Cathode Ray Tube
Envelope Or Container
Most people have seen a cathode ray tube or pictures of one. The "picture tube" in a television set is perhaps the most familiar form of a cathode ray tube. The outer shell that gives a picture tube its characteristic shape is called the envelope of a cathode ray tube. The envelope is most commonly made of glass, although tubes of metal and ceramic can also be used for special purposes. The glass cathode ray tube consists of a cylindrical portion that holds the electron gun and the focusing and deflection systems. At the end of the cylindrical portion farthest from the electron gun, the tube widens out to form a conical shape. At the flat wide end of the cone is the display screen.
Air is pumped out of the cathode ray tube to produce a vacuum with a pressure in the range of 10 -2 to 10 6 pascal, the exact value depending on the use to which the tube will be put. A vacuum is necessary to prevent electrons produced in the CRT from colliding with atoms and molecules within the tube.