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516 T. M. C. LANCE November
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 aluminum 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 aluminum 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 stabilized heater voltage with the heavy current of 14 amp 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 disk to prevent the light from the cathode falling directly onto the viewing screen.
13. A crucial point of the design is a thick polythene 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 disk of the same material which protects the scan coils against flash-over from the outer surface of the glass on damp days.
14. Is the deflection unit. This consists of four windings on an iron-toothed stator having 30 teeth. In order to obtain the insulation