Explanation of Cathode Ray Tube

Cathode Ray Tube (CRT)

A cathode ray tube is the heart of the CRO. The below fig shows the various parts of CRT.

1.       Glass Envelope: It is conical highly evacuated glass housing maintains vacuum inside and supports the various electrodes. The inner walls of CRT between neck and screen are usually coated with the conducting material called aquadag. This coating is electrically connected to the accelerating anode so that electrons which accidently strike the walls are returned to the anode. This prevents the walls of the tube from charging to a high negative potential.

Pic 1. Cathode Ray Tube

2.       Electron Gun Assembly: the arrangement of electrodes which produce a focussed beam of electrons is called electrongun. It essentially consists of an indirectly heated cathode, a control grid, a focussing anode and an accelerating anode. The control grid is held at negative potential with respect to cathode whereas the anode are maintained at high positive potential w.r.t cathode.

The cathode is made of a nickel cylinder coated with oxide coating and provides plenty of electrons. The control grid encloses the cathode and consists of a metal cylinder with a tiny circular opening to keep the electron beam small in size. The focusing anode focuses the electron beam into a sharp beam by controlling the positive potential on it. The accelerating with high positive potential. Therefore, the electrongun assembly forms a narrow, accelerated beam of electrons which produces a spot of light when it strikes the screen.

3.       Deflection Plate Assembly: The deflection of the beam is accomplished by two sets of deflecting plates within the tube beyond the accelerating anode as shown in the fig. one set is vertical deflection plates and other set is the horizontal deflection plates. The vertical deflection plates are mounted horizontally in the tube. By applying proper potential to these plates, the electron beam can be made to move up and down vertically on the fluorescent screen. The horizontal deflection plates are mounted in the vertical plane. An appropriate potential on these plates can causes the electron. Beam to move right and left horizontally on the screen.

4.       Screen: The screen is the inside face of the tube and is coated with some fluorescent material such as zine orthosilicate, zinc oxide etc. when high velocity electron beam strikes the screen, a spot of light is produced at the point of impact. The colour of the spot of light is produced at the point of impact. The colour of the spot depends upon the nature of fluorescent material. If zinc orthosilicate is used as the fluorescent material, green light spot is produced.