British Kinematography (1949)

184 at room temperature is blown across the face from an air nozzle. In the future it is planned to treat the outer face of the tube with an anti-reflecting coating. 5. Is the air nozzle, the design of which is quite critical to give uniform cooling over the whole face without noise. These two requirements go against each other, and a compromise has been found which is reasonably successful on normal television programmes. 6. Is the anode connection to the tube made through an internal wire connection welded to a large platinum disc pressed in to the inner surface of the glass envelope. The size of the disc is sufficient to ensure a low resistance contact to the graphite internal coating of the tube. The tube is exhausted through the side tube carrying the anode connection. 7. Is the lead-in cable which is polythene insulated. The diameter of this cable is surprisingly small, but on D.C. conditions is adequate for 60 Kv. without trouble. 8. Is one of the two getter tubes. This one contains 12 batalum getters which can be fired by high frequency. The first is fired when the tube Is manufactured and first sealed to the pump. At certain periods during life two or more additional getters are fired to maintain a good vacuum. 9. The second getter tube contains a zirconium wire getter which is continually heated during the running of the tube and the use of which has been found very advantageous. Unfortunately since the getter is near the anode coating it is necessary to maintain it at anode potential which involves a heating transformer insulated to full anode volts and two extra H.T. connections. 10. Is the electron-permeable aluminium coating applied to maintain the phosphor at anode potential and prevent " sticking," and which also considerably improves the contrast of the picture by obscuring internal reflection of light. The presence of the aluminium film also enables us to use a bright tungsten cathode, and the light from this, which is considerable, is also obscured. 11. Is the gun assembly which will be described separately, but attention is drawn to the heavy connectors necessary to maintain stabilised heater voltage with the heavy current of 14 amps, required by the tungsten strip cathodes. 12. Is a small air jet, which is directed into the pinch, for the purpose of cooling the pinch and copper lead-out wires. Note that the assembly ends with an obscuring disc to prevent the light from the cathode falling directly on to the viewing screen. 13. A crucial point of the design is a thick polvthene insulating sleeve tested to withstand 100 Kv. to earth, which provides the main insulation between the tube and the scan and focus coils which are at earth potential. Initially we relied on the glass neck of the tube to provide this insulation, but although each piece of glass was given a prolonged test at double working voltage we had many losses of tubes due to the puncturing of the glass after a few hours' run. The insulating sleeve is welded to a disc of the same material which protects the scan coils against flash-over from the outer surface of the glass on damp davs. 14. Is the deflection unit. This consists of four windings on an irontoothed stator having 30 teeth. In order to obtain the insulation required the minimum wall thickness of the sleeve under the scanning yoke is 6.5mm., and the problem of supplying sufficient scan current in these coils is very difficult, particularly at 15 Kc/s., which is the line frequency for 025 lines, and also as the coils appear at the end of a long cable. 15 Is the focus coil. This is a complicated and costly unit. It consists of a long solenoid embracing the electrode system of the tube and