CATHODE RAY TUBE 



supply (except a very high value resistor to discharge the capacitors after 

 switching off, provided in the interests of safety) which in consequence has 

 only to be able to supply a current of the order of 100 /^A. 



If the focusing is by permanent magnet, all one has to do is to adjust the 

 gap as required. If electromagnetic focusing is used, the coil may — if of low 

 resistance — be employed as an additional smoothing choke in the HT power 



HT+ 



HT+ 



n 



-Cathode 



X 



■Grid 



Cathode 



Grid 



HT- 

 Triode tube 

 Figure 32.32 



HT- 



Tetrode tube 



Figure 32.33 



supply, and the current through it controlled by a variable shunt resistor. If 

 it is of high resistance, it should be connected across the HT supply with a 

 variable series resistor. Various other small magnets are used with magnetic 

 tubes. One is the ion-trap magnet. 



Ion-trap magnet. In the expression for electrostatic deflection the ratio 

 elm cancels out ; that is, all charged particles follow the same path irrespec- 

 tive of their mass. In a practical tube heavy ions, both positive and negative. 



HT+ 



HT* 



TT 



Cathode 





Grid 





Triode tube 



Figure 32.34 



Cathode 



Tetrode tube 



Figure 32.35 



•Grid 



are present. In electrostatic tubes the negative ions are also projected at the 

 screen and strike it, at any instant, in the same place as the electrons. 



The deflection expression for magnetic tubes contains ejm, in such a way 

 that heavy particles undergo less deflection than light ones. The effect is 

 that the heavy negative ions bombard a restricted area of screen near the 

 tube axis, and eventually damage it and produce 'ion burn', an insensitive 

 dark patch in the middle of the screen. 



465 



