The essential parts of a hot cathode are shown in Fig. upPrinciple: It makes use of the following three principles: O) thermionic emission, (ii) deflection of the electron beam by the electric and magnetic fields, and (iii) fluorescence produced by the electron beam on a fluorescent screen.
Construction: It consists of a long evacuated glass tube (pressure nearly 0.001 Hg) containing the three main – i)Electronic Gun ii) Deflection System iii) Fluorescent Screen
Electronic Gun
it is the part of cathode ray tube which gives out a fine beam of electrons. Fig. 12.3 shows separately the electron gun. It consists of a cathode C which is indirectly heated by a filament F connected to a low tension battery (LT) of about 6 V. The cathode C emits electrons on heating (i.e. The electrons are emitted by thermionic emission).The electrons obtained from cathode are accelerated by a series of cylindrical anodes A1 and A, respectively. The anodes A1 and A2 have a small hole along their axes and they are maintained at a high positive potential with respect to the cathode, by means of a high tension battery (H.T) of about 1000v. The anodes A1 and A2 not only accelerate the electrons, but they focus them also in a fine energetic beam. In between the cathode and anode there is a grid G on which a variable negative potential (between -20 V to zero ‘volt) is applied with respect to the cathode C. By varying the negative potential on the grid, the number of electrons reaching the anode and striking the screen can be changed which changes the brightness of the pattern on the screen. This entire arrangement is known as an electronic gun.
The deflecting system
It is the part of cathode ray tube which deflects the electron beam. It consists of two pairs of plates. One pair of plates Y, Y is kept horizontal is called the Y-plates while the other pair of plates X, X kept vertical and is called the X-plates. The plates are placed with their planes parallel to the beam. When a potential difference is applied across any pair of plates, an electric field is set up between the plates which deflect the electron beam. The Y-plates deflect the beam in the vertical direction cd, while the X-plates deflect the beam in the horizontal direction. When no potential difference is applied across these plates, the beam strikes the centre of the screen and a steady bright spot is seen due to fluorescence on the screen. Sometimes a cathode ray tube is used where the electron beam is deflected by applying a magnetic field. The magnetic field is applied from outside by keeping the tube either ¡n between the poles of a horseshoe magnet or in between a pair of parallel coils carrying the direct current.
The fluorescent screen
It is the part of cathode ray tube on which the pattern is formed by the electron beam on striking it. The end of cathode ray tube opposite to the electron gun, is made fiat like a screen and it is coated from inside with a fluorescent material (such as zinc sulphide or barium platinum cyanide or zinc silicate containing some traces of manganese etc.).The electron beam on striking the screen gives bright spot on it due to the fluorescence of the material coated on it. If a varying potential is applied on the deflecting plates, the beam deflects according to the variation of potential on the plates and therefore the spot traces out a pattern on the screen in accordance with the variation of potential. Thus the electrical signal applied on the deflecting plates changes into the visual pattern on the screen. The tube is evacuated so that the electrons may move without being obstructed by the air molecules. Further the inner surface of the tube between the electron gun and the screen is coated with graphite and it is earthed. This shields the electron beam from any electric field outside the tube and also increases the life of the tube.
