VISUAL INDICATORS 



potential difference involved is V^^ and the energy acquired is e . F^^. We 

 have, if m is the mass of an electron 



ImvJ^ 



e-VA. 



y2 = -.2F. 



m 



On emerging from the third anode, the electrons encounter a lateral field, 

 due to the deflector plates, of magnitude Vjyjd. The lateral force on the 



Cathode □ 



Figure 32.11 



electron is (K^ . e)ld, producing an acceleration (Fp . e)ldm. The accelera- 

 tion acts for a time t, during which the electron is between the plates; but 

 t = Ijva. The lateral velocity which the electron attains is acceleration X time 



but 



2-V^/d 



It follows from the above that: 



(1) For a given deflection angle, a long tube produces a bigger deflection 

 per unit deflector plate potential difference — i.e. is more sensitive — than a 

 short one. 



(2) For good sensitivity the deflector plates should be long, or close 

 together, or both. In either case it is clear from the geometry that the 

 maximum possible deflection angle is severely limited by the beam striking 

 one or other deflector plate. Hence, electrostatic cathode ray tubes have to 

 be long in relation to their screen diameter. 



(3) For good sensitivity the EHT should be the lowest possible consistent 

 with securing a spot of adequate brightness and focus. The maker's figure 

 for cathode ray sensitivity is given in the form say, 800/ F^^ mm/ V, meaning 



454 



