DAVISSON CATHODE RAY TELEVISION TUBE 



863 



and a tantalum filament manometer was used for checking pressure. The 

 tubes used during demonstrations were sealed off, but the manometer was 

 retained as shown in the photograph in Fig. 6. In this case the hot tantalum 

 filament acted as a getter which successfully kept the pressure down to 

 about 10"® mm Hg. 



The aperture S' was .006" square. With a 5: 1 magnification this resulted 

 in a scanning line height on the screen corresponding to a 240 line picture 

 of about 7x8 inches. 



A stationary spot viewed on the screen showed a sharply defined rectan- 

 gular cross-section of approximately uniform brightness. Adjusted for no 



SLIT IMAGE 



APERTURE 



DIRECTION OF 

 SLIT TRAVEL 



MODULATING VOLTAGE — ^ 



Fig. 7 — Schematic diagram of modulation curve. 



overlap this resulted in a flat field with only a faint indication of line struc- 

 ture. If the image of the slit S falling on the square aperture 5' is perfectly 

 focussed, with edges parallel to the sides of the aperture; if no stray elec- 

 trons are present due to secondary emission or other causes, and if the 

 electrons in the beam all have the same thermal emission energy, then the 

 curve of beam current versus modulator voltage would be as shown in 

 Fig. 7. The current is zero until the leading edge of the slit enters the aper- 

 ture. The current then increases linearly until it reaches a maximum when 

 the slit fills the aperture. If the slit is wider than the aperture the curve 

 will have a flat top and will then decrease linearly as the trailing edge of 

 the slit travels across the aperture. 



The actual modulation curve did not show sharp corners, but was 

 rounded both at top and bottom as indicated by the dotted lines in Fig. 7. 

 The dispersion of thermal velocity of the electrons causes "chromatic" 

 aberration of the condensing lens system (Pi ; Pa) which therefore forms a 



