THE CATHODE RAY OSCILLOGRAPH 



17 



of the tube. In the 224 tube, with argon at the pressure of .01 mm. 

 the focusing occurs at about 20 microamperes in the beam. Less cur- 

 rent makes a large unfocused spot; more current produces a focus 

 before the screen is reached, with consequent spreading of the spot 

 again. Hence it is that the spot on the fluorescent screen is focused 

 by adjusting the heating current of the cathode. 



Fig. 21— General Electric, 1928." 



Besides focusing the electrons and preventing the accumulation of 

 charges in the tube, the gas plays various other roles. One very 

 curious effect of the gas is that it decreases the sensitivity of the tube 

 at small deflecting voltages. When uniformly varying voltage is ap- 

 plied to a pair of deflector plates so as to move the spot across the 

 screen, the spot seems to hesitate for an instant at the centre of the 

 screen, appearing to be brighter there. The effect has always been 

 observed but particular attention has recently been given to it by 

 Professor Bedell. ^o The explanation has to do with space charge be- 



" Lee, E. S., G. E. Rev., 31, p. 404, 1928. 



20 Bedell, F. & Kuhn, J., Phys. Rev. 36, p. 993, 1930. 



