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In a cathode ray tube, a series of electrodes produces a narrow, tightly focussed beam of electrons. This beam is then deflected horizontally and vertically, either by other electrodes or by external magnetic fields. The beam travels a distance, typically on the order of one or two feet, and strikes a flat screen covered with phosphors. The phosphors glow where the beam strikes, causing a bright dot of light.

In a cathode ray tube, a series of electrodes produces a narrow, tightly focussed beam of electrons. This beam is then deflected horizontally and vertically, either by other electrodes or by external magnetic fields. The beam travels a distance, typically on the order of one or two feet, and strikes a flat screen covered with phosphors. The phosphors glow where the beam strikes, causing a bright dot of light.

Cathode-ray tubes were very important electronic devices throughout most of the twentieth century. They found application in the early part of the century in oscilloscopes, a piece of technical equipment used in scientific research and electrical engineering. During the Second World War, they were used as radar screens. After the war, they became ubiquitous as television "picture tubes."  

Cathode-ray tubes were very important electronic devices throughout most of the twentieth century. They found application in the early part of the century in [[oscilloscope]]s, a piece of technical equipment used in scientific research and electrical engineering. During the Second World War, they were used as [[radar]] screens. After the war, they became ubiquitous as [[television]] "picture tubes."  

The deflection system is used to move the light rapidly across the screen. In an oscilloscope, the beam traces a wiggly line on the phosphor. In a television set, the beam traces a pattern of adjacent straight lines, called a "raster." In each case, because the deflection speed is so high, and because the phosphor glow persists for some time (milliseconds to seconds, depending on the phosphor), a viewer does not see the beam as moving, but sees a solid line or an illuminated picture (though usually with some detectable "flicker").

The deflection system is used to move the light rapidly across the screen. In an oscilloscope, the beam traces a wiggly line on the phosphor. In a television set, the beam traces a pattern of adjacent straight lines, called a "raster." In each case, because the deflection speed is so high, and because the phosphor glow persists for some time (milliseconds to seconds, depending on the phosphor), a viewer does not see the beam as moving, but sees a solid line or an illuminated picture (though sometimes with some detectable "flicker").

A curious application of the CRT was as a form of computer memory, the ''Williams tube.'' In this application, the CRT "writes" by depositing a pattern of charge on the screen, and "reads" by scanning the screen and detecting small differences in current caused by the pattern of charge deposited during the write cycle. Williams tubes were temperamental and unreliable, but enjoyed a brief vogue circa 1950.

A curious application of the CRT was as a form of computer memory, the ''Williams tube.'' In this application, the CRT "writes" by depositing a pattern of charge on the screen, and "reads" by scanning the screen and detecting small differences in current caused by the pattern of charge deposited during the write cycle. Williams tubes were temperamental and unreliable, but enjoyed a brief vogue circa 1950.