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BELL SYSTEM TECHNICAL JOURNAL 



glow lamp is a similar metal plate separated from the cathode by 



only a very small space (about one millimeter). At the proper gas 



pressure this space between the plates is with- 

 in the "cathode dark space" where no dis- 

 charge can pass. As a consequence, the glow- 

 discharge develops on the outer surface of 

 the cathode, where it shows as a perfectly 

 uniform, thin, brightly glowing layer. 



As an aperture in the disk moves across 

 the field, the observer, looking through at the 

 neon lamp behind the disk, sees the aperture 

 as a bright point. When the disk is rotat- 

 ing at high speed, the observer, owing to the 

 persistency of vision, sees a uniformly illumi- 

 nated area in the frame, provided that a 

 constant current is flowing through the lamp. 

 (The line structure that would otherwise ap- 

 pear in the field is largely eliminated by us- 

 ing apertures that slightly overlap in their 

 paths across the field.) 



The brightness of the neon lamp is directly 

 proportional to the current flowing through 

 it; and when a picture is being received, the 

 lamp is operated directly from the received 

 picture current. As a result of the system 

 just described, there is at any instant, in the 

 field of view at the receiving station, a small 

 aperture illuminated proportionally to the 

 brightness of a corresponding spot on the dis- 

 tant subject. Consequently, the observer 

 sees an image of the distant subject repro- 

 duced in the frame at the receiving station. 



Fig. 8 shows the external appearance of the 

 disk type of receiver in which the images ap- 

 pear. The disk rotates inside of a rectangu- 

 lar cabinet and the observer views the image 

 through the shielding window. The largest 

 disk, three feet in diameter, gives a 2 in. by 

 2]^ in. rectangular image. Each television 

 receiver is also equipped with a telephone re- 

 ceiver and transmitter; and it is possible for 



the observer to both see and converse with a distant person at the 



same time. 



Fig. 7 — Neon receiving 

 lamp. The rectangular 

 cathode is covered by a 

 uniform layer of glow 

 slightly larger than the 

 field of view on a televi- 

 sion disk 



