cathode ray Radiation emitted by the cathode of a thermionic electron valve containing a gas at low pressure. In 1897 J. J. Thomson identified the rays as streams of charged, elementary particles having extremely low mass, later called electrons. Some electrons are emitted because the cathode is heated but most because of collisions between the cathode and positive ions formed in the valve.

cath·ode-ray tube (abbr.: CRT) (also cath·ode ray tube) • n. a high-vacuum tube in which cathode rays produce a luminous image on a fluorescent screen, used chiefly in televisions and computer terminals.

cathode-ray tube Evacuated electron tube used for television picture tubes, oscilloscopes and display screens in radar sets and computers. An electron gun shoots a beam of electrons, focused by a grid. The electrons strike a fluorescent screen and produce a spot of light. In a television tube, an electrostatic or magnetic field deflects the beam so that it scans a number of lines on the screen, controlled by the incoming picture signals.

cath·ode ray • n. a beam of electrons emitted from the cathode of a high-vacuum tube.