VISUAL INDICATORS 



'pin-cushion' distortion is to produce a grid like Figure 32.14a or b dependent 

 upon whether Y plates or X plates are nearer to the cathode : this is clear 



I 



ff 



(a) 



(b) 



Figure 32.14 



for the former case from Figure 32.15. When the beam leaves the region 

 between the Y plates it encounters a fringe field possessing an axial compo- 

 nent which retards it. In consequence it suffers more X deflection, leading 

 to a grid of the Figure 32.14a type. 



Retarding Deflecting 

 force / force 



Figure 32.15 



Cossor double-beam electrostatic ft/Z^e— Cathode ray tube distortions may 

 be minimized by special shaping of the electrodes and by the introduction of 

 certain additional ones, e.g. the Fleming-Williams' curved anti-trapezium 

 plate''. By the use of such techniques the Cossor double-beam tube is 

 rendered feasible, in which the deflector plate drive is intended to be asym- 

 metrical. Part of this is sketched in highly simplified form in Figure 32.16. 

 The beam is divided into two, whilst still in a diffuse state, by a 'splitter' 



Splitter 

 plate 



Figure 32.16 



plate at A^ potential. The upper part is then vertically deflected by asym- 

 metric drive to the Y^ plate, to show one quantity, and the lower is deflected 

 by the Y^ plate to show another. Both beams then receive lateral deflection 

 from the X plates, to one of which the time-base waveform is connected. 



456 



