LOir VOLTAGE CATHODE RAY OSCILLOGRAPH 143 



The Focusing 



In some forms of Braun tube a sharp spot has been secured by using 

 a very high voltage, and therefore high electron velocity, so that 

 after the electrons have passed through one or two fine apertures 

 to make the beam parallel there is not time enough for the mutual 

 repulsion to spread the beam again appreciably before the electrons 

 strike the screen. With other tubes an external "striction" coil has 

 been used which maintains a strong longitudinal magnetic field in the 

 region between the anode and the cathode and which brings the 

 electrons to a focus on the screen. In the low voltage tube the spread- 

 ing of the electron stream is greater than in high voltage tubes because 

 of the greater time during which the mutual repulsion of the electrons 



Fig. 1 



acts, so that some means of focusing must be used. The electrons 

 can be brought to a focus by a longitudinal magnetic field so adjusted 

 that each divergent electron makes very nearly one complete turn of 

 a spiral and in travelling the length of the tube returns to the axis at 

 the screen. In this way a very sharp spot can be produced, but the 

 sensitivity of the beam to deflection is reduced very much by the 

 directing magnetic field. 



The method of focusing that is used in the present tubes grew out 

 of the suggestion by Dr. H. J. van der Bijl, that a small amount of 

 gas be introduced into the tube. This gas, at a pressure of a few 

 thousandths of a millimeter of mercury, serves to reduce to 1 mm. 

 diameter a spot which would be 1 cm. across in a high vacuum tube. 

 The sharpness of the spot depends also upon the current in the electron 

 stream so that the focus may be controlled by the cathode temper- 

 ature. The mechanism of this focusing action will be explained later. 



The presence of this slightly ionized gas also serves the purpose of 

 preventing the accumulation of charges on the glass, and it provides 



