DAVISSON CATHODE RAY TELEVISION TUBE 865 



coaxial terminal equipment so as to allow a small amount of 2500 kc carrier 

 leak to come through, the resulting vertical stripes were clearly visible, 

 in spite of the fact that the highest frequency in the video signal was only 

 about 800 kc. 



For the reasons given above no aperture equalization was employed for 

 the receiver; in fact, the excessive horizontal resolution was later on traded 

 for higher brightness as mentioned below. 



A slightly modified version of the Davisson tube was used later in the 

 1941 demonstrations of the transmission of 441 line, 30 frame television 

 signals over the 2.7 megacycle coaxial cable from New York to Phila- 

 delphia.! In this later tube the anode voltage was raised to 10,000 volts 

 instead of 5000 volts as used in the 1937 demonstrations. Furthermore, 

 the square aperture S' was changed to a rectangular aperture with the 

 horizontal side twice as long as the vertical. This alone of course doubled 

 the highlight brightness obtainable, and in the 1941 demonstrations the 

 received pictures had a highlight brightness of about 20 foot-lamberts, 

 using an aperture size of .0036" x .0072". 



Before concluding the description of the Davisson tube and its per- 

 formance, one more item should be mentioned. It was found that the glass 

 thickness of the tube's end wall gave rise to some halation due to internal 

 reflection from the outer surface, and this in turn resulted in a somewhat 

 degraded contrast range. A very much thicker glass wall would increase 

 the diameter of the circle corresponding to total reflection to a point where 

 the halation effect would be very much diluted and therefore negligible. 

 The effect of such a thick wall was obtained in the following manner.'' A 

 plate glass disk was mounted between one and two inches in front of the 

 tube fact and cemented to this by means of an airtight gasket. The inter- 

 vening space was then filled with Nujol, which has approximately the 

 same index of refraction as the glass. The resulting increase in contrast 

 range was very noticeable. 



It is interesting to note that at the time it was also proposed to add a 

 small amount of dark dye to the Nujol in order to further decrease halation 

 effects, and also to decrease the effect of ambient light. This is, of course, 

 the same principle that is now widely employed in present day ''dark glass" 

 kinescopes. 



Dr. Davisson designed this tube on the basis of his knowledge of electron 

 optics. At no stage did he depart from a design which would allow him 

 accurately to predict the performance. This accounts for the ''thin" lenses 

 used in the different focussing systems, for the small deflection angles em- 



t The transmitting equipment for these demonstrations was a film scanner employing 

 a Farnsworth image dissector and producing a 4 mc video signal. See reference'. 



