BELL SYSTEM TECHNICAL JOURNAL 



in a very short time, and the ray can follow faithfully potential varia- 

 tions at the plates that are very rapid. 



Let us see how the ray responds to voltage applied to the plates. 

 The ray normally travels with a speed v along the tube. Referring 

 to Fig. 4, the ray now passes between two plates of length / and sepa- 



Fig. 4 — Electrostatic deflection. 



ration d, between which a potential difference V is maintained. While 

 the ray is passing between the plates the electrons are subject to an 

 acceleration, 



m d 



m 



This continues during the time / = l/v. The transverse velocity ac- 

 quired is therefore 



lYLL 



m d V 



at = 



The ray then travels on in a straight line to the screen which it meets 

 at a distance D from the normal position. The deflection D bears the 

 same relation to the length of the beam, from the center of the deflect- 

 ing plates, as the transverse velocity bears to the longitudinal. 



This brings out an interesting point in connection with the design 

 of the deflector plates. For high sensitivity the plates should be long 

 and close together but the plates must not cut the path of the deflected 

 ray. If we want to get a certain maximum deflection D with a tube 



