(i) Deflection within the magnetic field, d 1 andĮquation (9.19) shows that the deflection sensitivity is directly proportional to the square root of the ratio of the charge to mass of an electron but inversely proportional to the square root of the final anode potential. Thus, the total deflection of the spot can be divided into two parts: As shown in the figure, the electron travels in an arc MN of a circle inside the magnetic field and thereafter it moves in a straight line. The configuration of the magnetic deflection system is shown in Fig. In this system the magnetic field is produced by passing current through the coil, whose axis is kept normal to the direction of the electron beam. The unit of deflection sensitivity is then expressed in inch/mA or mm/mA. The magnetic deflection sensitivity is defined as the amount of deflection of electron spot produced when one milliampere current flows through the deflection coil. It is also seen from equation (9.9) that the deflection sensitivity is inversely proportional to the final anode voltage. Let the plates be kept at a distance D from the screen and on entering the deflection plates let an electron move with a velocity v.įor a given CR tube, l, L and S are fixed and so varying the voltage applied to the plates, V d, or the final anode voltage V a, one may alter the deflection spot d. In the figure, V d is the potential applied between two plates, each of length l and spacing S. The geometry of the electrostatic deflection system is shown in Fig. The unit of deflection sensitivity is then expressed in inch/volt or mm/volt. source is applied to the deflection plates. The electrostatic deflection sensitivity is the amount of deflection of electron spot produced on the screen when a voltage of one volt from a d.c. The beam of electrons in a CR tube is deflected by two different means: Some commonly used phosphors are- zinc silicate, zinc oxide, zinc sulphide, etc. The fluorescent material coated inside the tube is termed as phosphor. Though the control grid is usually used to control the intensity of this spot, but it is also dependent on the fluorescent material. If the beam of electrons coming from the electron gun is not deflected it would form a luminous spot of light at the centre of the fluorescent screen. In television camera tube of iconoscope type the first anode potential is of the order of 300 volts while that of the second anode potential is 1000 volts. The first anode is always kept at an appreciably lower potential than the second one. The anode potentials in a CR tube are adjusted by considering the required beam power. voltage relative to the cathode, accelerates the electron beam. The second anode which is kept at a high positive d.c. In order to focus the beam of electrons properly, the voltage applied on the first anode is utilized. The indirectly heated cathode emits a stream of electrons whose density is controlled by the bias voltage applied between the control grid and the cathode.
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