DAVISSON CATHODE RAY TELEVISION TUBE 861 



is to produce an intense "focal spot" of electrons in the plane of the slit 

 aperture S (Fig. 1). 



The filament F is made of tungsten foil in the shape of a cross (Fig. 3) 

 and is fed by direct current with opposite ends of the cross connected to- 

 gether. This construction insures a nearly uniform temperature over the 

 center area of the cross and also minimizes magnetic fields set up by the 

 filament current, which would otherwise tend to disturb the electron opti- 

 cal symmetry. The filament resistance is approximately 1 ohm and the cur- 

 rent 10 amperes. The high emission current density from a tungsten fila- 

 ment was expected to result in a more intense focal spot than would be 

 obtainable with an oxide coated cathode. Also the large metallic structure 

 inside the tube might tend to cause deactivation or poisoning of an oxide 

 cathode. 



If the mechanical alignment were perfect the ''focal spot" of electrons 

 would fall directly on the slit aperture S. Because of unavoidable small mis- 

 alignments and particularly due to the fact that the filament cross is not 

 perfectly flat, such perfect symmetry cannot be insured without some cor- 

 rections. These corrections are supplied by a so-called collimating unit 

 CPi ; CCi . The unit consists of an electromagnet with insulated pole- 

 pieces (Fig. 4), and by applying small correcting voltages and currents to 

 the unit the "focal spot" is shifted until its most intense part falls on the 

 center portion of the slit aperture S. A second identical collimating unit 

 CP2 ; CC2 is mounted after the slit, in order to center the slit image on the 

 square aperture S'. 



The three circular aperture plates P2 , P2 , and P3 constitute the so-called 

 modulator lens system and serve to form an electron image of the slit S 

 on the square aperture S', with a magnification of 1:1. Accurate focussing 

 is accomplished by adjusting the potential of the plate P3 . 



The function of the modulator plates MiMi and M2M2 has already been 

 described. The photograph in Fig. 4 shows the modulator plate assembly. 



The set of three circular aperture plates P2 , Pa and PY comprise the 

 projection lens system, which forms an electron image of the square aper- 

 ture S' upon the fluorescent screen with a magnification of 5:1. Proper 

 focussing is accomplished by adjusting the potential of plate P4. 



Two sets of coils, mounted in the lateral corners of the tube housing, 

 were used to neutralize the earth's field. Each coil produces a field directed 

 at an angle of 45° from the vertical and, by properly adjusting the coil 

 current, it is possible to produce a practically uniform magnetic field, which 

 was used to center the beam on the screen. 



Because of the complicated mechanical assembly inside the tube there 

 was originally some difficulty in properly de-gassing the tubes and main- 

 taining vacuum. The earlier models were therefore continuously pumped, 



