Cathode ray tube

Cathode ray tube using electromagnetic focus and deflection (parts shown are not to scale)

Thecathode ray tubeorCRTwas invented byKarl Ferdinand Braun.It was the most common type ofdisplayfor many years. It was used in almost allcomputer monitorsandtelevisionsuntilLCDandplasma screensstarted being used.

A cathode ray tube has anelectron gun.Thecathodeis anelectrode(a metal that can send outelectronswhen heated). The cathode is inside a glass tube. Also inside the glass tube is ananodethat attracts electrons. This is used to pull the electrons toward the front of the glass tube, so the electrons shoot out in one direction, making acathode ray.To prevent gas discharge, which would prevent the tube from working, the air is taken out of the tube, making avacuum.

The electrons hit the front of the tube, where aphosphorscreen is. The electrons make the phosphor light up. The electrons can be aimed by creating a magnetic field. By carefully controlling which bits of phosphor light up, a bright picture can be made on the front of the vacuum tube. Changing this picture 30 times every second will make it look like the picture is moving. Because there is a vacuum inside the tube (which has to be strong enough to hold out the air), and the tube must be glass for the phosphor to be visible, the tube must be made of thick glass. For a largetelevision,this vacuum tube can be quite heavy.

The cathode ray tube was invented in 1897, and used as anoscilloscope(a machine to show waves). Later, along with other inventions and improvements, it was used for the first modern electronic television byPhilo T. Farnsworthin the 1920s.The CRT was the main type of television screen until theliquid crystal displaybecame popular in the early 2000s.

Related pages

• Liquid crystal display
• Vacuum tube