CATHODE RAY TUBE 



The construction of an elementary type of electrostatic tube is shown 

 in Figure 32.10. Electrons are emitted from an indirectly heated cathode 

 and are attracted to a relatively positive first anode, which is a disc per- 

 forated by a small central hole. The emission rate is controlled by con- 

 trolling the space charge around the cathode with a relatively negative 

 electrode which, by analogy with a triode valve, is called a grid, though 

 in fact it is cylindrical. The amount of grid bias used determines the bright- 

 ness of the trace. Some of the electrons emitted pass through the hole in 



First anode 



\ 



Third or 

 final anode 



X deflector plates 



Cathode Grid 



Second or 



focusing 



anode 



V deflector plates 



Figure 32.10 



the first anode and come under the influence of cylindrical anodes 2 and 

 3. The 3 anodes together form an electrostatic lens, which focuses the beam 

 so that an image of the hole in A-^ appears on the screen. Often ^^ and A^ 

 are connected together and to, say +2,000 V with respect to the cathode. 

 A2 is fed from a focus potentiometer with, perhaps, +700 V. 



The focused beam then comes successively under the influence of the 

 Y and X deflector plates. As normally used, the former impart a vertical 

 velocity to the electrons, bending the beam up or down, and are fed with 

 the signal to be displayed. The X plates impart lateral velocity to the 

 electrons, bending the beam sideways, and are usually fed with the output 

 of the time-base generator. 



Finally the deflected beam hits the screen, exciting the phosphor and 

 producing a small, round, bright spot. The electrons subsequently find 

 their way back to A^ via leakage paths within the glass envelope which is, 

 of course, evacuated. 



Deflection sensitivity of electrostatic cathode ray tube — Figure 32.11 shows 

 part of a cathode ray tube, in which A^ is positive to the cathode by an 

 amount V^^, and a pair of deflector plates, potential difference Vjj, distant 

 apart d, and / long, bend the beam an angle 6 so that the spot moves a 

 distance D. The length of the neck of the tube is L. 



Then 



— = tan = 



lateral electron velocity y, 

 axial electron velocity y„ 



The kinetic energy acquired by an electron, of charge e, in being accelerated 

 by an electric field whose boundary potentials are different by E, is eE. In 

 our case the electron is accelerated between cathode and third anode, the 



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