden or greenish glow develops on the glass at the anode end of the tube. The glow is caused by electrons, once known as cathode rays, striking the glass.

A form of Crookes tube containing a small metal shield between the cathode and the glass is often used to demonstrate the properties of electrons. The plate “casts a shadow” of its own shape on the glow or fluorescence, indicating that the electrons, like light, travel in straight lines. Shaping the cathode in a Crookes tube allows direction and concentration of the electrons at any desired place. Early X-ray tubes were Crookes tubes in which the electrons were focused on a metal target to produce X rays. See Cathode Ray; Cathode-Ray Tube; X Ray.