16 BELL SYSTEM TECHNICAL JOURNAL 



deflection for each volt applied to the deflector plates, or 1 mm. for 

 each ampere turn in a pair of small coils placed outside of the tube. 

 These figures are for the normal driving potential of vSOO volts. 



The flattened end of the bulb, on which the electron beam impinges, 

 is covered with a fluorescent material. The powder is a mixture of 

 zinc orthosilicate and calcium tungstate, both specially prepared for 

 fluorescence. The zinc silicate produces a green light of high visibility 

 and the tungstate a blue light of high photographic activity, so that 

 with the mixture the same tubes can be used efficiently for both visual 

 and photographic observations. 



As said before, there is some gas in the tube. One purpose of the 

 gas is to produce a small amount of ionization in the tube which pre- 

 vents any unduly large charges from accumulating on the glass walls 

 and screen. Electrons deposited on the glass are neutralized by posi- 

 tive ions produced in the gas. An electron current equal to the current 

 in the beam drifts back through the gas to the anode. The chief 

 result of this drift of electrons is that a negative space charge is formed 

 in the tube which decreases the speed of the electrons before they 

 strike the screen as if the driving potential had been lowered by about 

 50 volts. 



The other and more important purpose of the gas is to bring the 

 electrons of the beam to a sharper focus at the screen. The beam is 

 diffuse for two reasons, first because it is originally divergent, and 

 secondly because of the natural electrostatic repulsion that tends to 

 force the electrons apart. The gas serves to overcome these actions 

 in the following way: As the beam of electrons travels down the length 

 of the tube, some electrons collide with atoms of the gas and separate 

 the atom into an electron and a positive ion. The impact of the 

 electron does little to displace the massive positive ion from the posi- 

 tion it temporarily occupies while two electrons are immediately shot 

 out of the path. The result is a column of positive ionization down 

 the length of the beam, with a negative space charge surrounding it. 

 This produces a radial electrostatic field which tends to bend the path 

 of the outer electrons of the beam inward toward the centre. The 

 magnitude of this action depends on the degree of the differential 

 ionization. This, again, is the greater the higher the gas pressure and 

 the greater the current in the beam. The gas pressure must be low 

 enough so that the larger fraction of the electrons reach the screen, 

 and then the current in the beam must be such as to produce the de- 

 sired focusing action. The heavier the ions the lower can the pressure 

 be. The condition for a focus then involves the kind and pressure of 

 gas, the speed of the electrons, the current in the beam and the length 



