12-5] 



THE CATHODE RAY TUBE 



623 



Focusing of the electron beam is accomplished by either electric or 

 magnetic fields. A magnetically focused CRT is shown in Fig. 12-8. The 

 control grid is followed by a second grid which serves to screen the region 

 between the cathode and control grid from the high accelerating potential 

 of the anode and thereby reduces the control voltage necessary to cut off the 

 beam current. This electrode serves no focusing function. Focusing in a 

 magnetic system is accomplished by means of a longitudinal magnetic field 

 (of circular symmetry) which increases in intensity from the center to the 

 edge of the tube. External coils or permanent magnets are used to produce 

 this field. Electrons moving exactly along the axis of the tube pass through 

 the field with no deflection because they move parallel to the magnetic field 

 at all times. Electrons which are diverging from the axis of the tube as they 

 leave the cathode have a radial velocity component and are acted on by the 

 magnetic field in such a way as to cause rotation of these electrons about 

 axes parallel to the tube. The magnetic forces acting on the diverging 

 electrons produce the inward radial accelerations necessary to reverse the 

 initial diverging trajectory of the electrons to bring the beam to a focus on 

 the fluorescent screen. 



When an electromagnet is used to produce the field, the coil is wound 

 with many turns of wire on a soft iron ring provided with an annular gap. 

 The strength of the magnetic field and therefore the focusing is controlled 

 by adjusting the direct current passed through the coil. Instead of a 

 focusing coil, a permanent magnet may also be used. This magnet is made 

 in the form of a two-section annular ring. Focusing is adjusted by changing 

 the air gap between the pole pieces. 



The electrostatic focusing scheme utilizes an indirectly heated cathode 

 a control grid, a first or focusing anode, and a second or accelerating anode 



First or Focusing 

 Heater Anode 



Aquadag Coating 



Cathode 



Vertical 

 Deflecting Plates 



Horizontal 

 Deflecting Plates 



Fig. 12-9 Electrostatic Cathode Ray Tube. 



