12-5] THE CATHODE RAY TUBE 625 



beyond the second anode. The two sets of plates are so mounted at right 

 angles that the plane of one set of plates is vertical and the plane of the 

 other set is horizontal. The field is normal to the surfaces of the deflecting 

 electrodes. The lateral movement of the electron beam is a result of the 

 electric field which is perpendicular to the direction of the electron beam. 

 The magnitude of the deflection of the electron beam is directly propor- 

 tional to the deflecting voltage and inversely proportional to the voltage 

 applied between the second anode and cathode. 



Since the brightness of the luminescent screen increases as a function of 

 the electron volts which strike the screen and this energy is a direct function 

 of second anode to cathode potential, it follows that higher brightness is 

 attained at the expense of deflection sensitivity. This difficulty is overcome 

 by adding a postdeflection accelerating voltage (some defocusing is caused 

 by this method of increasing sensitivity while retaining high brightness). 

 The auxiliary high voltage anode placed near the screen consists of an 

 aquadag coating painted on the inside of the tube. Aquadag is a colloidal 

 solution of graphite. Although it forms a high-resistance coating, the 

 current density of this coating is small and the voltage drop is negligible. 



Some of the basic and important differences between magnetic and 

 electrostatic CRT's may be listed as follows: 



(a) At the excitation levels of intensity-modulated displays, magnetic 

 tubes provide better focus. 



(b) Electrostatic tubes are longer, but the overall equipment has 

 smaller size, weight, and power dissipation than the magnetic tube. 



(c) The grid modulation characteristic (beam current versus grid bias) 

 of most electrostatic tubes follows a square law, while that of the 

 magnetic tube follows a cube law with the reservation quoted 

 above. 



(d) Multiple presentations can be more easily displayed on electro- 

 static tubes because several guns may be accommodated in the 

 same glass envelope and operated independently without inter- 

 fering noticeably with each other. This cannot be done with 

 magnetic deflecting tubes because the magnetic yoke encompasses 

 the entire tube and would mutually affect multiple guns. 



Because of size limitations in airborne indicators, and because the usual 

 radar display uses multiple presentations of radar information and tactical 

 symbols, the electrostatic tube is most commonly used. 



Fluorescent Screens. Much of the abihty to display the signal 

 information content is dependent on the phosphor. Persistence, resolution, 

 light level, and contrast are functions of the phosphor quality. 



