770 



BELL SYSTEM TECHNICAL JOURNAL 



ployed to permanently deflect the beam for off-center displays such as the 

 B-type and certain modified sector scanning PPI forms. Here the ampli- 



CATHODE-RAY 

 TUBE 



'^/mac 



CENTERING- ^'^ u^^- MAGNETIC 



RING * ' SHUNT 



FRONT AND BACK 



POLE PIECES 



(a) 



Fig. 42.— Operational diagram of permanent magnet type of focussing and centering 

 structure for radar indicator. 



tude of deflection required is approximately equal to the radius of the screen 

 and this precludes the use of the usual beam centering structure located near 

 the gun structure portion of the neck of the tube. To prevent physical 



