| 摘要 |
Cathode unit intended for use in a fluorescent tube and designed in such a way that the service life of the tube is increased markedly as a result of a reduction in the loss of emmission material from the cathod (3). These material losses are caused by, inter alia, ion bombardment and by vaporization of the emission material. In order to achieve a marked increase in the reflection of released ions and molecules back to cathod surface, both ions and molecules which have been released through ion bombardment a ions and molecules which have been released vaporized from the cathod surface, the cathode (3) is surrounded by an electrically conductive cathode shield (6) in the form of a box-shaped casing which is, however, electrically insulated from the cathod. A opening (8) is made in the bottom of the shield for inserting the cathod (3). The open end of the shield (6) is sealed by means of a disc (9), preferably made of mica and provided with a centrally located hole (19). The hole diameter of the mica disc (9) should be selected so that it is as small as possible while bearing in mind the fact that the starting voltage for the tube should not exceed a predetermined value. The opening (8) in the bottom of the shield (6) should be of at least the same area as the hole in the mica disc (9). |
