VISUAL INDICATORS 



It will be clear from the foregoing that the successful use of magnetic tube 

 displays lies largely in the careful design and construction of the deflector 

 coil assembly. Reasonable care in winding, shaping and fitting the coils is 

 all that is needed to minimize all faults except biaxial pin-cushioning; the 

 necessary graded field to minimize this requires fairly advanced design, and 

 Cocking may be consulted, though in the writer's opinion the effect is not a 

 very serious one, and may often be ignored. 



( 



Figure 32.28 



X2 coil 

 Figure 32.29 



fs 



Figure 32.30 



Figure 32.31 



Ancillary circuitry — With magnetic cathode ray tubes it is helpful if a 

 negative HT supply is available, since the cathode can then be earthed, the 

 heater can be excited from the same a.c. supply as feeds the other valves, and 

 no strain is imposed on the heater-cathode insulation. Figure 32.32 is suitable 

 for a triode tube and Figure 32.33 for a tetrode. If there is no negative 

 supply. Figures 32.34 and 32.35 are possible, but the heater-cathode voltage 

 must be watched, and a separate heater winding in the mains transformer 

 may be required for the tube. No bleeder is necessary across the EHT 



464 



