34 BELL SYSTEM TECHNICAL JOURNAL 



to the average between the two values of feedback current which exist when 

 the beam falls (1) directly on a grid wire and (2) midway between two wires. 

 One may regard this bias as pressing the beam upward against a wire, resisted 

 by downward pressure associated with the feedback current. The latter 



Fig. 19. Interior of the coder tube, viewed from the gun end. 



increases as the beam approaches the wire, and equilibrium is reached when 

 the beam is about half way between positions (1) and (2) mentioned above. 

 The feedback current is actually intermittent, turned off and on by the 

 blanking pulse, and careful analysis shows it necessary to make the bias inter- 

 mittent also, with its wave fronts synchronous with those of the feedback 

 current. This is readily accomplished by deriving the bias from the blanking 

 signal itself. 



