CATHODE RAY TUBE 



(5) The advent of the transistor. The transistor is essentially a current- 

 controlled and -controlling device, and it seems reasonable to suppose that 

 future transistorized apparatus will employ magnetic cathode ray tubes. 



Construction — The triode magnetic cathode ray tube contains merely a 

 cathode, with associated heater, a grid to control the trace brightness, and 

 an anode. The assembly resembles cathode, grid and ^4^ in an electrostatic 

 tube, except that in the magnetic case the anode comprises also a deposited 

 graphite coating inside the flare of the envelope {Figure 32.22). The negative 



Anode yy 



connection yy 



Cathode 



i=rT- 



Grid Anode 



Tube neck 



Figure 32.22 



Figure 32.23 



side of the EHT supply is earthed. In the U.S.A. the tetrode tube has always 

 been preferred, and these are now common in this country also. The tetrode 

 tube possesses two anodes: ^4^ is a small metal anode near grid and 

 cathode, and is supplied with about 300 V positive; A 2 is the graphite 

 coating and is maintained at between 5 and 15 kV positive. 



Focusing with magnetic cathode ray tubes is achieved by an axial magnetic 

 field derived from an annular magnet {Figure 32.23) which fits over the neck 

 of the tube. It can be shown that any electron emerging from the hole in Ai 

 with speed v will be brought back to the axis of the tube in a distance 

 2 mvjHe in c.g.s. units, where H is the axial focusing field strength, so that a 

 focused spot is obtained. The experimenter does not have to concern 

 himself with this equation, since permanent-magnet focusing assemblies 

 may be bought, in which H is adjustable over the relevant range by mechani- 

 cally varying the air gap. The only point to watch is that magnetic cathode 

 ray tube necks are 23 mm, 35 mm or 38 mm in diameter, and it is important 

 to buy a focusing assembly of the correct size. 



The beam may be deflected vertically by passing it through a lateral 

 magnetic field {Figure 32.24) or laterally by passing it through a vertical 

 field. An exact expression for the deflection sensitivity is difficult to give 

 and in any case does not concern us. It is sufficient to know that the deflec- 

 tion is proportional to the field strength and inversely proportional to the 

 square root of the EHT. In practice one persuades a television service man 

 to give one a burnt-out ferrite-cored deflection-coil assembly, strips out the 

 old windings, inserts temporary new ones of known number of turns, and 

 measures the deflection sensitivity in terms of inches of deflection produced 



461 



